Sunday, October 4, 2015

The PPSh-41 Submachine Gun

In our last post, we studied the German MP40 submachine gun, an influential design. In today's post, we will study a submachine gun of Soviet origin, the PPSh-41.

The PPSh-41 derives its name from the Russian name Пистолет-пулемёт Шпагина which is pronounced as Pistolet-Pulemyot Shpagina (i.e. "Shpagin Machine Pistol"). Shpagin refers to the designer of this weapon, Georgy Semyonovich Shpagin. The number -41 refers to the year that this weapon was first produced.

A PPSh-41 with a drum magazine. Click on the image to enlarge.
Image released under a Creative Commons Attribution-Share Alike 3.0 Unported license by Lposka

The gun was nicknamed the "Peh-Peh-Sha" (after the Russian pronunciation of the first letter of each word), or sometimes as "Papasha" (the Russian word for "Daddy"). During the Korean war, US and UK forces who encountered this in the hands of North Koreans and Chinese troops, nicknamed this the "burp gun" on account of the "brrrap" sound it produces when firing in automatic mode.

The origins of this weapon have to do with two reasons. First, when the Soviets fought against Finland in the Winter War of 1939-40, they found that Finnish forces armed with Suomi KP-31 submachine guns could take on superior numbers of Soviet forces fighting with bolt-action Mosin-Nagant rifles at close quarters. The Soviets already had a submachine gun of their own, the PPD-34, but it was hard to produce economically and quickly. Many of the metal parts of the PPD-34 were produced by milling and machining operations, which made it take a long time to produce.

Then, in the summer of June 1941, the Germans launched Operation Barbarossa and invaded the Soviet Union. A large number of Soviet small arms as well as a number of the factories capable of manufacturing small arms, fell into German hands. The Soviets needed a weapon that could be produced quickly and without the use of sophisticated manufacturing technologies.

Enter Georgy Shpagin, a son of a Russian peasant, who had started off life as a carpenter, before being drafted into the Russian army and becoming a gunsmith and later a weapon designer. He took the PPD-40 (an improved version of the PPD-34) as the starting point of his design. While the PPD-40 used mostly machined parts, his new PPSh-41 design largely relied on stamped steel parts. Most of these parts could be manufactured by unskilled workers using equipment that is commonly seen in small workshops and automobile repair garages. Parts were joined together by welding. The PPSh-41 design used less parts than the PPD-40 and could be manufactured in about half the time it took to make a PPD-40. Only the barrel needed a little more sophisticated factory equipment to manufacture and even this was simplified, as will be discussed shortly.

The cartridge used for his weapon design was the 7.62x25 mm. Tokarev pistol cartridge, which was already in service with Soviet forces for some years previously. To simplify the production of barrels, he often used existing barrels designed for the Mosin-Nagant M1891 rifle, which were already bored for 7.62 mm. A single long M1891 barrel would be cut into two shorter barrels for the PPSh-41 as they both use 7.62 mm. diameter cartridges, and then each shorter barrel would have a chamber machined to accept the 7.62x25 mm. Tokarev cartridge. The barrels were chrome lined for extra reliability, but most of the other parts were much more cheaply produced.

The weapon itself fires from an open bolt using the blowback action (similar to all the designs we studied in the last few posts). Earlier models had a selector lever to select the firing mode and later models were designed to fire in full automatic mode only. The firing rate of this weapon is about 1000 rounds per minute, which is the highest of all the submachine guns during World War II. The receiver is hinged to make disassembly and cleaning the weapon in the field easy to do. The chrome lining in the barrel enables it to use corrosive ammunition and not be cleaned for a while in the field.

The first prototypes went into production in November 1941 and by the spring of 1942, the factories were making them at an incredible rate of 3000 per day and by the end of 1942, over 1.5 million of these were made.

