Originally Posted By AcridSaint:
Either should thread into the same lower. My understanding is that all lowers use the same thread, but the commercial tubes aren't cut as deep as mil-spec.
You are correct.
While MIL-SPEC receiver extensions are not as thick as commercial tubes, real MIL-SPEC extensions are made with a process that makes them potentially much stronger, and instead of cut threads, their threads are rolled, which is also stronger. The price difference isn't that much; given the choice, I go MIL-SPEC.