Mil-Spec, or “Military Specification”, is the criteria used by the military to meet the standard for their firearms. It includes material used, dimensional specifications, even how they are tested and function.
While most AR15 components tend to meet or exceed Mil-Spec standards, there is one portion of an AR15 that may have a different set of spec standards: the receiver extension, more often referred to as the “butt stock.” Manufacturers have the choice of using Mil-Spec or Commercial-Spec. Both extensions have 1-3 and 16-16 thread rolls, however, the process of threading is different between the two, so the Commercial-Spec threads are slightly weaker than Mil-Spec. Other than that, the Commercial-Spec receiver extension will have an angle to it in the rear, making it a little longer, in essence, and it has an external diameter of 1.168 inches, where the Mil-Spec diameter is 1.148 inches. CMMG® uses all Mil-Spec components on its AR15s.
There is a specific set of standards for an AR15, but the same cannot be said for a large-frame platform, such as the .308 Winchester (or its NATO counterpart, the 7.62x51mm cartridge). As there is no specific “spec” standard for a .308 platform, it is generally broken down into two separate groups. The Armalite “AR10” and the DPMS “LR-308,” but there are also specification sub groups beyond that, which we will cover a little further down.
The Armalite “AR-10” is well known in the firearm community and is what most people have come to know any AR-platform .308 WIN as, but this is a common misconception. The most distinctive feature on an AR10, and it is pretty much a dead giveaway as to what set of specs it uses, is the shape of the upper and lower receivers, or to be more precise, where the upper receiver would pivot into place and connect to the lower receiver (the “takedown pin” area). Instead of it having a rounded 90-degree corner, it has a slanted surface in between the horizontal and vertical edges. Components on an AR10 will not be cross compatible with components on an LR-308, unfortunately.
Both, the AR10 and the LR-308 can utilize SR-25 pattern magazines, but only the AR10 has the distinction of also being able to utilize FAL magazines in some models (models before 2012, mainly). DPMS is the more common specification set you will see on modern .308 chambered ARs. The caveat to this is that the DPMS style comes in either a Gen1 or a Gen2 set-up.
Gen1 is the more prevalent of the two sets of specifications, despite it being longer and therefore heavier than Gen2. Gen1 and Gen2 are not compatible with each other, either. Gen1 also has a set of sub-specs for its upper groups. They are commonly referred to as High Profile and Low Profile, referring to the height of the tang on the Picatinny rail, where the charging handle is installed. High Profile and Low Profile handguards and upper receivers are not cross compatible with each other, either. For the record, CMMG’s Mk3™ is a Gen1 DPMS High Profile platform, and it is available in .308 WIN (this can also utilize the 7.62x51 NATO cartridge) and 6.5 Creedmoor.
As always, if you have any further questions, our Tech Support Team is more than happy to answer them and can be reached via email at firstname.lastname@example.org.