Initially, the PPSh-41 was designed to use a drum magazine of 71 round capacity, which was an exact copy of the Finnish Suomi KP-31 submachine gun that the Soviets had encountered in the Winter War. The drum had a spring loaded feed mechanism that the user would have to wind up like a watch to use. However, the drum magazine had reliability issues if it was loaded to its full capacity of 71 rounds and therefore, users would often load less than 64 or 65 rounds to avoid this problem. The drum magazine was also made of a thin sheet metal of 0.5 mm. thickness. This meant that it was pretty easy to bend, which could also cause feed failures. Also, the springs inside the drum magazines would often weaken after a while and make feeding unreliable. On top of that, it took a long time to load a drum magazine. Therefore, newer versions of the PPSh-41 were supplied with 35 round box magazine instead, after about 1942.

A PPSh-41 with a box magazine. Click on the image to enlarge.

The box magazines were easy to carry around in pouches and more reliable than the drum magazines. The thickness of the metal used to make magazines (both box and drum magazines) was later changed from 0.5 mm to 1 mm., which made them much harder to bend accidentally. Many infantrymen would carry one drum magazine in the gun and several box magazines in pouches.

The Soviets made about 6 million of these weapons by the end of World War II. Some of these were later gifted to other communist countries and some African countries. The design of the PPSh-41 was also shared with some countries, most notably, the North Koreans and the Chinese, who made their own versions of this weapon during the Korean war. The Chinese also gave their design to the Vietnamese, who made their own version during the Vietnam war. US and UN troops who faced this weapon in Korea and Vietnam nicknamed this the "burp gun" on account of the noise it made when firing. The weapon still remains in use by some irregular forces around the world.

Saturday, September 26, 2015

The MP40 submachine gun

In our last post, we studied about the German MP38 submachine gun. In today's post, we will study its successor, the MP40 submachine gun.

The MP38 submachine gun was a pretty successful design and performed the tasks that it was designed for well. However, it could not be manufactured quickly because of the way that some of the parts were made (many parts were machined). The original manufacturer of the MP38, Erma, gave the design to C.P. Haenel to help with production, but even these two companies together could not meet the demand for these weapons from the German Army. As a matter of fact, they could not even meet their own production targets that they had set themselves! By 1939, the German War Office asked for changes in the production methods to speed up manufacturing.

Both Erma and Haenel were relatively smaller arms manufacturing companies, so larger companies, such as Krupp, Steyr and Merz Werke, were recruited to help. The engineers from these larger companies were experienced in modern mass production technologies and they made changes to the MP38 model to speed up its production rate. The new model was introduced in 1940 and was called the MP40.

A MP40 with the stock folded. Click on the image to enlarge.
Image licensed under the Creative Commons Attribution-Share Alike 3.0 Unported license

A MP40 with the stock extended. Click on the image to enlarge
Image licensed under the Creative Commons Attribution-Share Alike 3.0 Unported license

In the MP38, many parts were machined, which made the process of manufacturing expensive and slow. In particular, the receiver took multiple machining operations on a block of steel, to make the final product. Many of the parts of the MP40, on the other hand, was made by stamping out steel parts using dies and high speed presses and then joining the parts together by using brazing and spot-welding techniques. Only the barrel and the bolt were machined in the MP40. This made the production rate much faster. Over 1.1 million MP40 submachine guns were produced between 1940 and 1945.

Therefore, the MP38 was one of the last submachine guns to be largely manufactured with machining technologies and the MP40 was one of the first submachine guns designed specifically to be manufactured quickly and cheaply. This is why the quality and finish of the MP38 is usually superior to the MP40, but the MP40 is just as effective as the MP38.

One weakness of the MP40 is that it uses the same double-column single-feed magazines as the MP38 and therefore, it has the same issues with dirt and jamming.

The MP40 comes in multiple variants as the Germans continued to improve the design to reduce the manufacturing time. Erma, Haenel and Steyr were the main manufacturers and a number of the parts were made by other subcontractors. For instance, one of the manufacturers of the plastic grips was the German electrical giant AEG (these days, they are known as AEG/Electrolux). Krupp and Merz Werke produced a lot of the stamped parts (tubes, frames etc.) for both Erma and Haenel (Steyr produced their own stampings), Mauser Werke supplied barrels and so on.

