Many people don’t know how different the AR-15 and AR-10 are. Despite their external and internal similarities, building an AR-10 from an 80% lower is quite different in a mechanical context, yet the machining methods are quite similar. There is one primary reason for this.
Nearly all AR-15 parts are now considered mil-spec. This is a measurement system that ensures all parts work together regardless of maker, as they are all created from the same molds and manufacturing methods. The one part of an AR-15 that isn’t always mil-spec is the buffer tube, which is either commercial or mil-spec. Outside of that one part, every standard AR-15 part can work with any other part, regardless of who made it. Parts can be easily swapped between AR-15s because they are almost always mil-spec.
AR-10 vs AR-308
When you’re building an AR-10, most of that does not apply. There is no “military spec standard” because AR-10s are not used in the military. In fact, there are two different kinds of weapons that are commonly available.
First, there’s the original AR-10. This type was designed by ArmaLite, who also designed the AR-15. This version of the .308-chambered AR has been around since the late 50s, but it’s not common today.
Today, most ARs in .308 are of the DPMS variety. These are new introductions to the AR world, but they’re much more compatible. In fact, most .308 80% lowers and .308 AR parts are referred to as “DPMS compatible,” while true ArmaLite AR-10 parts are hard to come by, despite the fact that the weapon is still sold.
There are quite a few differences between the two, including how the upper fits to the lower, the different bolt carrier groups, and the head space for the bolt carrier group. As you can imagine, you really can’t mix the two together at all.
To muddy the waters even more, DPMS released a Gen 2 .308 AR in 2014, which shrunk the size of the weapon to nearly the size of a standard AR-15.
Due to the lack of one set standard for .308 ARs, it can be difficult to build one, especially from an 80% lower. That’s why we’re here!
Building a.308-chambered AR-10 from an 80% Lower
Now that we’ve covered the basics about the different types of .308 ARs, we will briefly talk about actually building yours from one of our 80% lowers and 80% lower jigs.
The big contributors in the .308 AR market don’t make 80% lowers themselves. These have to be specifically produced by manufacturers that specialize in 80% lowers. However, because the multiple sizes that we have talked about, it is important to pay attention to which type of lower you are buying.
Most of the 80% lowers that are available are DPMS-compatible. If you look at our store, we have clearly marked them as DPMS Gen 1 lowers. This will make the rest of your build go smoothly. No matter where you are shopping, make sure you get some clarification about which type of lower you are getting.
Essentially, once you start a build with one of our 80% lowers, you need to stick with other DPMS Gen 1-compatible parts. It is plainly marked on the manufacturer’s website when you are looking at AR-10 parts.
Compatibility between DPMS Gen 1 and Gen 2 platforms is hit or miss. Palmetto State Armory says that their Gen 2 parts will fit Gen 1 stuff, but it’s no guarantee. Other manufacturers say that they may not fit at all. While you may be able to file down the parts to make them fit, your best bet is always to keep your compatibility without having to modify anything – for obvious safety reasons. This will make your build much easier! You can’t go wrong buying from CMMG, DPMS, Fulton Armory, and Aero Precision AR-10 parts.
As you can see in our product description of the .308 80% lower, we recommend finishing your build with one of the aforementioned uppers and CMMG’s MK3 .308 Lower Parts Kit. Magpul magazines work best in this lower.
Now go Build Your AR-10!
Despite the differences between the weapon systems, we hope that this guide has cleared up some of the confusion surrounding the different types of .308 ARs.
To make it extremely simple: buy our 80% lower, which is DPMS Gen 1 compatible, mill it with one of our jigs, finish it with CMMG’s MK3 .308 Lower Parts Kit, and match it to an upper from DPMS, CMMG, Fulton Armory, or Aero Precision.
It really isn’t as confusing as people make it out to be. Keep the same compatibility groups, and you’ll only have to make minor changes.
For an article that gives more of a step by step about actually building the .308 AR, be sure to check it out here (link to the other article).