A friend just got a DPMS .308 AR. It is the M4 style one with colapsable stock and carbine gas system.
I was wondering if the buffer tube and receiver back plate is the same on those as a regular AR-15? He was asking me questions about changing parts but I wasn't sure if they were the same from one caliber gun to the next.
The DPMS LR-308 carbine with collapsible stock uses a standard AR-15 RE. The difference is the buffer length/weight and the spring. As far as I know DPMS is the only manufacturer using the AR15 RE. The other manufacturer .308 AR's use a longer RE with collapsible butt stocks. Slash (heavybuffers.com) makes short .308 buffers so you can use any AR15 RE/stock set.
Also, I’m not sure of the exact history and it can be a bore anyway, but suffice to say that Remington’s 308 AR, and Knight Armament’s 308 AR (SR-25 and M110) were/are both made with AR-15 compatibility in mind. Things such as stocks, grips, non-free-floating fore ends, trigger groups, and other parts are AR-15 interchangeable. Also, a good point is Bushmaster/Remington/DPMS’s 308 ARs will take Magpul's 308 PMAGs.
Very little compatibility.
Most .308AR's use a collapsible stock RE that's a half inch longer then the AR15 RE. This is to compensate for the longer BCG. Floating hand guards do not interchange with any AR15. There are differences between .308 AR barrel nuts by brand and they require specific hand guards for correct fit.
There are several different magazine platforms, G3, FAL, M14 and SR-25. The SR-25 was become the defacto standard.
Hand grips, fixed A2 style butt stocks and RE's, FCG's and safety levers are all interchangeable between AR15's and .308 AR's. Other small parts like mag release, bolt catch, buffers and buffer springs are very specific for the manufacturer.