The MP40 is really the prototype for a number of other submachine guns that have been made since. It influenced the development of the US M3 Grease gun and the British Sten gun, as both the US and the UK saw the need for a rapid-firing weapon that could be produced quickly and cheaply. It is one of the first weapons to use plastics instead of wood. The forward folding stock design was first designed for the MP 40 and later copied by other people, notably the AK family.

Interestingly, the MP40 was often referred to by Allied troops as the "schmeisser", named after the famed German designer, Hugo Schmeisser. In reality though, he had very little to do with this design, as most of it was done by Berthold Geipel and Heinrich Vollmer and later improved by mass-production specialist engineers at other companies. About the only contribution of Hugo Schmeisser to the design was the magazine, which Hugo Schmeisser had designed and patented for another weapon, but the German War Office insisted on using Schmeisser's magazine design for the MP38 and MP40. Interestingly, Hugo Schmeisser did design the MP41, which is largely a MP40, but using a wooden stock and a selector lever from the MP-28, which Schmeisser had worked on earlier.

Even though the German factories stopped producing the MP40 after World War II, they remained in service with other countries for several years since. For instance, France used the MP40 during the 1950s and 1960s and Norway still used them until about 1990 or so.

Monday, September 21, 2015

The MP38 Submachine Gun

In our last couple of posts, we traced the development of the American M3 Grease Gun and the British Sten Gun. In today's post, we will study the gun that preceded them both and inspired their creation. Today, we will study about the German MP38 submachine gun.

A MP38 submachine gun. Click on the image to enlarge.

The full name of this weapon is Maschinenpistole 38 (i.e. "Machine Pistol 38"). The origins of this weapon have to do with advancements in military doctrine after World War I. It was during and after this war that military concepts such as tanks, troop carriers, paratroopers etc. started becoming popular. There was need for a rapid firing weapon that could be used from inside vehicles such as tanks and trucks, as well as carried by paratroopers. General infantry crews also had use for a small, light, rapid firing weapon to protect themselves, while transporting their heavier machine guns to other locations. The German Wehrmacht was beginning to develop its theories of lightning warfare (i.e. the Blitzkrieg), where the idea was to use rapidly moving mechanized forces on the ground, operating together with air support. The German Army Weapons Office published a requirement asking for a suitable submachine gun to be designed in 1938.

The German arms company, Erfuter MaschinenFabrik Gmbh, (translation: Erfurt Machine Factory Inc.), better known by its shorter trade name, Erma, began to develop a weapon to meet this requirement. Instead of developing a weapon from scratch, they modified an existing design that they were already working on, the MP36. The MP36 was actually a compact version of the Erma EMP (Erma Maschinen Pistol (translation: Erma Machine Pistol)).

Erma EMP. Click on the image to enlarge.

The Erma EMP was actually developed by Heinrich Vollmer, a German arms designer who had his own small arms manufacturing company. He had designed this gun in 1930, based on an earlier design he had worked on in 1925 and 1928, but his newer design used a side-feeding box magazine (a feature later seen on the Sten and Sterling submachine guns) and a telescoped return spring (a feature used in the MP38 and MP40). However, the German military stopped supporting his company in 1930 and since he didn't have the financial capability to manufacture these, he sold the design and manufacturing rights to Erma, who started producing his design in 1932 as the Erna EMP. Some of these Erma EMP guns were sold to Spain and some South American countries.

The Erma company had started to make a compact version of the EMP, which they called MP36, which was a selective fire weapon. When the requirement came from the German War Office, they took this prototype and simplified it a little more and called in the MP38. One of the requirements was to use plastics in the furniture instead of wood, and the MP38 uses bakelite for its handguards and grips. After the German War Office announced that they had accepted the design, Erma started to produce the weapon in 1938 and a couple of years later, C.G. Haenel also started producing the weapon.

