The AR10 series gives you two basic choices for design and the parts are not interchangable as in the AR-15 series at this time. DPMS has their parts patterns and magazine and Armalite has their pattern, which because of the availablity of magazines uses a modified M-14 mag. (Blackthorne, Hesse, or Vulcan whatever their name is this week matches the Armalite pattern avoid them like plague/Demies.) Either company offers an excellent target rifle version. DPMS may be a bit less expensive and I think currently offers more caliber selection. My brother has the DPMS SASS version, cannot give a first hand review of function and accuracy cause it is still gathering dust. My choice is building one on an Artmalite lower leaning (should be finished late next month) and towards the flat top upper upper scoped for accuracy to provide a .308 option different from the G3's, M-14's, FAL's etc.
As to what parts interchange with AR-15's: the basic lower on the AR10 uses a different front take down pin, mag release, and bolt stop. They can use A2 stocks, but with a different buffer and spring.
A little birdie tells me that LCW may be offering a 22-250 version very soon, they are waiting on barrel production at this time. As soon as this project is complete I'll provide a report.
You can also find more information on www.ar15.com
on the AR10 series.