Hey guys... sorry if this is a repetitive question, but I haven't been able to find the answer via search here or google/etc and I am on a time crunch to order parts.  I'm building a 16" DPMS pattern .308 carbine...  I have an LMT SOPMOD on my AR-15 that I'd like to transfer over, so I need a receiver extension it will fit on.

So is the standard AR-15 buffer tube exactly the same and compatible as long as I use AR-10 buffer & spring?  Or is that a shoddy fix and I should just buy the DPMS or Armalite carbine buffer tube?

Thank you for your time and sorry again for being a noob to 308 ARs.

The SOPMOD stock will fit on any milspec carbine receiver extension tube.  

You have two main choices:

AR-15 (from many manufacturers)
AR-10 tube from Armalite or Vltor.

The only practical difference is that the AR-10 will make your collapsed stock 5/8" longer.

AR-15 tube:   get a stubby (2.5") buffer from DPMS or Slash.  
AR-10 tube:   get an standard AR-15 sized (3.25") H3 carbine buffer, or 9mm buffer
Both can use an Armalite buffer spring, you may need to trim.

See:
http://www.ar15.com/forums/topic.html?b=3&f=121&t=444682