Defensive AR15 Discussion *Picture Heavy*

[William B.]

There seems to be a lot of disinformation in gun rags and on the internet these days as to what features and processes make an AR15 "mil-spec" or combat-worthy. This is probably due to the amount of AR manufacturers/assemblers that exist today, many of them marketing their product to a generation of hobby shooters who were introduced to firearms through video games, movies, and air-soft. There is nothing wrong with this since, as we know, wherever there is a demand an entrepreneur will rise to meet it. Problems begin when consumers cannot differentiate between these products because of deceitful marketing practices. Then you have LEO's, PMC's, defense-minded civilians, etc. believing that these sub-standard products are "just as good as..." and purchasing them as serious defensive tools. The purpose of this thread is to generate some discussion about "mil-spec" and what you should look for when purchasing an AR15 that you may use to defend yourself, your home, and your family.

Mil-spec stands for "military specification". These specifications set forth the minimum standard to which a manufacturer must adhere in order to produce a truly mil-spec firearm. Some of these criteria are features and others are processes that the parts and completed firearm must go through. That being said, no civilian AR15 can truly be mil-spec since it will lack a select-fire FCG and most will have an extra 1.5" added to the barrel. Regardless, the mil-spec sets a pretty high standard and is a good yardstick by which you can measure a product. Some manufacturers may take shortcuts and cause their product to fall short. However, you may find that some manufacturers go beyond mil-spec and produce an even better firearm. Knight's Armament is a very innovative manufacturer that comes to mind. I am not an expert on this subject by any means. The knowledge that I have came from self-education and this is my attempt to share it with some fellow firearms enthusiasts on this forum. I'm going to start at the muzzle and work my way back as I explain (as best as I can) the features and processes that I made reference to above.

[William B.]

The M4 profile barrel (shown below) is 14.5" long. Most AR15's will feature a 16" barrel to comply with federal law, but you can also have an extended muzzle device permanently attached to your 14.5" barrel that will bring its total length to the legal limit.

Mil-spec barrels are made from 4150 chrome-moly vandium steel per MIL-B-11595E. I'm no metallurgist, and I don't imagine most of you are either, so here's a "Barney-style" thread about barrel steel. Most quality AR makers disclose what materials they use on their website.



The notch that you see in the barrel profile between the muzzle and the FSB is pointless for most civilians. Its purpose is to accomodate the attachment band on an under-barrel grenade launcher. Unless you plan to mount your registered M203 or a 37mm flare launcher this feature may be something that you can overlook.

Next you have the front sight base/gas block. Many end-users choose to replace the factory FSB with a low-profile gas block so that they can run a longer rail system out over it. If you choose to stick with a standard FSB/GB there are a few features that you may want. First, you should make sure that your AR15 has been equipped with a correct-height FSB. A flat-top upper with an optic rail requires a different height FSB than an A2 style upper with a fixed carry handle. If your AR15 has a flat-top ensure that your FSB is marked with an "F" like the one below.


The FSB should be pinned in place using taper pins. Their tapered contour will ensure that they do not back out. Some manufacturers may not use taper pins, so this may be a good question for you to ask when shopping. Most gun store employees will not know the answer to this question, so you may be best served by giving the manufacturer's CS dept a call and asking them instead. Many aftermarket FSB's/GB's feature shoddy attachment methods, such as clamp-on FSB's. These are not ideal and should have Loctite applied to the screws at the very least. I personally had a FSB of this type that would rotate during heavy firing sessions. If you choose to replace your factory FSB you may want to look into having your new one pinned as well. Most competent armorers are able to perform this modification on most FSB's/GB's.

The bayonet lug is another one of those "male nipple" features like the M203 notch and doesn't benefit most civilian users. While it may be more aesthetically pleasing than the look of a neutered, post-ban FSB it really serves no purpose on a 16" barreled AR15 since you can't mount a bayonet on it anyways.

Now I'll move on to the gas system. There are several different length gas systems on the market right now. Rifle length, which is usually on a 20"+ barrel such as the M16 A1, A2, etc.; carbine length is featured on the M4; and mid-length, which isn't currently fielded on any standard issue military weapons, but seems to serve most end-users very well. Most AR15's seem to feature the carbine-length gas system, although the mid-length is gaining quite a following right now. There are arguments to validate any of these systems depending on which side of the fence you're on. Knight's Armament actually has an outstanding proprietary length system that is featured on their SR15/SR16 line of carbines.



