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'The Chart' - by Rob Sloyer


So, what is an M4-pattern carbine? The true M4 is a select-fire military-issue shortened version of the M16 with a collapsible stock, 14.5" barrel, and flat-top upper (with Picatinny rail system) in place of the old A2 carry handle. obviously what we are discussing here are non-NFA firearms which means that they are not select-fire and have a barrel length of at
least 16".

[b]Bolt Carrier Group[/b]
The first six items listed on the chart pertain to the bolt and/or carrier, commonly referred to as the "Bolt Carrier Group" or BCG. This is the part that moves back and forth in the upper receiver chambering fresh rounds, extracting and ejecting empty rounds, and generally ensuring that your rifle or carbine operates as it should. As such, in terms of reliability, the BCG is the heart of the gun, and having as high a quality BCG that is assembled correctly out of the correct materials and which has been properly checked for flaws is key to the continued reliability of the AR-platform firearm. The first four items refer to the bolt itself, while the remaining two deal with the carrier.

[b]Shot Peened Bolt[/b]
There are many resources available online as to the exact process and how it affects the structure of the part at the molecular level, but essentially the goal is to increase the resistance of metal to fatigue. The bolt, in the performance of it's duties, is put under a considerable amount of stress as the gun fires. Ensuring that this part lasts as long as possible is key to the continued reliability of the firearm over time, and increasing the resistance of the part to fatigue increases it's service life.


[b]High Pressure Test (HPT) Bolt[/b]
This is also sometimes referred to as "proof loading" or "proof firing". Essentially it is a test fire of the bolt (and barrel) in order to subject the part to a pressure that could cause it to fail in use. This is a preparatory step in order to prepare the part for the next step. Not all companies perform this step and prefer to "batch test" their bolts and barrels, or to test a representative sample of each batch.


[b]Magnetic Particle Inspection (MPI) Bolt[/b]
Like shot peening, there are resources available online as to what the exact process is and how it works, but the intended purpose is to check for surface cracks in the part that may not be detectable by the naked eye and that may have been caused by the HPT. Not all companies perform this step and prefer to "batch test" their bolts and barrels, or to test a representative sample of each batch. This is a crucial step following the HPT in order to observe the results.


[b]Black Extractor Spring Insert[/b]
The crucial element here is, in fact, the 5-coil extractor spring which the black insert indicates as there is some debate as to the actual chemical or physical properties of the insert itself as compared to the rifle version which comes with the 4-coil spring and is blue. The shorter gas system of the carbine makes for a sooner an more violent operation of the BCG which can cause the extractor on the bolt to jump over the rim of the case and not properly extract the empty case. To some degree this can be mitigated by gas port size, but beefing up the spring tension to cause the extractor to clamp more tightly on the case ensures proper function.


[b]M16 Bolt Carrier[/b]
The M16 bolt carrier serves two functions. The first is that the firing pin is fully shrouded so that the hammer is cocked by the carrier and not the firing pin itself. The second is that the M16 bold carrier is heavier and therefore increases "lock time" (or the amount of time that the empty case after the primer is struck by the firing pin) which aids in extraction. The heavier carrier also reduces the felt recoil impulse which in turn reduces wear and tear on the other internal parts of the carbine.


[b]Properly Staked Gas Key[/b]
The gas key on top of the bolt carrier is the part that the gases (which have been re-directed through the gas port, then the gas block or front sight base, and into the gas tube) impact on in order to push back on the carrier, unlock the bolt and cycle the firearm. As such it is under tremendous pressure and is critical to the continued operation of the firearm. The key is held on to the top of the carrier by two screws, typically allen but sometimes torx, that are tightened to a specified torque. After tightening the metal of the key should be "staked" in such a way as to prevent the screws from loosening. In order for the staking to perform it's job properly it must deform the metal of the key sufficiently to make contact with, and perhaps even deform a bit, the attachment screws. Use of Locktite is not sufficient, as virtually all versions of Locktite are weakened by heat.


[b]Barrel[/b]
The barrel rivals the bolt in terms of how critical it is to the long term reliability and functionality, as well as accuracy of the firearm. Clearly, accuracy may be sacrificed to some degree in favor of longevity in terms of a chromoly and chrome-lined barrel. The features that are included in the barrel section, the next eight items on the chart, are all related to the longevity of the barrel as well as the reliability of the firearm, with some features being somewhat optional as they pertain to use of certain projectiles and other shooter-defined needs.

