JDW,

Yes, with just one round at a proof house, you can prove a barrel safe. Keep in mind that a proof round is 2x the safe working pressure of the gun, so if it can withstand proof, it can work all day long shooting normal loads.

However, also keep in mind that any time a gun is modified (long cones, chokes, refinish, honing, nitro proof, chamber lengthened, etc), the gun must be "reproven" back in the UK. We have no such moral imperative in the states so you see guns out of proof that are dangerous, such as the gun we're discussing.