A rifle-length gas system can be equipped with about a 12" handguard or rail with no modification to the FSB; a mid-length gas system can be equipped with about a 9" handguard or rail without modifying the FSB; and a carbine-length gas system can accomodate about a 7" handguard or rail without modification to its FSB. Daniel Defense manufactures an excellent extended rail system (see pic below) that has an opening from which the factory FSB can protrude. This means you don't have to spend extra dough modifying or replacing parts. A few other manufacturers are beginning to follow DD's lead as well.


If you look back up at the first picture you will notice that that barrel profile becomes skinny underneath the handguards. This is a weight-saving measure that may shave a few ounces off of your carbine. It may not sound like a lot, but ounces add up to pounds pretty quickly when you begin mounting items to the front of your rifle such as suppressors, flashlights, bayonets, forward grips, NV aiming devices, etc. There is also what is called the SOCOM profile, which is very similar to the M4 profile (M203 notch, skinny underneath the handguards) except that it stays fat underneath the handguards. A lot of quality manufacturers are starting to make lightweight barrels (AKA skinny bbl, pencil bbl, etc.) that have a continuous thin profile that shaves even more weight than the M4 profile. There are always arguments against this concerning barrel heat, but unless you are performing full mag dumps from your Beta C-Mag on a regular basis I don't think it's really a big issue. Another argument concerns barrel whip. I won't get into that as it is outside my area of knowledge, but you can use your Google-Fu to research that subject if it concerns you. I, personally, plan to eventually try out a pencil barrel since, as a grunt, I'm a big fan of lighter gear.

The mil-spec barrel twist rate is 1/7, or 1 complete turn every 7" through the bore. This twist rate is more ideal for stabilizing longer bullets such as 75gr TAP or 77gr MK262. Many AR manufacturers choose to equip their product with 1/9 barrels as well. This twist rate seems to be more ideal for stabilizing shorter, lighter projectiles such as varmint rounds in the 40-50gr range, 55gr M193, and even 62gr M855. Different users have different results, though. I will probably catch some flak for saying this, but I will anyways. It seems to me that in some cases 1/9 twist is an indicator that the mfg. may have cut corners in other areas such as chamber dimensions, chrome lining, barrel steel, MPI, and proof testing. This brings me to my next point...

Chrome-lined barrels. If you are shopping for a firearm that will see serious use and maybe even some abuse there is no reason not to have a chrome-lined barrel. It is more durable (this means more barrel life and more for your money) and more resistant to the elements than chrome-moly or non-chrome-lined barrels. Some argue that you can squeeze more accuracy out of a non-chromed barrel, but remember that the mil-spec is a set of criteria. If the weapon that you're considering adheres closely to the TDP it will not be an inaccurate weapon by any means. It must be capable of at least 4 MOA with mil-spec ammunition. This is combat-accurate and I can say that I have seen Marines qualify "Expert" on the rifle range shooting targets as far as 500 meters with iron-sighted M4's. I have also heard of instances (from reliable sources) where SOF soldiers acheived hits on point targets with their M4's past 500 meters.

[William B.]

M4 feedramps are an important feature on a rifle that is equipped with a carbine gas system. Due to the gas port's closer proximity to the carrier key, this gas system runs at higher pressures than its longer cousins. The higher pressures cause the cycle of operation to happen faster and M4 feedramps will help to ensure smooth feeding, especially with defensive loads such as soft-points and ballistic-tipped ammunition. As you can see below the M4 feedramps consist of two cuts in the bottom of the barrel extension that mate up with corresponding cuts in the lower receiver.



There are a couple of common manufacturing mistakes that you'll want to inspect for if you're buying a carbine with M4 feedramps. M4 feedramps on the barrel extension, but no M4 cuts on the lower: this isn't ideal, but you can get away with it. Rifle feedramps on the barrel extension with M4 cuts on the lower: not good.

Many manufacturers build their AR15's with .223 chambers instead of true 5.56 NATO chambers. There isn't anything wrong with that if that's how the gun is marketed and you understand the difference between 5.56 and .223, but a lot of manufacturers mark their .223 rifles "5.56" or ".223/5.56". Some just have plain out-of-spec chambers that aren't .223 or 5.56. I had an AR15 that suffered from this problem and would have multiple stuck casings every range session with different types of ammunition. The best way to avoid this is to stick with a manufacturer who is known for producing quality carbines. If you're the lucky one and wind up with an out-of-spec chamber find a competent AR armorer and he should be able to use a chamber reamer to alter the dimensions of your chamber.

