I have a Colt "CM" marked A2, auto sear reliefed upper. From the documentation I've read, this is a large hole .316 upper. This looks like it will make a nice auto sear drop in project later, but what lower will work with this upper? I figured a large hole Colt lower, but anything else? And where, can I find large hole lowers? I am under the impression large hole anything is tuff to come by.

From what I've read this is considered a NON- "mil-spec" upper. Does this mean I have to take note on what bolt groups, barrels and other parts I buy for this?

Thanks,
David