I'm in this one late, but there some info that everyone can use:
Rifle receiver extensions are all the same between all platforms (AR-15, .308AR, A1, A2). Extra parts convert the A1 config to the A2 config. MagPul PRS stocks use the A1 extension.
Rifle buffers are not the same for AR-15 and .308 ARs. There's a difference in buffer length. .308 AR rifle buffers are shorter, due to the carriers being longer.
Carbine Receiver Extensions come in two lengths. AR-15 stuff has a 7" internal depth, while Armalite AR-10 (.308) are 7 5/8" internal depth. The VLTOR A5 recoil system uses a carbine receiver extension that matches the Armalite .308 length.
AR-15 Carbine Receiver Extensions come in two diameters: 1.14" for mil-spec size, and 1.17" for commercial size. What collapsible buttstock you run must match your AR-15 carbine receiver extension.
Carbine buffers are not the same for AR-15 and .308 ARs. The .308s use a shorter buffer to make up for the longer BCG length of the .308, and still fit that stuff into a buffer that's 7" internal depth.
^^^Well, now, except for Armalite. Their .308 Carbine Receiver extensions are longer, so they can use regular old AR-15 carbine (heavy) buffers in them. They use AR-15 H3-weighted buffers in them.
There is the difference. Run any old AR-15 carbine receiver extension on your .308, but make sure you're using the shorter .308 AR buffer. Unless you run an Armalite (or the same-size VLTOR A5) receiver extension - then you run AR-15 carbine buffers in your .308 AR, but make sure they're H3 weighted, at a minimum.