.308, The Heavy Hitter POSTED AT THE BLACK RIFLE COMPANY by John Harrington
.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.
DPMS .308
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…
MakeModelSystemMagazineNotes
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.
Credit: http://gunwiki.net/Gunwiki/Ref308ARs
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