Thursday, April 25, 2013

Parts of the Firearm: The Barrel

In our previous post, we studied the bolt carrier group. In this post, we will study about the barrel of a firearm. We've actually talked a lot about barrels in a number of posts in the past, but never really discussed the parts of a barrel in detail. We will do that in this post.

A barrel is simply a tube through which a bullet comes out of. Most firearms these days have a single barrel, though shotguns do come in double barrel versions. This is in contrast to previous centuries when multi-barrel firearms did exist. In most cases, the barrels are cylindrical tubes, but there are also polygonal barrels and oval barrels which have been used in history. With that said, let us discuss the various terms that describe barrels.

Bore: This is a word that describes the inside of the barrel tube. The inside surface may be rifled (more modern) or smooth (i.e. smoothbore, which is what the first firearms were like).

Breech: This is the rear part of the barrel (i.e.) the end that is closest to the firing mechanism. Most modern firearms are loaded via the breech end of the barrel.

Chamber: The chamber is at the breech end of the barrel. This is the area where the cartridge is placed into, prior to firing it. The most pressure exerted on the firearm upon firing occurs in the chamber area, hence the walls must be thick enough to withstand this pressure. The manufacturing of the chamber has a lot to do with the precision and reliability of the firearm. If the chamber fits the cartridge very tightly and precisely, then accuracy of the firearm is improved, but the reliability of feeding a new cartridge into the chamber is reduced. Conversely, if the cartridge fits the chamber loosely, then the feeding of a new cartridge into the chamber is much easier, but when the cartridge is fired, it will move around in the chamber and affect the accuracy of the bullet. Therefore, a good chamber design tries to strike a balance between these two factors.

An exception to this are revolvers, which have multiple chambers in a separate cylinder. In this case, the chambers are not part of the barrel.

Freebore: In the case of rifled barrels, this is the area just forward of the chamber, but before the area where the rifling starts. It is a smooth area that guides the bullet forward where it engages the rifling.

Muzzle: This is the part of the barrel which the bullet comes out from. For early firearms, they were usually loaded via the muzzle end of the barrel. The pressure generated by the burning gases decreases as it approaches the muzzle. Hence, some manufacturers make the walls of the muzzle end of the barrel thinner than the breech end, since it doesn't have to withstand as much pressure and they can reduce the overall weight of the firearm this way.

Devices such as flash suppressors or compensators may be screwed on to the muzzle end of the barrel.

The length of a barrel depends on the type of firearm. Rifles and shotguns have barrels starting from around 17-18 inches in length and go all the way to 60 inches or longer. Revolvers and pistols tend to have barrels in the 3-5 inch range, though there are exceptions to this rule of course.

When a cartridge is fired, the expanding gases act on accelerating the bullet as long as it is still within the barrel. Once the bullet leaves the barrel, the expanding gases no longer act on it and the bullet is no longer accelerated. Hence, if the barrel is longer, then the expanding gases act on the bullet for a longer time and allow it to come out with greater velocity than if the barrel is shorter. On the other hand, longer barrels are harder to aim with. Hence, the design of a firearm must strike a compromise between these factors.

Cross-sections of three barrel types. Public domain image

In the above image, we see the cross-sections of three different barrel types. The left one is a smooth bore barrel, the middle one is a rifled barrel and the right one is a polygonal barrel.

Wednesday, April 24, 2013

Parts of the Firearm: The Bolt Carrier Group

In our previous post, we studied the parts of the gun that comprise the fire control group (a.k.a. the trigger control group). In this post, we will study another group of components, the bolt carrier group (or BCG).

