Previous Shawn shared a rant from someone about parts commonality in 308 rifles.
Now let me work from memory. This is just my stray memories, not a comprehensive researched list.
For better or worse, the AR15 standard is well known. The data package was leaked, and people can look it up. Still, companies cut corners or make parts that are not right. With .308 ARs, there is no single standard.
Let say you were going to buy mags for a future .308 AR. There are the SR25 pattern mags, which are most popular and available from Magpul, KAC, Larue, Lancer, etc. But those won’t actually work in an original Armalite AR10. Now, the Eagle Arms-Armalite rifle might be using the SR25 pattern mags, or a mag based off the M14 mag. There are also rifles that use FAL mags, but those are kind of rare. Then specialty 308 AR rifles like the HK 417 or MR762 use a proprietary mag.
At least the SR25 pattern mag has pretty much become the standard now. Magpul even makes a 50 round drum in that pattern. The availability of cheap good mags like the Magpul pmags and awesomely high quality mags like the Larue and KAC mags in SR25 pattern makes that the pattern I prefer to use.
There are different receiver patterns. There used to be two major versions referred to as AR10 pattern and DPMS pattern. Those were identified by a round receiver cut on the DPMS and a slanted cut on the AR10.
You can sometimes force a slant cut upper to work on a round cut lower, but there will be a gap even if it works. Even between lowers of the same style, they don’t always interchange. An Armalite AR10, LMT MWS, Larue OBR/UU/PredatAR/PredatOBR, KAC SR25, etc uppers should interchange, but they sometimes don’t. The push pin/take down pin locations sometimes vary slightly and/or are different diameters.
I’ve heard many stories that go like, “You can put Brand A on a Brand B, or Brand B on a Brand C, but you can’t put a Brand A on a Brand C.” I think the order was Armalite, Larue, KAC, but I may be mistaken. As I said, I am working from memory here.
Now, there are even more styles that do not interchange with the other brands of uppers.
It gets more fun. Lets imagine you buy a upper and a rail and you go to put it together. It might not work. There are different threads on some of the different barrel nuts. Larue used to sell a rail for the .308s. He had offer two barrel nut options as the Eagle Arms/Armalite and the DPMS were not compatible.
It gets worse. There are different receiver heights. Even among the same brand. DPMS changed the height of the rail on their uppers. So an old upper and a new hand guard won’t line up right. A quick online search points out that the old receivers were 2.063 inches tall and the new ones 2.003 inches. That is about a hair less than a 1/16 of an inch for those of you not used to the decimal inch. 1.542 mm in the wrong measurement system.
Some companies started offering their hand guards in both heights. Double check if you are building or customizing a DPMS pattern gun.
There used to be barrel & bolt interchangeability issues as well. It was recommended you get both at the same time from the same source.
The bottom is an AR15 carbine buffer and spring for reference. The other three are .308 AR buffers and springs. The middle two have different buffer weights and spring strengths. But those two are interchangeable.
308 AR rifles that use a collapsible stock tend to use a longer receiver extension (buffer tube) than the AR15. This longer tube is called an “A5” if you are using it on an AR15. (Brilliant move, take something already on the market and renaming it) The springs and buffers reflect this longer length tube. So you can’t just pull off your stock and install something like a Magpul UBR with out having to replace your buffer with something like a Slash Heavy Buffer for the AR15 length receiver extension.
Some of the old school .308 ARs had a really small cut in the carrier for the hammer, so a standard AR15 trigger group would not work with them. Now, any modern 308 pattern AR will most likely work with any modern trigger. But you should be careful that it isn’t a trigger so light and short that you will accidentally fire multiple shots due to recoil.
Really simple so far, right?
The gas tubes are not even the same length. Some use an AR15 length gas tube. Others a longer one. Some use a custom even longer gas system.
Some guns, like the SIG 716 use proprietary parts that you might or might not be able to buy from them. They have already discontinued several versions of that rifle. I wouldn’t want to own one because if a major part broke you might not be able to get a replacement. It was something like $300 for an ‘internal parts kit’ and they were not selling gas system parts. Or something like that. I think SIG makes good products, I don’t think they support them well enough.
A tangent. I was considering buying a SIG MPX, and they already discontinued the model I was looking at. Turns out they had something like 3 generations of MPX guns. All discontinued now that the Copperhead version is out. Sig introduces, sells, and discontinues models of guns before I ever hear about them.
Back to the topic at hand. I’m not saying don’t build a 308 AR. I love em. Just be really careful should you decide to build one your self.
Right now, I think one of the best values for a higher end gun would be the Larue UU kit. About $1200 for EVERYTHING minus a stripped lower. Your choice of calibre and barrel length. You can get it in AR10 or DPMS pattern. Then just slap it together on a Larue stripped lower or any cheap lower of your choice. That said, there are countless options. If you don’t plan on heavily customizing the gun and want to buy a complete gun, there are some great deals on the Colt 901 models. Shop around.