.308 AR Builds: Things to Know
WHAT AM I GETTING MYSELF INTO?
Thanks to CJ for bringing this topic to our attention. For first timers looking to build a .308 Win/7.62x51mm AR rifle, there is lack of agreement when it comes to specifications and parts requirements. The .308 AR has thus been one of the more confusing endeavors when it comes to home-building. The similarities between .308 caliber ARs and the classic AR-15 are vast if not uncanny, but all of the differences are absolutely critical. And given that .308 parts tend to be significantly pricier than AR-15 parts, it’s not the kind of build you want to be left with a bunch of spare parts for.
Eugene Stoner, the venerated designer of the AR-15 as he is known, actually designed the AR-10 first. The AR-15 came about at the request of the US military and was adapted to scale for the .223 Remington cartridge by Stoner’s assistants. I’m sure Stoner himself also played some instrumental role in doing so, but just figure by then the heavy hitting was done. All of the systems that were already present in the AR-10 were preserved for the AR-15. The only real change was scale. It’s probably not fair to simply say that was all that changed, because not everything changed in scale. Some parts remained identical.
Now since the military adopted the smaller of the two rifles for their use in the form of the M16, the AR-10 was not taken in by the military and given an “M” designation (at least not yet). Though there was limited use by armed forces, it was technically not “mil std”. Because the standards were not written down in RFQs for contracts or purchase standards, manufacturers sought to put their own stamp on this particular breed of AR by making their own modifications to the design.
Armalite Model AR-10
Before we get started, you should know that this led to a number of different available standards and this is an extremely important consideration when going out and searching for parts. Even though there are several different “standards”, there are two major component patterns that we refer to as “Armalite” and “DPMS”. You can guess why they are called these. That said, folks also tend to mix around terms like “AR-10” when they really mean to refer to a .308 caliber AR and not necessarily the Armalite AR-10 which Armalite vigorously keeps close to itself. In the article I will refer to the guns in general as .308 ARs but will occasionally refer back to Armalite or DPMS style builds.
Keep in mind there are exceptions like LWRC’s REPR .308 rifles that have completely proprietary receivers and charging handles but still have some parts that can be shared between guns. Cobb, Rock River, and a number of other manufacturers have made their own standards and it’s important to keep an eye on what you’re buying. But since Armalite and DPMS parts and dimensions cover 95% of what’s out there, that’s what we’ll stick with for today.
The following is a chart from GunWiki which will be extremely helpful when it comes to hunting for parts…
AeroprecisionSPR-2ArmaliteAR-10These are identical to Armalite’s AR-10 receivers.
ArmaliteAR-10ArmaliteAR-10This is by far the most popular 308 AR platform in existence. Until DPMS started cutting into their market, Armalite refused to license its receiver design.
BushmasterBushmaster 308RRAFALBushmaster designed a 308 AR-10 style rifle, then lost interest and sold it to RRA.
CMMGMK-8DPMSDPMS, G3Use of G3 mags requires modification of upper (slight widening) at a machine shop on standard uppers. JD Machine, of San Diego, offers pre-widened uppers.
CobbMCR-200CobbG3Cobb’s system is completely proprietary, and is not compatible with any other lower manufacturer. Since Cobb was purchased by Bushmaster, the fate of the MCR system is up in the air.
DPMSTAC-20DPMSDPMSDPMS came up with their own system, but have allowed other manufacturers to make compatible receivers.
Fulton???DPMSDPMSFulton’s 308 receivers are milled from billet, like POF’s, and carry a comparable price tag.
HesseHAR-25ArmaliteAR-10Roughly made receivers which, after some gunsmithing, can become functional Armalite-type receivers.
JD MachinePR3DPMSDPMSJD upper receiver needed to use G3 mags in CMMG directly
KaiserworksKR-7DPMSDPMS, MagpulDo not confuse with Kaiser Defense
LaRueStealth OSR???SR-25/DPMSMags are compatible with M110, which is SR-25 compatible.
NoveskeN6ArmaliteAR-10Noveske is a premium rifle manufacturer. Expect quality to meet or exceed that of a basic AR-10.
POFMP-308DPMSDPMSThese receivers are compatible with the TAC-20 system, however they are expensive because they’re made from billet.
Quentin DefenseQD-10DPMSDPMSAvailable as either 100% or 80% offerings.
RemingtonR-25DPMSDPMSAvailable at present only in oak camouflage. Multiple calibers available.
