The AR-10 is not a govt rifle-it's a commercial gun, so making proprietary parts is a good thing from a marketing view, if it breaks; you have to buy "MY parts at MY prices".
I picked an MP-10 up last year, and ran a boxes of rounds through it. Shot OK with the factory magazine and the 20 round Pmag. I did LIKE the rifle, as it has an ambi safety, mag release, and bolt release from the start. It's also a bit lighter than the DPMS rifles, and has an 18 inch barrel.
I ended selling as the accuracy was pretty steady at 1-2 inches, and not much room for improvement without a major re-build.
Couple of points on the Smith, it was close, but mine was NOT DPMS compatible. The upper pin lugs were longer on the Smith than the DPMS rifles. My Smith upper would not close on a DPMS lower without some fitting, I believe the rear lug is wider to prevent the installation. I don't know what the gap would be between upper and lower, we didn't go that far once the rear lug clearance was identified.
A DPMS upper would close on the Smith lower to about 30 degrees, maybe a bit less, and then you could feel the metal-on-metal bind of the shorter DPMS front lug against the Smith lower. The upper would not close any further without forcing it.
RRA rifles - Up front, I do like RRA rifles in 5.56. The .308 line is a another rifle that only fits RRA upper and lowers, in other words, it's own company parts. Read the parts disclaimer on the RRA website.
The RRA takes FN-FAL mil-surplus magazines, and they make their own. The RRA marketing thought in 2005, after the first Assault Weapons Ban 94-04 expired, was to design a rifle to use a surplus magazine that is available world-wide, think World War Z and cheap magazines.
The FN FAL magazines can hit/miss on quality, think South Africa manufacturing vs. German engineering -you have to know what you are buying.
RRA uppers are about an inch longer in length than DPMS pins, so they will not mate up.
Because the FN-FAL magazines latch differently, the magazine release is forward under the trigger guard. It also serves as the bolt release. Operators claim it works fine, but I would have to retrain between rifle controls, hard to break 25 years of training on AR rifle, less of a problem on other designs.
Armalite - separate rifle, with its own specs and parts. The Armalite uppers will mate with a DMPS lower, BUT they have an angled cut on the back of the upper that leaves a gap at the back of the upper receiver that exposes the buffer/bolt rear. The same cut prevents a DPMS upper from mating with an Armalite lower.
Most Armalite parts are NOT compatible with the DPMS rifles, especially bolts and carriers.
DPMS - IMHO, the "Chevy" of the AR-10 world, more produced, more copied than any other. Doesn't make it the "best", just the easiest to work on, get parts for, upgrade, etc.
Remington, DPMS, CMMG, AR-Stoner, and "most" aftermarket parts company provide parts in the DPMS pattern than any other. IMO, this is the best choice for a truly unique build that is "yours".
The real wrinkle in all of this is the BCGs. I haven't found/experimented with the bolts enough to say whose bolts fit what .308 gun.
Rule of thumb: stick the same BCG as the upper/barrel, should prevent the most amount of problems.
great info
I'll throw in - a lot of the newer .308 AR's seem to come with 16" barrels. Let me caution you that a 16" .308 is an acquired taste. They are very loud, especially if they have a brake. When I started shooting mine at the range, it would usually have people clearing out around me, if they weren't complaining.
I eventually put an 18" barrel, and a Vortec flash hider on one of the AR-10's, which makes them more tolerable. The other one is a bull barrel 20" gun.
I have plenty of mags and spare parts for mine, but eventually I will get another gun, that can use PMAGs. Colt builds one, the LE901, that can accept standard AR-15 uppers, but I have not seen one for sale in a while, and the .308 barrels are all 16". When I buy one I'll probably ante up for a LaRue, LWRC, or similar rifle. It may or may not have a piston upper.