[b]Milspec Barrel Steel[/b]
No term used in the chart has elicited more of a negative reaction than "milspec". As such every effort has been made to remove it from The Chart wherever possible. In the case of the barrel, however, it remains. The short version of the story is that barrels are typically made from two grades of chromoly steel, 4140 and 4150, with the latter being a slightly higher grade that withstands heat slightly better than the former. The long version involves very specific types of steel, much longer numbers, and is in fact generally considered to be of little consequence. There are, however, other grades and/or types of steel that meet or exceed the properties of 4150 and are therefore acceptable.


[b]High Pressure Test (HPT) Barrel[/b]
This means the same thing, and is done for the same reason, as the HPT of the bolt.

[b]Magnetic Particle Inspection (MPI) Barrel[/b]
This means the same thing, and is done for the same reason, as the MPI of the bolt.

[b]Chrome Chamber and Bore[/b]
Chrome-lining of the chamber and bore serve to protect both from corrosion due to the heat of combustion of the gunpowder in the bullet as well as "neglect" in humid or other harsh environments. The chamber and bore are directly related to the accuracy potential of a firearm, and damage to either in the form of pitting will negatively impact the accuracy potential. The trade off is that chrome is often applied unevenly, at the microscopic level, meaning that it may negatively affect the accuracy potential in and of itself. The potential for damage due to other factors is generally considered greater than the small amount of uneven application, and so chrome-lining is generally considered desirable. No, it cannot be added after the fact as barrels intended for chrome-lining are first slightly overbored with the lining then reducing the internal diameter to the proper dimension.

[b]5.56 Chamber[/b]
There is a common misconception that .223 and 5.56 are the same thing. They are not. 5.56 is often loaded to a higher pressure, among other things, which is the most critical issue. There are other dimensional differences pertaining to throat, bullet seat, etc. but what it comes down to in practical terms is that you can shoot .223 in a 5.56 chamber but the reverse is not a good idea. Generally speaking the barrel will be marked with one or the other but unfortunately those markings cannot always be trusted. If you think you may ever shoot 5.56 ammunition it is a good idea to get a 5.56 chamber from a maker that can be trusted.


[b]1:7 Rifling Twist[/b]
Another common misconception is that bullet weight determines the optimal rifling twist. This is incorrect in that it is actually bullet (projectile) length that should be used to determine the twist rate. Generally speaking, however, the heavier bullets are also longer so while technically incorrect it is common to say that a 1:7 twist is more desirable for the heavier 75 and 77 grain projectiles. Therefore, choosing a barrel twist really comes down to first choosing your projectile weight, and more correctly, length. If you work for a department that mandates or issues a certain ammunition then this should be your guide when choosing a rifling twist rate. A good rule of thumb is that 1:9 will stabilize bullets in the 45 to 62 grain range, and 1:7 will stabilize bullets in the 55 to 77 grain range. Like all things this is not a given, and any barrel should be tested with the intended ammunition to ensure the desired results are achieved.


[b]M4 Feedramps[/b]
M4 feedramps refers to the feed ramps in the barrel extension being matched up to feed ramps cut into the upper receiver. The alternative is Rifle feedramps which stop at the end of the barrel extension and do not continue into the upper receiver. Longer projectiles, soft-point projectiles, and carbines with faster cyclic rates tend to benefit from the extended M4 feedramps. There are no known downsides to having the extended feedramps.


[b]"F" Height Front Sight Base[/b]
Front sight bases come in two basic varieties. One is the front sight base intended for use on carbines and rifles with fixed A2 uppers,the other (The "F") is intended for carbines with flattop uppers. In order to ensure compatibility with the various aftermarket rear back up iron sights, the "F" is more desirable. Not all "F" height front sight bases are marked with the "F" (LMT for example), and some that are marked are not true to the correct height.


[b]Taper Pins at Front Sight Base (FSB)[/b]
A minor issue, but taper pins hold the front sight base better and tighter than a straight pin. Some makers attempt to make up for this by using slightly oversized straight pins that can be extremely difficult to remove.
More information on taper pins can be found here


[b]Parkerize under FSB[/b]
Parkerizing under the front sight base (or FSB) is done by very few makers. Most prefer to attach the base to the barrel and then parkerize the assembly as a whole. If parkerized as an assembly, both the outside of the barrel under the rings of the base and the inside of the rings themselves do not get the protective coating of the parkerizing process. There have been some reports of these parts rusting in the unprotected area, but it is unlikely that rust in these locations will affect the function of the carbine.

