A lot of calibers will fit the 5.56 magwell. Length is the problem, the overall length of a 5.56 with a 77-80 grain bullet is about all you can go.
A .308 and other long rounds will require an AR-10 style rifle which isn't compatible with the AR-15, unless you spring for a Colt 901 Modular Carbine .308/.223.
The biggest issue with assembling an AR isn't so much getting the parts together right - it's getting the right parts together.
US Army 1966-69, VFW Life Member, Retired Geek