The MP38 uses a blowback action and features a folding butt, which reduces its length considerably when folded. It uses the 9x19 mm. parabellum cartridge, which was also used by the Luger pistol in World War I. All the critical operating parts are contained in the receiver. There is only one mode of fire: full automatic mode. However, due to the slower rate of fire, it is possible to fire single shots by pulling and releasing the trigger quickly. The magazine holds 32 rounds and is a double column, single-feed type. Uniquely, on the underside of the barrel, there is a "lip" or "resting bar". This is designed to keep the weapon steady, when firing over the side of an open-top armored personnel carrier. The idea is that the lip latches on the side of the vehicle's wall and prevents the recoil from driving the weapon back into the vehicle's compartment. A cooling fin at the end of the barrel helps dissipate some of the heat. The pistol grip and handguard are made of bakelite plastic to save weight and this is the first submachine gun in history to feature plastic parts.

Interestingly though, the receiver of this weapon is made of machined steel instead of being stamped. Therefore, it took longer to make the receivers. So while the gun was a success, it could not be made fast enough and production only lasted a couple of years before they developed the MP40 in 1940. The MP40 was made of stamped steel and used spot welding technologies to speed up production. Production of the MP38 did continue until 1941 though.

A weakness of the MP38 and MP40 was the magazine, which tended to be affected by dirt. The same magazine design was copied by the British for their Sten gun, so that they could reuse German magazines and therefore, the Sten gun inherited the same weakness as well.

One more interesting fact is that the Allies incorrectly referred to the MP40 as the Schmeisser, named after the famous German weapons designer, Hugo Schmeisser. In reality, he had very little to do with the design of the MP38 and MP40. If anything, his sole contribution was a patent he held on the magazine, which he had actually designed for a different weapon.

The MP38 may be considered as the one of the last submachine guns that was built out of machined parts. Therefore, it has a better quality and finish, compared to the MP40 that followed it. We will study the MP40 in the next post.

Saturday, September 12, 2015

The Sten Gun

In our last post, we saw how America adopted the M3 a.k.a the Grease Gun. In today's post, we will look at one of the guns that inspired it, the British Sten gun. This was a gun that was designed to be manufactured cheaply and easily and we will study its origins and design today.

Different Sten gun models. Click on the image to enlarge.

First, we must go back in history to Europe in the summer of 1940. German soldiers were sweeping through Belgium and France and allied troops were in a desperate situation and trapped in the tiny port of Dunkirk. The British deployed every boat and ship available to rescue the stranded Allies and in nine days (27th May - 4th June), over 300,000 soldiers (British, French, Polish, Belgian, Dutch etc.) were evacuated to England. However, this rapid evacuation also resulted in soldiers leaving their equipment behind and large amounts of firearms fell into the hands of the Germans. Shortly after that, the Battle of Britain started and many factories in England were bombed. As a result of all this, there was a shortage of small arms in Britain. The British were buying Thompson submachine guns from the United States, but the factory could not keep up with the demand (and after 1941, many of those Thompsons went to the US military, so they couldn't supply anyone else anyway). Therefore, a decision was made to design a submachine gun that could be made in England quickly and cheaply.

The task of designing this new weapon fell to Major R.V. Shepherd of the Design Department at the Royal Arsenal, Woolich and Mr. Harold J. Turpin, of the Design Department of the Royal Small Arms Factory, Enfield. The design they came up with was called the STEN. The "S" and "T" in the name came from the first letters of the designers last names (S from Shepherd and T from Turpin) and the "EN" came from the first two letters of "Enfield".

From the beginning, the aim was to design a cheap gun that could be manufactured with a minimum of machining operations. It had to be capable of being manufactured in small workshops and produced as quickly as possible. It also had to be capable of single shot and automatic fire and designed for close range fighting. It was designed to use the 9x19 mm. Parabellum Luger cartridge, which was also used by the Germans. The Sten was also deliberately designed to fit German 9 mm. magazines from the MP-38 and MP-40, so that they could use captured German ammunition and equipment if needed.

