This subject periodically crops up on the Muzzloading Forum with respect to modern US made barrels used by longrifle builders such as myself, which are not routinely proofed. Personally I would not 'proof' a 19th c barrel. It's almost certainly been done already, when the gun was made (and if it's not an originally proofed piece, then it shouldn't be fired anyway). Since then the barrel has only got weaker. Even if your barrel appears to have survived your proof firing, you could have stressed it in a way only discernible by microscope. Why do this? The fact is that firing any antique involves a degree of risk at the outset, until you're confident it's OK, but you're at greatest risk if the first shot you fire with it is a 'proof' load.