Luckily, chances are, if you have already bought a 308 AR pattern rifle in the last 15 years, it is most likely a DPMS pattern which means it will be easy for you to find parts. However, it is still important to know the differences. One quick way to determine if a rifle is ArmaLite or DPMS pattern, is to look at the rear where the upper and lower receivers meet. An ArmaLite AR-10 rifle will have a diagonal cut where the receivers meet, and a DPMS AR-308 rifle will have a rounded cut.
An important thing to consider is that the bolt carrier groups are also not interchangeable and will not headspace correctly if they are not mated to the barrel extension they were designed for. The parts are close enough dimensionally that it looks like they might work, but this is a dangerous lie. Make sure you are buying the correct parts. If it doesn't say it is the part you need, verify with the retailer or the manufacturer if the retailer can't or won't tell you.
Another important thing to note when upgrading parts on your AR .308 rifle is handguard dimensions and how they relate to the receiver. The DPMS LR-308 pattern rifles have two types of upper receivers they can come with. Low profile and high profile. This refers to the height of the tang or where the flat top picatinny rail will meet the picatinny rail on your handguard. The low-profile upper receiver has a tang height of 1/8th of an inch while the high profile is thicker with a 3/16th inch height. The most common you will see is high profile versions and all of our AR 308 handguards are labelled to make it easy when you upgrade.
Here is a complete list of parts that are generally not interchangeable between the AR-308 and AR-10 platforms:
· Upper and Lower Receiver
· Buffer Assembly (tube, buffer, spring)
· Gas tube (The AR10’s is too long)
· BCG (Head spacing differences)
· Barrel nut
· Magazine catch
However, some non-essential parts can be swapped between the AR-308 and AR-10 such as:
· Dust Cover
· Charging Handle
· Gas Block
· Trigger Assembly
· Takedown springs and detents
· Pistol Grip
· Trigger Guard
· Castle Nut
· Safety Selector Lever
Because the large platform AR market is not as set in stone in terms of parts tolerances as the AR-15 market, we generally recommend buying a complete AR 308 rifle and upgrading parts as you go instead of building one from the ground up. If you buy a DPMS pattern rifle, stick to DPMS pattern parts and the same for ArmaLite and you will be a happy camper. Additionally, sticking to the same brand will help prevent a lot of headaches when you get to the range.
One of our favorite brands that is commonly available and utilizes the DPMS platform is Aero Precision. Piecing together an M5 upper and lower from Aero Precision is just as reliable as buying a rifle whole. If you stick with them, you can easily get an AR308 rifle that will be reliable.