The design that they came up with was a submachine gun using a blowback mechanism and firing from an open bolt. When the weapon is cocked, the bolt remains at the rear of the weapon. When the trigger is pulled, the bolt is pushed forward by spring pressure and strips a cartridge from the magazine, chambers it and then fires it. The firing pin is fixed in front of the bolt. After the cartridge discharges, the bolt moves rearward against spring pressure and inertia of the heavy bolt and then recocks itself. The working components of this weapon are housed in a basic tubular metal receiver with a barrel on one end and a wire shoulder support welded to the other end, with a simple trigger mechanism in between

The Sten gun, Mark I

The first version of the Sten gun, the Mark I model, came with a conical flash hider and contained some wooden parts (the foregrip and part of the stock). The front pistol grip could also be rotated to make the firearm smaller and therefore, easier to pack. Production started in late 1940 and about 100,000 of this model were made.

Compared to the Mark I model, the Mark II model was much more stripped down. The flash hider was removed and the folding front pistol grip and all the wood were eliminated as well. This made the Mark II smaller and lighter than the Mark I model.

The Mark II variant was the most commonly manufactured model and about 2 million of these were produced. Some Mark II models were made with integral suppressors attached and were classified as Mark II (S)

The Mark III variant was even more stripped down than the Mark II model and was first produced in 1943. In this model, the receiver and the barrel shroud are made from a single tube, by wrapping a sheet steel plate into a cylindrical shape and welding the top. This model is also a bit lighter than the Mark II model.

The Sten Mark III model was the second most commonly produced model of the Sten gun family and was the most stripped down model of the series, and therefore the lightest version.

By 1944, the threat of a German invasion of Britain was over and the Sten gun quality improved. Models Mark IV and Mark V had better quality fit and finish and even came with wooden parts.

The Sten gun model Mark V

The Mark IV model was a paratrooper's model with a folding stock, but never got off the prototype stage. The Mark V model had better sights and finish and came with a bayonet attachment as well.

The Sten was designed to be manufactured quickly and easily. This is why most of the components could be manufactured by stamping sheet metal and doing some minor welding. From the beginning, many of the parts were subcontracted to small workshops, with final assembly being done at the Enfield factory. This was especially useful as the larger factories were being bombed from the air by the German Air Force, early on during the war. The design was made simpler with each generation and the Mark III model only had 47 parts. Interestingly, one of the largest manufacturers of the Mark III model was a toy company called Lines Brothers. The Sten was really cheap to manufacture and only cost about $10 to make, which was much cheaper than the Thompson submachine gun, which cost about $200 then.

While Sten guns were cheap to manufacture, they occasionally had jamming issues as well. The gun was designed to use the same magazine as the German MP-38/MP-40, so that people could reuse captured equipment. However, it also inherited the problems of the German magazine, in particular dirt could cause it to jam. In the absence of a pistol grip and forward grip in the Mark II and Mark III versions, some soldiers would hold the magazine with the supporting hand, causing it to wear out the magazine catch and cause failure to feed issues. The safety device was rudimentary and there was a danger of accidental discharge upon dropping the weapon, especially since many were crudely made. The Mark V model attempted to fix some of these issues.

The Sten was loved and hated by its users at the same time. Many didn't like its peculiar appearance and reliability (at least for the Mark II and Mark III models) and it was nicknamed the "Plumber's Nightmare" and the "Stench Gun". However, they liked its cheap cost and short range firepower. It was manufactured during World War II by many British companies, as well as workshops and factories in Canada, Australia, France, Poland, Denmark, Norway etc. It was responsible for the US manufacturing its own cheap submachine gun model: the M3 grease gun. Towards the end of World War II, even the Germans got in the act and made over 28,000 copies of the Sten gun. After World War II, many were made in small workshops Israel in 1948. The Sten gun is still in use in some countries around the world.

Wednesday, September 9, 2015

The M3 Grease Gun

In today's post, we will study a submachine gun that was in US service for 50 years and is still used by some military forces elsewhere in the world. Today's post will study about the famous M3 and M3A1 submachine guns, more popularly known as "grease guns".

