As far as I know America does not proof guns. They over build them to make sure they are safe.
Any gun sold in this country (not antiques for collections) needs to be proofed. It is illegal to sell an un-proofed gun. (or a gun out of proof).
Guns coming in from Europe have their own proof houses and are legal to sell. Guns coming in that are not proofed have to be proofed first before they can be sold.
You can have the gun proofed. The Proof house will advise you how to do it. They only want the pressure bearing parts. ie the barrel in the case of a muzzleloader.