Rock River ArmsLAR-8RRAFALLPK unknown; design acquired from Bushmaster, name changed from LAR-10 due to Armalite lawsuit. Inch or metric mags usable; some 308 Bren mags as well.
VulcanHAR-25?ArmaliteAR-10Roughly made receivers which, after some gunsmithing, can become functional Armalite-type receivers. Vulcan sold the rights/inventory to Hesse.
THE RECEIVER IS DIFFERENT
As mentioned and made apparent by the wiki chart above, there are a number of different standards and the receivers are no different. The chart shows that the majority of guns conform to either the Armalite or the DPMS systems. Be aware that the differences are much harder to spot than the similarities.
Notice the sharp corners on the back of the Armalite receiver.
Differentiating Armalite receivers from DPMS ones is actually very simple. Just look at the curve on the back end of the receiver set just in front of the receiver extension. If there is a radial curve, the receiver is most likely a DPMS-pattern receiver. If there is no curve but rather two angles and three straight edges (like the corner of a stop sign), you’re likely looking at an Armalite-pattern receiver. Due to this very simple difference in geometry, upper and lower receivers cannot be mixed between patterns. If you’re building a DPMS pattern gun, do not buy an Armalite upper (or Noveske, Aeroprecision, etc.).
Now also referring to the chart above (which really only includes the majors by the way), you’ll notice the companies that produce Armalite pattern guns number fewer than the group that produce DPMS pattern guns. Noveske has for the time being suspended production of their N6 line of Armalite pattern .308 rifles as of early 2012. So what you are left with is of course Armalite itself, and Aeroprecision who incidentally cuts receivers for Armalite. Because you only have two major manufacturers of repute making this pattern, it is very possible that parts will become more scarce and consequently more costly (although you can bet Armalite will always be a good source for these parts). That is not to say that the Armalite is not worth building, especially if you can get your hands on a Noveske N6 upper. Remember the barrel and the bolt is where all the performance is built into. Scoring a winner like a Noveske N6 makes building it out worthwhile. DPMS pattern parts are in abundance and mod parts are widely available since this pattern has become the more popular among manufacturers. Most new .308 AR rifles are built with mostly DPMS pattern components.
THE MAGAZINE IS DIFFERENT
“Armalite pattern .308 guns use Armalite’s own magazine standard.”
Magazines for DPMS pattern rifles are referred to as DPMS pattern magazines. However, Knights Armament’s SR-25 uses a magazine with the same or similar dimensions as the DPMS magazine and are therefore cross-compatible. Magpul Industries’ LR series PMAG magazines are produced and labeled as SR25/M110 compatible. (M110 refers to the as of late adopted .308 AR standard adopted by the military as a semi-automatic sniper rifle called the M110 SASS).
Armalite pattern .308 guns use Armalite’s own magazine standard.
CMMG Inc. experimented with their DPMS pattern Mk3 rifle and adapted it to fit magazines from the Heckler & Koch G3 rifles (which were among the most abundant and affordable magazines in the world), and that resulted in the creation of the Mk-8. The Mk-8 is no longer in production but the receivers are still floating out there. JD Machine & Tool in San Diego produces modified upper receivers for the Mk-8 lower that will accommodate the G3’s magazine. By the way, JD Machine’s Mk-8 upper will work with any DPMS pattern lower.
THE BUFFER SYSTEM IS DIFFERENT
This one is a doozy. This is the part that seems to confuse the greatest number of people and for good reason… it’s confusing.
The .308 AR’s receivers are longer than on an AR-15. Therefore the bolt carrier is also longer as the systems are identical except in scale. The length of the carrier, the amount of travel required to cycle the gun, the length of the buffer, and the compressed length of the buffer spring all factor in to whether or not your gun will function. See my previous article “Different Strokes” for reference.
So to make things simple I’ll divide this up into categories:
The following two setups are becoming continually more rare…
.308 AR Receiver with a .308 Rifle buffer tube – Chances are this proprietary setup came complete with the buffer and spring. If it didn’t, these extended tubes have additional length and can usually accommodate a standard AR-15 rifle buffer with a standard A2 length spring.
.308 AR Receiver with a .308 Carbine buffer tube (collapsible) – This one also likely came as a complete stock with the buffer and spring. The only difference is the length of the tube itself. The buffer and spring are the same as in an AR-15.
These are much more common nowadays…
.308 AR Receiver with an AR15 A2 Rifle tube – There is a specific buffer made for this setup as well as a specific spring. Both are required.