The origin of this design has to do with the onset of World War II. In Europe, the Germans had developed the MP-40 submachine gun and the British had the Sten submachine gun. Both these weapons were chambered to fire the reliable 9x19 mm. Luger cartridge. In 1941, the US Army Ordnance Board conducted some studies on the effectiveness of these weapons on the battlefield and determined that there was a need for similar weapons for the US military as well. At that time, the US military had already adopted the Thompson submachine guns (which were developed at the end of World War I) into service in 1938, but the Tommy guns were relatively expensive to manufacture. The Ordnance Board wanted a weapon that could fire the same .45 ACP cartridge of the Thompson, but which could be manufactured much cheaper and could be fired as accurately as well.

The final list of requirements included:

  1. Weapon to be made of metal completely, with largely sheet metal construction, in order to speed up manufacturing.
  2. Weapon designed to fire .45 ACP cartridges, since the US military was already using this cartridge (The Germans and British were using 9 mm. Luger cartridges for their submachine guns)
  3. Weapon to be designed for fast production, with a minimum of machining operations.
  4. Weapon to be designed to be cheap to manufacture (cheaper than the Thompson submachine gun)
  5. Weapon to be designed as reasonably accurate. It was required to demonstrate that this weapon could put 90% of shots fired in a standing position in full-automatic mode into a 6x6 foot target at a distance of 50 yards. 
  6. Weapon to be capable of firing in both semi-automatic and full-automatic modes. This requirement was later removed during development of the weapon and it was designed to fire full automatic only.
The task of designing this weapon fell to George Hyde of General Motors' Inland Division. George Hyde was actually of German descent and had emigrated to the US in 1927, whereupon he changed the spelling of his last name from "Heide" to "Hyde". He was a well known gun designer in 1941 and was put in charge of designing the new weapon. Meanwhile, another engineer named Frederick Sampson was put in charge of preparing the tooling for mass production of this weapon. During the design phase, a conversion kit was also designed, which would allow the user to quickly convert the gun from firing .45 ACP to firing the 9 mm. Luger cartridge.

The prototypes were completed in late 1942 and approved for mass production. The Guide Lamp division of General Motors (the same people that manufactured the FP-45 Liberator pistol) was put in charge of manufacturing it. While the weapon was given the official designation of "U.S. Submachine Gun, Caliber .45, M3", many people noticed that it resembled grease guns used by automobile mechanics and therefore, the new weapon was nicknamed the "Grease Gun".

The M3 Grease Gun. Click on the image to enlarge.
Image licensed under the Creative Commons Share-Alike 3.0 Unported license by Curiosandrelics

The M3 was designed to be cheap. The manufacturing cost of one of these in World War II was around $18. Only three parts, the bolt, the barrel and the firing mechanism, were precisely machined. Most of the other parts of this gun were manufactured by using metal stamping and pressing technologies. Spot welding and seam welding were used to join most of the parts together and some other parts were riveted together. These processes allowed the gun to be manufactured very rapidly and with low cost. The stock was simply made from a single steel rod, which was bent into shape. The ends of the stock were drilled and tapped, so that it could be used as a cleaning rod, as well as a disassembly tool. In the M3A1 model, the stock also had a tool welded to it, to be used to load a magazine. The safety was a projection on the inside of the ejection port dust cover, which locked the bolt into the forward or rear positions. However, since the metal around the dust cover was so thin, it could get bent easily and the safety mechanism wouldn't work any more.

The two halves of the receiver were made by stamping sheet steel and then the two halves were welded together. The bolt was made heavy and made to run on two parallel guide rods, which had twin return springs. This allowed the receiver to be made to looser tolerances. The barrel was cold swaged to save time and cost. The weapon was designed to be striker fired, with a fixed firing pin inside the bolt.