[b]Double Heat Shield Handguards[/b]
Obviously this is not an issue of the end user intends to replace the handguards with an aftermarket part, but the proper M4 handguards are larger and contain a dual-layer of aluminum heat shields inside each half. The added diameter and extra shield serve to keep the shooter's hands cooler over prolonged periods of fire.

[b]Receiver Extension[/b]
The receiver extension is the part of the rifle, often mistakenly called the "buffer tube" that extends out from the back of the lower receiver. It not only holds the stock on the rifle but also does act as a tube for the buffer and bolt carrier to move back and forth inside of when the rifle cycles. This receiver extension is held in place by a castle nut which also holds the receiver end plate in plate, which in turn holds a spring and detent in place inside the lower.

[b]1.14" Diameter Receiver Extension[/b]
This is often referred to as the "milspec" receiver extension. The alternative to a "milspec" receiver extension is the "civilian" or "commercial" receiver extension. There is some debate as to whether or not the milspec extension is actually stronger or "better" than the commercial, but for most users the real choice comes down to availability of aftermarket stocks. Some companies, like Magpul with their CTR stock, offer versions for both extensions, but many do not. If you know that your intended stock is available for the commercial receiver extension or if you are happy with the stock your rifle comes with it is most likely not an issue. If, however, you want to change the stock or just keep your options open then the milspec extension is preferred.
Dimensions for a "milspec" receiver extension can be found here [url]/url]
[url="http://www.magpul.com/pdfs/buffertube-Milspec-M4.pdf"]http://www.magpul.com/pdfs/buffertube-Milspec-M4.pdf[/url]
Dimensions for a "commercial" receiver extension can be found here
[url="http://magpul.com/pdfs/buffertube-civilian-M4.pdf"]http://magpul.com/pdfs/buffertube-civilian-M4.pdf[/url]

[b]Staked Castle Nut[/b]
The castle nut is the nut that holds the receiver extension in to the lower receiver and prevents it from backing out. If it backs out, the buffer retainer spring inside the lower can come loose, which in turn can render the carbine inoperable. The best case scenario if your castle nut comes loose is that your stock becomes loose which is also not a good thing. Calling it a "staked castle nut" is somewhat of a misnomer as the part that is staked is actually the receiver endplate. The castle nut itself has small notches on the forward side, and large notches to the rear. The large notches are used for tightening, and the small notches are there so that the receiver endplate can be staked to displace metal into the notch on the castle nut, thereby keeping it from turning. With the proper castle nut wrench the staking can be easily overcome to change out the parts, but without the tool the nut will not come loose. Locktite, once again, is not a viable solution as this part can heat up and Locktite is weakened by heat.


[b]"H" Buffer[/b]
The buffer is the weighted part that moves back and forth inside the receiver extension when the rifle cycles. It is held forward by a long spring called the buffer spring, and is kept from moving too far forward by the buffer retainer pin (which is in turn held in place by the receiver extension, see "staked castle nut" above). The buffer and spring provide resistance to the bolt carrier as it cycles and the spring then pushes the bolt and carrier back into battery after the empty case is ejected. A heavier buffer can increase lock time (see "M16 bolt carrier" above) which reduces wear and tear on parts and increases reliability in carbines. The heavier buffer can also decrease felt recoil. It is, however, possible to install a buffer that is too heavy which will not allow the rifle to cycle properly. Typically the "H" buffer is used on carbines with barrels 10-16" with carbine (7.0) length gas tubes.


[b].154" Diameter Fire Control Group (FCG) Pins[/b]
Colt is the only maker of complete rifles that uses slightly oversized fire control group (trigger and hammer) pins to prevent the installation of M16 full-auto or burst parts from being installed in the lower. Virtually all aftermarket trigger makers (Timney, Geissele, McCormick, etc.) make trigger groups that utilize the larger pin size so finding aftermarket parts is a non issue. The odd-size pins do become an issue if you own rifles from Colt and other companies as the spare parts in question will not be interchangeable. In addition, finding the proper diameter pins, and the FCG parts they hold in place, for spares from any vendor other than Colt may prove difficult.


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