.308 AR Receiver with an AR15 carbine buffer tube – This is also meant to be used with a specific buffer and spring. Take care to use the carbine spring to ensure sufficient depth during the ejection stroke. With this length it is important to use an extra short buffer. These can be especially pricy (at around $100), but they work and there isn’t much else that does. Be wary of taking shortcuts with this setup.
For reference and for clear guidance on which buffer setup to select, I recommend visiting www.heavybuffers.com. They have this whole thing down pat and on both patterns to boot.
“The .308 AR buffer is shorter or the tube is longer primarily because the bolt carrier must travel farther back than on an AR15.”
The .308 AR buffer is shorter or the tube is longer primarily because the bolt carrier must travel farther back than on an AR15. It is not always apparent when doing your function testing. Your bolt seems to lock back fine, it releases properly when pressing the bolt catch release, the cartridges eject properly when hand cycling. It should be good to go right? Well… maybe.
When using a buffer that is too long, or a tube that is too short, the carrier may have just enough travel available to it that it can eject a cartridge and lock back. But it doesn’t lock back on the bolt catch face, it locks on the back of the magazine follower and not consistently. So during function testing it seems to work just fine, because hand cycling can’t replicate the kind of speed that the gun operates at. When you take it to the range to shoot it, the first round is loaded with the charging handle by hand and fires just fine, but upon ejecting the first round, you’ll find that the chamber is empty and the second round is still in the magazine. The bolt no longer has the run up distance to allow the cartridge in the magazine to rise enough for the bolt face to catch it on its way forward. The bolt carrier bottoms out in the tube before it can gain the necessary distance, hence the empty chamber.
SO WHAT ELSE IS DIFFERENT?
Charging Handle – It is indeed longer. The bolt carrier and upper receiver are longer. The charging handle is therefore larger. These are available in not only standard designs but in fancy aftermarket ones as well.
Muzzle Device – The threads and bore are different for the .308 Winchester rifles. Most are threaded in 5/8” x 24 tpi (threads per inch) instead of the AR-15’s standard 1/2” x 28 tpi. These are also commonly designated as being for 7.62x51mm or simply .30 caliber. They are also the muzzle device size commonly used in intermediate calibers like 6.8 SPC ii and 6.5 Grendel. As the muzzle device is larger, so are the crush washer, shim rings, or peel washer.
Handguards – The barrel nut on .308 ARs are larger than on the AR-15. Thus, most free-floating handguards will not be cross-compatible. This also has to do with the height of the top rail as the .308 receiver is larger and would not be on the same plane as a AR-15 handguard. Chances are, you’ll need to find something made specifically for .308. If you are using a plastic drop-in handguard, there is word that you can use AR-15 handguards on .308 barrels with .308 delta ring assemblies. HOWEVER, it is also noted elsewhere that the heat shields leave very little room for the larger barrel and will mark or scratch the barrel surface if they are not modified.
Gas Blocks – It makes sense that these would be the same as barrel diameters are similar and porting size is dependent on the port in the barrel and not on the gas block itself. In fact even the gas tubes are the same. But, the profile of the upper receiver is different and ends up at a different height than the AR-15 version. Whenever you install a gas block you should always double check the geometry of the plumbing to make sure the barrel vents properly. Either way, if you are building a .308, get a .308 gas block of the proper diameter for the gas block seat of your particular barrel.
Gas Tube – On an AR-10 (Armalite) the Rifle length gas tube is slightly longer than the AR-15’s tube. You would have to get an AR-10 specific tube for this build. However, DPMS-pattern AR 308 builds retain the use of the AR-15’s rifle length gas tube. AR-10’s carbine length more closely resembles the AR-15’s midlength than the carbine length. Double check the position of your gas port to make sure you’ve got the right gas tube.
Lower Parts Kit – This one is pretty important. It can get confusing. This is the area where you’re most likely to end up with some spare parts. Generally speaking, the difference in parts between an AR-15 lower parts kit and a .308 lower parts kit can include: 1) the bolt catch, 2) the takedown pin, 3) the pivot pin, and 4) the mag catch and 5) the hammer. This is NOT always the case though. Note that I wrote “generally speaking… can include”. They all seem to be a little different. But if you’re planning on buying an AR-15 lower parts kit and just replacing the parts needed for your .308, remember that some of these are not the cheaper components of an LPK. Though manufacturers usually conform to the norm, little compromises are made here and there. For instance, Mega Arms LLC ships their MA-TEN upper and lower receivers as a set only with the pivot pin and takedown pin pre-installed. This is great in that it minimizes the confusion involved in locating a set that fits properly if they are not standard. If you are building an Armalite pattern gun, chances are the Armalite AR-10 lower parts kit will contain everything you need and will fit just fine. It is the DPMS-pattern guns where several manufacturers have taken more liberties. It is a good idea to ask the manufacturer of your lower/upper to see which parts will be compatible. Sometimes still it may be best to just start out with a DPMS .308 parts kit.