When the M3 first entered service, no replacement parts were supplied. The weapon was originally intended to be a low-cost tool that could be discarded when it ceased working. However, due to a temporary shortage of M3s in the middle of 1944, the US Army Ordnance workshops manufactured some spare parts to keep existing weapons operational. There were also a number of issues with the M3 and several improvements were incorporated into the M3A1 model, which went into service in December 1944. During the Korean war, existing M3 guns were converted to the M3A1 standard.

While it was hoped that this gun would be produced in large numbers to replace the Thompson submachine gun in US service, this did not happen and there were about three times as many Tommy guns in service than M3/M3A1 at the end of World War II.

Due to its cheap cost and portability of the weapon, it was issued to paratroopers, tank crews and truck drivers. Even though it was withdrawn from frontline service in the US military in 1957, they were still issued to tank and truck drivers and were carried by US tanks during the 1991 Gulf war. The M3A1s were finally replaced starting in 1992, with the M4 carbine. That means it served in the US military for 50 years (1942 - 1992). It is still used by military forces in other parts of the world.

Thursday, September 3, 2015

How to Measure the Chamber Dimensions

Assume that you've just purchased a firearm and brought it home and found that you don't know what cartridges the firearm takes. This could be because the firearm was manufactured a long time ago, when manufacturers did not stamp the cartridge dimensions on the outside of the weapon. Or, the previous owner could have a custom barrel made for the firearm. Perhaps a previous owner liked to experiment with wildcat cartridges. Or perhaps, you would like to measure the dimensions of the chamber to make sure it isn't too worn out.

As you can see, there are plenty of reasons why someone might need to measure the dimensions of a firearm chamber. So how is it done? We will study that in this post.

One of the most common and reliable ways of measuring chamber dimensions is to use a metal alloy called "cerrosafe chamber casting alloy". This is an alloy made of 42.5% bismuth, 37.7% lead, 11.3% tin and 8.5% cadmium. Cerrosafe alloy was originally used to produce castings of toy soldiers and because of its low melting point, it was also used for fuse links in fire sprinkler heads.

A bar of cerrosafe alloy. Click on the image to enlarge. Public domain image.

Cerrosafe has some properties that make it very useful for the job of measuring chamber dimensions:

  1. It is relatively cheap to buy and easily available. A bar of cerrosafe weighing 1 lb. (0.45 kg.) can be purchased for prices ranging from $25 to $45 or so, from various sources.
  2. It is generally reusable, unless the user overheats it too much. Therefore, the alloy can be reused multiple times for many years.
  3. It has a low melting point and melts at temperatures between 158 to 190 degrees Fahrenheit (or 70 to 87.77 degrees centigrade). Note that this is below the boiling point of water (212 degrees Fahrenheit or 100 degrees centigrade). This means it can be melted by using devices commonly found in every kitchen or garage (e.g.) stoves, hot plates, blow torches, small lamps, a double boiler etc. This also means that hobbyists can use it without purchasing any special equipment.
  4. An unusual property of cerrosafe is the way it shrinks and expands as it cools down. Initially, the cerrosafe shrinks slightly during the first few minutes of cooling, just like any other normal metal or alloy does. This makes it easier to remove from a firearm chamber. However, after about 30 minutes have passed, it starts to expand while cooling. After about an hour or so, it returns back to about the original dimensions of the chamber and after about 4 days (96 hours), it expands to slightly larger than the dimensions of the chamber it was cast in (it expands about 0.0025 inches per inch of size after 96 hours, which is actually pretty negligible).
  5. One more useful property of cerrosafe is that it does not bond itself to the barrel metal like plain lead or tin do. This also makes it easier to extract out of the barrel.
In case the user thinks that cerrosafe is too expensive, there is another product made by Rotometals Inc., called Rotometals chamber casting alloy. This is practically the same composition as cerrosafe, but doesn't come in a nice stamped bar with letters on it, but is sold as a conical ingot instead. However, it has identical properties as cerrosafe, but costs about 50% less (A 1 lb. ingot of  Rotometals chamber casting alloy only costs about $15 to $20 from various sources).

An ingot of Rotometals chamber casting alloy. Click on the image to enlarge.

