but the op asked about a milspec part...not a milspec rifle. most likely because he has a milspec stock, that fits a milspec tube. by boldly dismissing anything MILSPEC when the 308 is involved, you are just confusing the issue for noobs. hence my clarification. a milspec AR15 extension is correct on a 308ar, with the proper 308 specific buffer and spring allowing any milspec sized stock to be used.