[harpseal]

Given that the lower reciever is the serial numbered part what law would make this illegal to own if one had it in conversion kit form? In other words the M82A1 in .50 BMG and the upper reciever, barrel and magazne that attach to the .50BMG lower reciever?

[Sabretooth]

Do you mean a 20mm upper?

If so yes that would be legal.

[Trailerpark Badass]

Given that the lower reciever is the serial numbered part what law would make this illegal to own if one had it in conversion kit form?

You'd be making a Destructive Device by making a rifle with a bore diameter of over .50. Still legal to do on an ATF Form 1, if DDs are legal in your state and the CLEO will sign your Form 1.