The bolt carrier group is usually found in firearms that have a gas operated action. These are the parts that control extracting the old cartridge, cocking the firearm and loading a new cartridge. There are several parts that comprise the bolt carrier group:

Disassembled parts of the bolt carrier group from an AR-15 rifle

In the above image, we see the main parts comprising the bolt carrier group of an AR-15 rifle. There are seventeen parts that comprise the bolt carrier group for the AR-15. They are:

  1. Bolt (A)
  2. Ejector (B)
  3. Ejector spring (C)
  4. Ejector roll pin (D)
  5. Extractor (E)
  6. Extractor pin (F)
  7. Extractor spring (G)
  8. 3 gas rings (H)
  9. Bolt carrier (I)
  10. Bolt cam pin (J)
  11. Bolt carrier key (K)
  12. 2 Bolt carrier key screws (L). These attach the bolt carrier key to the bolt carrier.
  13. Firing pin (M)
  14. Firing pin retainer pin (N)

There are eight basic operations that are done by the bolt group on an AR-15 or M-16:

  1. First, the bolt is in its rearmost position. The action spring then pushes the bolt forward and as it moves forward, it picks up a bullet from the magazine and pushes it towards the chamber via the feed ring.
  2. As the bolt carrier moves forward, the bolt passes through cuts in the barrel extension and a cam pin causes the bolt to rotate, so that the locking lugs on the bolt are locked as the bolt reaches its forward most point.
  3. When the user pulls the trigger, the sear releases the hammer (we covered these parts in our last post about the trigger group). The hammer spring then rotates the hammer with force into the back of the firing pin (which is part of the bolt carrier group). The firing pin passes through a hole in the middle of the bolt carrier and the bolt and the other pointy end of the firing pin strikes the primer of the cartridge, thereby firing the weapon.
  4. Fourth, as the bullet leaves the barrel, some of the gases behind it are tapped into a gas tube. The hot gases travel down the tube, down through the bolt carrier key and are redirected forward, pushing the gas rings on the bolt. This pushes the bolt forward slightly and the bolt carrier to the rear. The rearward movement of the bolt carrier pushes against the cam pin that caused the bolt to lock in step 2. This cam pin now causes the bolt to rotate in the opposite direction and unlock the lugs that were locked in step 2. The bolt is now free to move backwards.
  5. As the bolt carrier group moves to the rear, the extractor removes the old cartridge case from the firing chamber and pulls it backwards.
  6. As the bolt carrier continues to move backwards, it re-cocks the hammer.
  7. As the bolt moving backwards goes past the ejection port, an ejection spring forces the now empty cartridge case to be pushed clear of the extractor and out of the ejection port.
  8. Once the bolt has reached backwards to its rearmost point, the action spring pushes the bolt forward again, as described in step 1 and the entire cycle repeats.
The following video gives a decent animation of how things work:

As with the fire control group, it is possible to purchase the parts individually or purchase an entire pre-assembled bolt carrier group part which is ready to be dropped into a firearm. There are aftermarket bolt carrier groups that are plated with hard chrome or titanium nitride for better lubrication and reliability. Some are heavier for slower cycling and others are lighter for reduced loads and faster cycling. Still others are built to much more precise tolerances for consistent locking etc. There is a large selection for users to choose from, depending upon needs and preferences.

Tuesday, April 23, 2013

Parts of the Firearm: The Fire Control Group

In our last post, we looked into the parts of the firearm that would form the receiver. Today, we will look at the parts that comprise the fire control group, otherwise known as the trigger group.

The fire control group (a.k.a. the trigger group) is located inside the receiver of a firearm. The fire control group comprises those parts of the firearm that handle the motion of the trigger lever (springs, levers etc.), any trigger safety locks, the hammer and the sear (this is the part that holds the hammer back until the trigger is pulled firmly).

In the above image, we see the fire control group for a Ruger 10/22 rifle in a disassembled state with its various components.

While a person can buy the individual components from various sources, there are also several manufacturers (e.g. CMC Triggers, Geisselle, Timney, Tapco etc.)  that make complete replacement trigger groups for various firearms, that are in a pre-assembled housing that just needs to be dropped in to the receiver.
A Timney Fire Control Group. Click on image to enlarge.