SO WHAT IS THE SAME? YOU SAID IT WOULD BE THE SAME. ALL I SEE IS DIFFERENT.
Trigger Group – The trigger group is the same. Now there are trigger manufacturers that sell .308 specific triggers and parts. While they do carry some platform specific benefits, you will welcome the idea that you can move that expensive Geissele or Timney into your new .308 build. This is especially welcoming considering AR-15 triggers are far more common and have many more options. While there are numerous variations in fit between triggers, you can still consider the larger .308 version to have the same fitment considerations as the AR-15. Be wary of Jewell triggers made for the AR-15 has they have been known to occasionally light strike in .308 ARs.
Buffer Assembly – If you choose to use AR-15 parts for this very important part, just remember that the receiver extension (also known as the buffer tube), castle nut, and endplate are all the same as on the AR-15. Likewise for the A2-type fixed buttstock, the tube, screw and spacer are all the same. Keep in mind the previous section on buffers and buffer springs though. If you decide to use a .308 specific stock, you can just throw this paragraph out.
Grip – The pistol grip is identical to the AR-15’s. In fact the entire fire control area, from the trigger well to the heel, should be dimensionally the same as on the .308 AR. That means you can use any Hogue, Ergo, or Magpul grips made for the AR-15. There may be a minor difference however in the distance between the grip and the safety selector. A longer spring is sometimes necessary with certain aftermarket grips like Magpul’s MIAD.
Lower Parts Kit (excluding the parts mentioned earlier) – The parts kit itself contains the same internals for the most part. Your trigger group, springs, detents and buffers are all cross-compatible with the AR-15. There are occasional exceptions with the trigger guard in that many .308 lowers are cut from billets and the design incorporates an integrated trigger guard.
Gas tubes (DPMS) – These are not proprietary in length and use the same diameter as do the AR-15 gas tubes. The gas tube itself allows for the transfer of a great deal of pressure from its standard design and thus is very flexible from one caliber to the next. Remember, this does not apply to Armalite AR-10 gas tubes (which are longer).
Ejection Port Covers – .308 ejection port covers are larger than on AR-15s. In most cases, you will choose to purchase an upper that comes complete with the ejection cover installed. But if you don’t, be sure to purchase a cover made for the .308.
Even though all of the basics are identical to a .308 AR, knowing how to build an AR-15 really only brings you as far as the assembly of the parts. The actual function of the gun requires a little more attention to detail and that focuses primarily on the receivers, the buffer system, and the magazines. Because the operating core of the gun is different dimensionally from the AR-15, you’ll need to exercise extra care in acquiring your parts. Because there are different available standards, unlike the AR-15, you’ll need to be extra observant as to which patterned parts your build will require.
Most first time builders and many veteran builders will end up with some extra leftover parts. Whether that is due to an economic decision (e.g. from perhaps buying a parts kit instead of individual parts), or because certain components were purchased in error, it shouldn’t bother you too much. Think of it as the great adventure that is always worthwhile and one that results in some spares you can save in your parts bin for a future build.
You can get help at almost any time. Intrepid builders have gone ahead before you and made all of the mistakes that you could possibly encounter. Their advice can keep you from making any major errors. Seek them out on gun forums, YouTube, or your local gun club. Seek out buffer advice from HeavyBuffers.com. Whenever you need specific advice about your parts, try to get some help from the manufacturer of your item. Most, if not all of the time, they will be very helpful. After all, they want to show you that their parts work! As always, a quick Google search can shed a lot of light on simple issues. Give Google a spin. Of course, we here at Shield Tactical will be happy to tackle any questions you might have as well. You can always ask your questions or state your concerns and even your own personal tips via the comments section right here on Saved Rounds.
Naturally, always make safety your first consideration when engaging in any shooting (or building) activity. Have fun with the build and enjoy your new .308 AR rifle!