I have a .44 Pietta Colt Cattleman single action muzzle loading black powder revolver. I bought it second hand from a chap that had removed the original nipples from the cylinder and replaced them with nipples that take shotgun primers. From various discussions I now have reason to suspect that this pistol will happily cope with a nitro load of about 5 grains of Bullseye. I telephones the Birmingham Proof House and spoke to one of the supervisors there about the possibility of getting the pistol proofed for such a load. At first he seemed to think it was OK but then changed his mind to say he doubted it could be done.
Apparently, the reason it could not/ would not, be done was that if proofed for 5 grains of Bullsey, the chambers could be filled with a significantly greater quantity of Bullseye which if fired might dismantle the pistol into it's component parts and distribute them around the area. There was no suggestion that the 5 grain Bullseye load would of itself be unsafe.
The logic of this seemed to deserve more thought. Consider, my Savage .308 has been proofed. I load it with a max load of 46 grains Viht N140 and that's about OK pressure wise.OTOH, a 100% load of Bullseye, things are definitely not alright. Some other powders, according to Quickload, are much much more not alright.
I am starting to wonder about the whole proofing thing. Sure, some testing needs to be done, but I would like to think that any testing would have a rationally thought out basis.