I am trying to find out if anybody knows the facts regarding building out an AR lower stamped 5.56....i am wanting to know if i can still buy an upper of a different caliber or if it can only ever be 5.56....thank you in advance for any info:1zhelp:
You can buy an upper in any caliber. The lower is just stamped that since it is designed to be used with .223 Rem / 5.56 NATO; hence it will not work with a .308 Win upper.
When you buy an upper, buy a 5.56 NATO upper so that you have a NATO chamber. Other options are 6.5 Grendel, 6.8 SPC, .204 Ruger, .50 Beowulf, .458 SOCOM, etc.