For the 260 Rem, You will need the AR 10 platform. Based on .308, So all and any .308 sized cartridges would work. .243, .260, 7-08, 308, the new .338 fed, and .358 as well for a build up and use standard AR10 mags. .308 based cartridges will not fit in the AR15 Mag.
There may be a factory gun out there that is what you want or at least 80% and then add the options you want. DPMS lists a .260 in their RFLR-260L for $1504.00. It may be cheaper to start with a factory gun and add the few options you want then to start with a naked lower and build up from there. I forget what Armalite makes. But uppers are two pins away from going from .243 to .300WSM. Also most uppers for the AR will fit from manufacturer to manufacturer as the receiver is pretty standard.
DPMS builds a whole rifle in a few configurations from .243 to .300 WSM for the AR10 Platform. With the AR15 you are limited to .204 ruger, .223, 6mm X45 Saw, 6.8 SPC, or the .450 Bush. I have also seen uppers in .50 Beowolf and .50 A/E.
www.dpmsinc.comwww.armalite.comI have an Armalite AR 15A2 and have only had one small problem. It was with Factory ammo and Armalite took the rifle fixed it and sent it back with a new charging handle at no charge, in short order. The thing runs great and I use it for three gun, bowling pins, and High power matches.