The A1 stock is 3/8" shorter than the A2, but the use the same extension. The longer A2 gets a 3/8" long spacer on the back end to make up the empty space, and a longer screw. The tube is standard, but you still need the 308 specific buffer and spring.
Any carbine stock will fit on any carbine extension as long as it is the correct diameter (commercial =bigger/milspec=smaller)
DPMS uses standard AR15 carbine extensions with a modified, shorter buffer to compensate for the longer carrier.
Most guys feel that buffer is too light and go with a heavybuffers setup, consisting on an armalite AR10 spring and a weighted AR10 buffer.
Not all 308 platforms use the same carbine extension and parts, but rifle extension are standard.
Dpms just introduced totally new proprietary parts and threw all standardization back in the toilet.
ok, fact check that for me fellas. Its early. Just tried to stick all pertinent info into one post :p