The following table shows how cerrosafe (or rotometals) chamber alloy's dimensions change with time, as it gradually solidifies.

Time Contraction/Expansion per inch
2 minutes-0.0004 inches
6 minutes-0.0007 inches
30 minutes-0.0009 inches
1 hour0.0000 inches
2 hours+0.0016 inches
5 hours+0.0018 inches
10 hours+0.0019 inches
24 hours / 1 day+0.0022 inches
96 hours / 4 days+0.0025 inches
200 hours+0.0025 inches
500 hours+0.0025 inches

As you can see, the alloy initially shrinks for the first 30 minutes or so, then it starts to slowly expand. After approximately 1 hour, it returns back to the original dimensions of the chamber and the alloy continues to expand, until about 4 days later, when it reaches its maximum size.

So how does the user measure the dimensions of a chamber using this alloy then?
  1. First the user disassembles the firearm as needed, to get access to the chamber of the firearm.
  2. Next, the user cleans the barrel and puts a small amount of oil in the chamber and then pushes a cleaning patch into the barrel so that it is just ahead of the throat of the barrel. The cleaning patch serves to block the barrel after the chamber.
  3. The cerrosafe (or rotometals) bar is heated until it melts. It is only necessary to ensure that the bar is not directly heated by open flame. This means it can be heated in a small iron ladle or coffee can, using a stove, electric hot plate, propane torch, oil lamp, candle etc. It can also be heated by placing a small container containing the cerrosafe into a larger container of water and then boiling the water. Remember that the melting point of cerrosafe is well below that of the boiling point of water.
  4. The molten cerrosafe is then poured into the chamber of the firearm, until the chamber is full. Since the melting point of cerrosafe is so low, it doesn't affect the barrel or the cleaning patch that is blocking the barrel on the other side of the chamber. The cerrosafe alloy is then allowed to cool until it turns a shiny silver color. As soon as the alloy has cooled enough that it is no longer a liquid, it can be pushed out of the chamber using a cleaning rod or wooden dowel (remember that cerrosafe initially shrinks slightly as it cools for the first few minutes, which makes it easier to push out).
  5. After about an hour, the casting expands back to the original dimensions of the chamber. It is then carefully measured at various points to determine the exact dimensions of the chamber, using a vernier caliper or a dial gauge. The user can then use these measurements to figure out what cartridge size the firearm was designed to use. Reference books that list the exact dimensions of various cartridge types are readily available, therefore the exact cartridge model may be easily determined. A good reference book that is commonly used is "Cartridges of the World".
After this, the casting can be stored somewhere until it is needed to measure the chamber of a different firearm, as the alloy can be remelted and reused multiple times.

One of the nice things about this process is that the user doesn't need to be very experienced to do this and the tools are also generally available and cheap.

Here's a movie from the one and only Mr. Larry Potterfield, showing how to use cerrosafe to measure a chamber.

Happy viewing!

Monday, August 24, 2015

M16 Operation and Functioning Cycle

In our last post, we studied the similarities and differences between the AR-15 and M16 trigger mechanisms. Today's post will be another educational movie courtesy of the US Army (your tax dollars at work!). We will study the methods of operating a M16 rifle and its functioning cycle.

Today's movie was produced sometime in the early 1960s and describes the operation of the M16 rifle, specifically the earlier models. It also shows the functioning cycle of operation of a M16 rifle (i.e.) Firing, Unlocking, Extracting, Ejecting, Cocking, Feeding, Chambering and Locking and describes in detail how each stage works.

Note that this movie is pretty old, it actually refers to the rifle as the XM16E1. This is the first model of the M16 family that was adopted by the US Army. The earlier M16 rifle model was first adopted by the US Air Force, but the US Army insisted on a forward assist lever to be added to the rifle and therefore, the Air Force model was called M16 and the US Army version was called XM16E1 and later renamed to the M16A1 model in 1967. Also, in the early days, M16 magazines only held 20 cartridges and this can be seen in the movie as well.

Happy viewing.