While shooting smokeless is not something I do nor would recommend, it is something I have mulled over in a theoretical sense. The pressure of a properly applied smokeless load is not neccessarily higher than a black powder load. If, for example, you were to load the equivilent of a 410 shotgun load of powder and an equal or lighter bullet vs the shot charge in a .50 rifle, the resultant load would only produce pressures in a similar range to what many inline muzzleloading guns already shoot i.e. <25000psi. I doubt however that the resulting velocitywould be much different than what is acheived with BP. BUT, the burning temp of most smokeless powders is higher than BP and BP subs so the problems come in the form of more gas cutting of components that were not designed for that type of exposure. They will eventually fail, sometimes much sooner than expected. This is my opinion of why it is a bad idea to shoot smokeless in a gun not designed to shoot it. I don't have solid proof of this due to not doing the testing myself, but I have discussed it with a gunsmith friend that has seen guns that smokeless has been used in. Generally the nipple will burn or blow out unless a stiff load is used, then its anybodys guess as to what will happen. I personally don't want to guess about it so I stick to the right stuff.