In the above image, we see a fire control group made by a manufacturer called Timney, designed to be used with AR-15 rifles. This part is designed for people who have very little gunsmithing experience and offers a better feel than the original AR-15 trigger. Timney also makes replacement fire control groups for other rifles, such as the Remington 700, Ruger 10/22. Mosin-Nagant M1891 etc.

Here's a video demonstrating the replacement of the entire fire control group with a new one. Note that as the user disassembles the old fire control group, he pulls out the parts individually (because the original parts were not assembled as a single component), but when he installs the new group, he installs it as a single component.

By the way, one of the major differences between a semi-automatic rifle and its fully automatic version (e.g. AR-15 and M-16) is the fire control group. This is what makes the AR-15 a semi-automatic rifle, whereas the M-16 is capable of selecting between multiple firing modes.

Sunday, April 14, 2013

Parts of the Firearm: The Receiver

A recent question from a reader of this blog made me aware of something that I wasn't aware of for a long time -- it looks like some people are not really familiar with different parts of a firearm and terminology, so I figure we should put together a series of posts discussing the various parts of firearms and what they are used for. In this post, we will discuss what a receiver is.

Briefly speaking, a receiver is the part of the firearm which contains all the operating parts of the firing mechanism. This includes the trigger mechanism, the bolt mechanism, hammer, the part that holds the magazine to the firearm, the firing chamber etc. It does not include parts that are secondary to the firearm (e.g.) the stock, sights, gun sling, magazine, barrel etc. Think of the receiver as the parts of the firearm that are absolutely necessary to operate the firearm. It is called the receiver, because it "receives" the ammunition and fires it.

Per US federal laws, the receiver is considered the firearm and therefore, anyone purchasing a receiver has to undergo a background check and complete paperwork, the same as though they had purchased a complete firearm.

In the case of pistols and revolvers, in most cases, the receiver is housed in the firearm body or frame.

Parts of a pistol, including the receiver. Click on the image to enlarge.

In the above image, we see some parts of a pistol. The receiver is the major part of the body of the firearm, as it contains the trigger, hammer, firing pin etc. Of course, there are some exceptions: in some handguns, such as the Sig P-250, Ruger Mark I, Mark II and Mark III etc., the receiver is actually a separate part that can be detached from the body and swapped out.

With rifles, the situation is a little different and the receiver only constitutes a part of the firearm body. In some firearms, such as the M-16, AR-15, Heckler & Koch HK-416 etc., the receiver is actually in two parts, an upper part and a lower part. In such cases, US law (and some other countries) considers that the receiver part that has the serial # of the firearm stamped on it is considered the controlled part of the firearm (i.e. the part that needs background checks and paperwork to be done for a sale)

An AR-15 and its receiver. Public domain image.

In the above image, we see a complete AR-15 rifle and its receiver below. Technically, the AR has multiple receivers (an upper and a lower receiver) and the part we see above is the lower receiver part, but since the lower receiver has the serial number, it is considered to be the controlled part for legal purposes in the US.

In the case of other rifles, such as the FN-FAL and some other Heckler & Koch products, the upper receiver is the one that is considered the controlled part. Other rifles have only a single receiver part (such as bolt action hunting rifles or the AK family of rifles), so that is considered the controlled part.

When we say controlled part, this means the part that is monitored by law agencies of a country. In the US, this means that if a person buys a receiver from a licensed dealer, they have to undergo a background check, the same as the one they have to go through if they bought a complete firearm. However, if they bought other parts such as barrels, slides, magazines, sights etc., no background check is needed. Some people circumvent this check by purchasing a partially completed receiver (which is not considered a receiver technically and therefore doesn't need a background check and paperwork) and finishing the rest by themselves in a workshop. How much of the receiver needs to be complete before it is considered to be a receiver depends on the firearm model and the Bureau of Alcohol, Tobacco and Firearms (BATFE). The term "80% receiver" is often seen on internet forums (the notion being that if a receiver is 80% complete, it isn't technically a receiver), but this is just a marketing term and not a legal one. The BATFE decides on a case-by-case basis where the dividing line between a partially complete receiver and a complete receiver is. Some states may also enforce additional laws on top of the federal ones.

