There are many misconceptions as well as misinformation out there when it comes to .308s in the AR platform. There are two primary options on the market today and most of the parts are not interchangeable between the two. This post will likely evolve over time but let's get started.
Identifying the difference between AR-10 and LR-308 rifles.
Don't listen to the guy behind the counter at your local FFL or even rely on the name of the item you purchased.
The absolute easiest way to positively identify if you have a DPMS style LR-308 or an AR-10 is the angle from the rear take down hole to the top of the receiver threaded for your buffer tube. Aside from it's size, an LR-308 will look just like an AR-15 with a nice smooth gradual curve. The AR-10 will have a much more noticeable sharp angle. Please see highlighted portions of the above images.
Obviously, the same will go for your upper receivers, and hopefully you can also see that because of this an LR-308 lower is not compatible with an AR-10 upper and vice versa.
Do I have a high or low profile upper receiver?
This is another common question. DPMS receivers are most commonly available in two heights. There are technically three but the third is relatively uncommon and is referred to as a "Slick Side" or "High Rise" receiver and is awkwardly high. The common two are aptly named high and low profile.
Simply measure the height of the tab that juts out above the charging handle. Low profile receivers measure 1/8" and high profile receivers measure 3/16". If it is larger than that, good luck finding a handguard to pair with the top of the receiver; I don't know of any. Monstrum Tactical's 308 handguards are designed for the top rail to line up to low profile receivers as they are by far the most common. You can find our LR-308 hanguards here.
Will Monstrum Tactical's handguards install on a high profile receiver? Yes, absolutely. The top rail of the of the handguard will just be 1/16" lower than the top of the rail on the receiver. Not really a bad thing as most people are running dedicated optics on their .308s and never plan on using iron sights anyway.
Are there any other .308 variations?
Yup. DPMS came out with their Gen2 series. Unfortunately if you have one of these and would like an after market handguard, you are likely out of luck. As of this writing I am not aware of any manufacturers that are producing the proper sized barrel nuts for these rifles.
List of barrel nut sizes:
LR-308 - 1 7/16" with 16 TPI
AR-10 - 1 7/16" with 18 TPI
DPMS GEN2 - 1 5/16" with 18 TPI
AR-15 - 1.250" with 18 TPI
Given the different diameter of the barrel nuts, your standard AR-15 armorer's wrench will not be able to install the barrel nut on an LR-308. We do have the LR-308 armorer's wrench here.