Here is a (probably) silly question about black powder v. nitro double rifles from someone who knows very little about either.

I assume the lower pressure handling capability of BPE rifles is due to the strength of steel in the receiver and barrels, so that one cannot convert a BPE rifle to handle nitro cartridges in the same caliber just by adding a new set of barrels. That is, the relative weakness of the receiver steel cannot be overcome.

Is that a fair statement or does the truth lie somewhere in between?