Note that all the above is per US laws only. Other countries may have different laws (for example, some European countries consider the barrel as a controlled part).

Tuesday, April 2, 2013

The Charleville Musket

After our last couple of posts where we studied the development of the first truly American rifles, we will study an influential French musket called the Charleville Musket. Don't worry, there's an American connection to this as well, which will be mentioned later down this article.

The Charlevilles were actually a family of muskets that were used by the French military. The first in the series was the Model 1717, which was adopted (as you may already have guessed) in the year 1717 AD. This model was the first in the French military to be built to a standard pattern specification. It was a musket with a flintlock firing mechanism, a smooth bore barrel of .69 caliber (17.5 mm. diameter) and weighed somewhere between 9-10 lbs. (4-4.5 kg.). While the concept of rifling was already understood at this time, the decision to use a smooth bore barrel over a rifled one was deliberate. In those days, the fouling in the barrel caused by black powder took a while to clean and infantry commanders noted that the smoke produced by the firearms tended to negate the rifle's longer range and accuracy anyway. Also, rifling was more expensive to manufacture as well. Therefore, the designers chose to use a smooth bore barrel with this firearm. However, the smooth bore barrel lacked accuracy beyond 50-100 yards or so, therefore the solution was to line up men in a massed formation and tell them to fire at a target, hoping that at least some of them would hit something. A good trained person could fire only about three times in a minute, therefore there was a bayonet included for close range fighting. The stock was made of walnut wood and the butt was shaped so that it could be used as a club at close ranges.

Improvements to the musket led to new models in 1728, 1743, 1746, 1763, 1766, 1770-1776, 1777 etc. and they were produced until the 1840s or so, when they were replaced with firearms using percussion locks. They were used during the French revolution and also by Napoleon's soldiers.

Model 1866 musket. Click on image to enlarge.
Image licensed under Creative Commons Attribution-Share Alike 2.0 Generic from

These muskets were officially called the French infantry musket, but the reason we know them as "Charlevilles" is because of America. These muskets were originally produced by firearm factories located in various towns of France, notably St. Etienne, Tulle, Maubeuge and Charleville (in the Ardennes region of France). Although St. Etienne was the factory that produced the most muskets, it was the factory at the town of Charleville that contributed to this series of firearms being labelled as "Charlevilles". The reason was that during the American revolution, the French decided to aid the Americans. Due to the influence of the Marquis de Lafayette, large numbers of French muskets were shipped over to America during the early days of the revolution. Many of the US troops fighting in the revolution were armed with the model 1773 and 1776 muskets and the French forces aiding the US troops were armed with model 1777 muskets. It is a common misconception that American forces were mainly using Kentucky rifles that we studied about in the previous post. In reality, the majority of the firearms used by US soldiers during the American revolution were French in origin. Some of these muskets were made by the Charleville factory and had markings that indicated where they were made. Pretty soon, American soldiers started to call all muskets as "Charleville" whether they were made by the Charleville factory or not, and the name stuck.

The Charleville musket later went on to heavily influence the design of the Springfield Model 1795, which was the first firearm officially produced by the US Government for use by American soldiers. Like the Charleville, the Springfield Model 1795 was a .69 caliber muzzle-loader with a smooth bore barrel and a flintlock firing mechanism and weighed between 9-10 lbs. Interestingly, one of the contracts to manufacture this model was given to Eli Whitney in 1798 and led to him developing the concept of interchangeable parts and modern production techniques.

The Charleville musket design was also copied by Austria, Belgium, Prussia and Russia and they remained in service until around 1845. There are modern replicas still being produced today by various companies.