I have several stripped AR receivers and misc components, but I have yet to finish one or buy barrels yet. I have been reading this forum a bit and I saw some people asking about the 308 caliber ARs. Can any AR lower be built up for 308? What would be the expense compared to a 223/556? If not, how about a 762x39. That would make a good carbine caliber, while the 308 would make a good rifle for me. I like the 556, but only as a varminter cartridge. In lamens, what do I need, if anything?