The bolt carrier group (BCG) is an extremely important group of components in any AR15. It consists of the bolt itself, the firing pin, and the bolt carrier. Most consider it to be the heart of the AR15. Mil-spec bolts are machined from Carpenter No. 158® Steel. Most quality AR makers disclose what materials they use on their website. Once again, your best insurance is to stick with a quality manufacturer. There will also be QA stamps on bolts and bolt carriers. They will be different depending on what make your BCG is, but I will get into that later.

The extractor is a crucial part of the bolt assembly that grips the rim of the casing during each cycle of operation. Mil-spec (Colt) extractor assemblies should come with a copper-colored, 4-coil extractor spring and black insert (see below). Many end-users add a Crane O-ring, which fits around the extractor spring and increases tension on the case rim. This may be beneficial to you if you fire a lot of steel-cased ammunition or if you AR has a short barrel and/or short gas system. Any of these features will make the extraction process more difficult and may lead to stuck casings and shorter service life for your extractor spring. It is important, however, that you not unnecessarily modify your AR15. More engineering went into this weapon than most of us imagine and too many unnecessary mods may cause problems. Always have a good understanding as to why you need a particular modification and exactly how it will affect the cycle of operation. In a nutshell, "If it ain't broke; don't fix it." This train of thought is applicable in many other areas as well and will save you grief as you modify your new AR15 to suit your needs.



The bolt carrier and carrier key are machined from steel and should be chrome-lined. There are several types (see below) of popular AR15 bolt carriers on the market. The M16, or "full-auto" (as it is most commonly known) bolt carrier features a fully shrouded firing pin channel and more material along the bottom of the carrier, making a full circle. In no way does a "full-auto" carrier put you in violation of any laws. The monicker "full-auto carrier" seems to give many people that impression and I prefer not to use it for that reason. Many quality manufacturers such as Daniel Defense, BCM, and Colt market and sell AR15's that feature "full-auto" carriers to the general public. "Semi-auto" carriers do not have as much material along the bottom of the carrier and may leave the firing pin un-shrouded. This may cause damage to the firing pin as the bolt carrier re-cocks the FCG during the operation cycle. If you look hard you may also find the notorious "half-circle" carrier. This carrier does not have any material along the bottom of its rear (thus the name). The most desirable of these carriers is the M16 carrier, largely due to the fact that it is the carrier that the rifle was originally designed with. Because of the added weight of the extra material it provides an ideal amount of locking time for chamber pressure to drop before the extraction process begins.



The carrier key is the pipe-shaped piece that you see fixed to the BCG in the above photos. Its job is to channel gas from the gas tube into the BCG during the cycle of operation. Most carrier keys are steel and chrome-lined, however Lewis Machine & Tool (LMT) manufactures their keys through a process known as metal injection molding (MIM). MIM is a somewhat controversial method in the internet gun world right now, but for all of the arguments against it I've never heard of an LMT owner who had his carrier key fail. You should ensure that your potential AR15's key is secured using the proper screws and that it has been properly staked (see below). Improperly staked carrier keys may come loose during the cycle of operation causing failures to cycle or even causing the weapon to go down. Needless to say, this would not be ideal in a training course or even worse- in a real defensive scenario. If you discover that your AR15 has an unstaked or improperly staked carrier key, have it properly staked by a competent, AR-savvy armorer or gunsmith.

Properly staked carrier key:


Poorly staked carrier key:


Unstaked carrier key:


Now I will move on to the buffer and receiver extension. Mil-spec M4-style AR15's will come from the factory with an "H" buffer. This buffer is easily indentified by the "H" on its face. M16-stye AR15's should feature a rifle buffer that is heavier and significantly longer than the "H" buffer. Rifle buffers are not compatible with M4-style buffer/receiver extension assemblies. Also available are "H2" and "H3" buffers. These buffers are several ounces heavier than the "H" buffer, but lighter than a rifle buffer. They are marked accordingly. Some end-users experiment with these buffers until they find the weight that they prefer. Many prefer the heavier buffers due to the softer impulse, increased locking time, and possibly increased service life (this is due to the slower carrier velocities that are a result of using heavier buffers) that they may provide. The downside is that your firearm may not cycle properly when using more affordable, but underpowered ammunition such as Wolf or Tula. Lower quality AR15's may feature a carbine buffer which is unmarked and is lighter than any of the buffers mentioned above. It is desirable that you use at least an "H" buffer on M4-style AR's to ensure adequate locking time and slower BCG velocities during the cycle of operation. Vltor recently released their proprietary A5 buffer/extension assembly that provides a reliable, collapsible stock for the rifle-length gas system. It was developed specifically for the M16 gas system, but most end-users seem to be successful running it on AR15's of all type.

