I have an older A2 Colt CAR 16" upper receiver group with a lightweight profile barrel. It has the larger hole for the perm/ screwable front pivots pins. The lowers I have are drilled for the smaller mil-spec pin. My question is whether there's an adaptor out there that will allow me to mate this my lower to this upper? Like a sleeve that would go inside the upper and fit snuggly around the front pivot pin it would allow to pass through. Does anyone else have experience with this issue? Thanks.