They look the same, they function the same way, and they even use the same damn parts (for the most part, pun intended): What's the difference between the AR-15 and the AR-308? Or AR-10? Which is it, even?
The answer's more complex than you think. There is no single type of AR chambered in .308. In fact, there are a few. And each one is just slightly different. Frustrating? Yes. But today we're going to explain it all away. We'll compare all the rifles, including which parts can be shared between the AR-10-308-LR-whatever and the AR-15. We'll also clear up all this "KAC SR-25", "AR-10", "DPMS .308" confusion.
The Types of AR-308s:
The AR-10 is owned and manufacutred solely by ArmaLite. It was designed by Eugene Stoner in 1955. This is the first AR-platform rifle ever built, even predating the AR-15 by one year.
The SR-25 is a slightly modern equivalent to the ArmaLite AR-10. It was also developed by Eugene Stoner in the 1990s, when he began working for Knight's Armament Company. Today, the U.S. Military has adopted the KAC SR-25 as a service-issued Designated Marksman Rifle (DMR). After some modifications, SOCOM adopted the SR-25 as the Mk 11 Mod 0 in May 2000. Beginning in mid-2011, SOCOM began divesting the Mk 11 Mod 0 from their inventory and replacing it with the SSR Mk 20.
Differences between the AR-15 and AR-10
There are two primary differences between the AR-15 chambered in 5.56/.223, and the AR-10 or AR-308: The .308 rifle is physically larger, and it fires a bigger caliber. Everything else about the two rifles are mechanically the same:
- Both use direct-impingement gas systems
- Both use detachable box magazines
- Both use a buffer and recoil spring
- Both use the same trigger setup and parts
- Both use the same BCG design (.308 is larger)
Study the comparison of the two rifles above, and you'll find they appear to share many of the same parts, too. There are just a few parts that the AR-15 and AR-308 cannot share. This parts compatibility list applies to all AR-15s and ARs chambered in .308, whether it's an ArmaLite AR-10, an LR-308 from DPMS, or an SR-25 from Knight's Armament Company:
Parts shared with AR-15, ArmaLite AR-10, DPMS .308, & SR-25:
- Bolt stop pin
- Bolt stop plunger
- Bolt stop spring
- Buffer tube (Tube & buffer compatibility below)
- Buffer detent
- Buffer detent spring
- Buttstock assembly
- Buttstock screw
- Buttstock spacer
- Detent (two per rifle)
- Detent spring
- Disconnector spring
- Ejector/Safety spring
- Gas tube (DPMS and SR-25 only, excludes ArmaLite)
- Hammer and trigger pin
- Hammer spring
- Mag catch button
- Mag catch spring
- Pistol grip
- Recoil spring (using AR-15 spring will increase felt recoil)
- Rifle receiver extension
- Safety detent
- Safety selector
- Trigger guard assembly
- Trigger guard pin
- Trigger spring
AR-15 and .308 Buffer and Tube Compatibility
All AR-308 lower receivers (ArmaLite AR-10, DPMS LR-308, and SR-25) use the same diameter buffer tube housing with the same threads, which is also the same as the AR-15! Importantly, this means that the buffer tube is interchangeable between the AR-15 and any AR-308. However, the .308 bolt carrier group (also shared between all AR-308s) is longer than the AR-15 BCG and there are five different types of buffers that can be used with the AR-308 platform:
- DPMS .308 "shorty" buffer
- ArmaLite AR-10 carbine buffer
- (identical to #3 but heavier)
- AR-15 carbine buffer
- AR-15 rifle buffer
- ArmaLite AR-10 rifle buffer
The different buffers (and their lengths) mean you need to pair up the right buffer with the right tube, to accommodate the longer .308 BCG:
"Which buffer and tube do I use for my AR in .308?"
These rules apply the same to all AR-308s:
- If you want to use a DPMS "shorty" buffer, you'll need a standard (carbine) AR-15 buffer tube.
- Want to use an AR-15 carbine or ArmaLite carbine buffer? You'll need an ArmaLite or VLTOR A5 medium-length tube.
- If you wish to use an AR-15 rifle or ArmaLite AR-10 rifle buffer, you'll need a rifle-length AR-15 tube.
AR-15 Parts Incompatible with AR-308:
- Upper receiver
- Lower receiver
- Bolt carrier group
- Charging handle
- Firing pin
- Gas tube (ArmaLite only)
- Gas block
- Barrel + extension
AR-10 & SR-25 vs. DPMS (LR-308)
Fun fact: The AR-15 is technically the descendant of the AR-10. Eugene Stoner designed the AR-10 in 1955, before he developed the smaller carbine rifle in 1956. The SR-25 was also developed by Eugene Stoner in the 90s, when he joined Knight's Armament Company. As such, the modern DPMS-type .308 rifle platform (which largely feeds the commercial AR-308 market, much more than the AR-10/SR-25) contains one important physical difference that prohibits compatibility with the the other two:
It is this ever-so-slight physical difference in the shape of the rear of the upper receiver that prohibits the AR-10, SR-25, and LR0-308 platforms from truly working together. A DPMS upper receiver will not fit on an AR-10/SR-25 lower, and vice versa. Simple (but disappointing, we know).