[William B.]

Now let's discuss receiver extensions, or buffer tubes if you will. The receiver extension houses the buffer assembly and also provides kind of a spine on which the stock is fixed. As with most other AR15 parts, all extensions are not created equally. The TDP calls for M4 receiver extensions to be machined from 7075 T6 aluminum. Many commercial-spec extensions, however, are made from 6061 aluminum, which is not as durable as 7075. It is important that your receiver extension be of high quality. If it becomes dented or bent it will restrict the buffer assembly's full range of motion and, in turn, keep the BCG from cycling. This will render your AR inoperable and ruin your day whether you're at the range or defending against a home invader. Mil-spec and commercial extensions are usually threaded differently as well. Mil-spec extension threads are created through the rolled-thread process, while many commercial extensions have their threads cut into their body. This means that the commercial extensions must have a larger diameter in order to achieve threads of the correct dimensions using the cut-thread process. These extensions will have a diameter of 1.17", but mil-spec extensions which have rolled-threads will have a diameter of 1.147". This will be an important factor when selecting a butt-stock for your M4-style AR15. Many companies such as Colt, BCM, and LMT only manufacture stocks that are compatible with mil-spec receiver extensions. They will not be able to slide over a commercial extension due to the increased diameter. Many other manufacturers, however, do make stocks that wil fit 1.17" extensions. These stocks will also fit onto mil-spec extensions, but will have a certain amount of wobble due to their increased dimensions on the 1.147" extension.

You should also make sure that your castle nut is properly staked. This will ensure that it does not become loose and back off during a heavy range session. Here is a good thread that discusses receiver extensions and specifically the merits of castle nut staking.

Mil-spec M4's/M16's must be high pressure tested (HPT) or proof tested before they make it into military hands. Many quality AR15 manufacturers put their product through this same process. This consists of firing an over-pressured round through the weapon. The round used is the M197 Proof Cartridge, which is rated at 70,000psi. In comparison, the 55gr M193 ball round has an average chamber pressure of about 55,000psi. After the M197 Proof Cartridge has been fired the firearm is magnetic particle inspected (MPI). This is done to ensure that the over-pressured round did not create any flaws or cause any damage to the firearm. Parts that are typically MPI'd are the bolt, bolt carrier, and barrel. Quality manufacturers do throw out parts that fail to pass inspection. Passing parts are marked with QA stamps. You can usually find these stamps on the bolt, bolt carrier, and barrel. Exact markings differ from manufacturer to manufacturer, but are usually somewhat similar. Most barrels are marked "HPT MPI". Bolts may be marked a variety of ways. "MPI", "MPT", "MPF", "C" (Colt), etc. Carriers are usually marked this way as well. Below are some examples of QA marks commonly found on Colt AR15's.

[William B.]

I hope that somebody has been able to get something out of this and that it will generate some positive discussion. I haven't discovered or said anything new here. It is merely a compilation of information. Here is a list of websites and books from which I have learned:




The following websites are a wealth of knowledge on the AR15.

• m4carbine.net
• tacticalyellowvisor.net
• everyspec.com

The following books provide insight into the history, development, employment, and maintenance of the M16 family.

• Future Weapons- Kevin Dockery
• AK47: The Weapon that Changed the Face of War- Larry Kahaner
• Green Eyes and Black Rifles: Warrior's Guide to the Combat Carbine- Kyle Lamb
• The TM

[Gibson]

Nice thread. Looking forward to your next installments.

Just bought an LE6920 recently, I opened it and examined it but afterward just stuck it back into the plastic and then popped it back in the box. . . need to get it out and let it 'sound off'.

[William B.]

Nice thread. Looking forward to your next installments.

Thanks. I'm glad you're enjoying it.

Just bought an LE6920 recently, I opened it and examined it but afterward just stuck it back into the plastic and then popped it back in the box. . . need to get it out and let it 'sound off'.

Congrats on the new stick. Shoot the hell out of it before you decide how to modify it.

[William B.]

Well, it's finished for the most part. I'll probably edit it along and along, though. I hope you guys enjoy it.

[Scouts Out]

Freakin awesome write up!!!!!

[MHL555]

6920, Awesome write up. I am blessed to have 4 colts, two SP-1's rifle/carbine, 6601, 6520. I just purchased a M16A2
upper in mint condition. God speed & Semper Fi, Mark

Happy 4th of July all.