Is there somewhere in the USA to have a gun nitro proofed? I have a nice old 450 BPE double rifle, rotating lever lock, fluid steel barrels. A friend who was a gunsmith for Purdey looked at it, said he thought it was made around 1875. It is black powder proofed. My friend said that the barrels are solid and the action is strong and tight enough that he believes it would be safe to fire NE rounds.