Thanks, but assuming the rifle and design are not strong enough for nitro, will a new set of barrels alone make it strong enough? (Of course I realize the cost of new barrels will easily be $15k plus). Is the depth of the rifling significant pressure wise? I know the weight is important and nitro barrels are significantly heavier.

I'm struggling to get at the fundamental question, because I don't know what it is.

The rifle I am thinking of is something like one of the Fraser hammerless doubles from around 1880.