That's interesting about the 45-120 and brings me right back to my question, if not black then why not just use a bulky low pressure smokeless load and not bother with the subs? A huge case like the 45-120 would seem questionable for smokeless loading IMHO but there's lots of load data out there for different smokeless loads and no rash of 45-120 blow-ups being reported so it would seem using that data would be safer than your experience with Pyrodex, of course in huge cases like that BP would seem to be the logical choice for several reasons. 45 Colt or most any BP pistol cartridge and even big cases like the 45-90 and a host of other BP rifle cartridges there are case filling smokeless loads of the right powders that shoot quite well and at about the same (and sometimes even lower) pressures as a BP load so why Pyrodex?
This is not to suggest using smokeless and abandoning BP but, once more, the point is that if BP is not going to be used for whatever the reason why not use a smokeless "sub" instead of the highly corrosive and cleaning critical concoctions? For a Cap&Ball or a ML one of the subs might make sense in the absence of real BP but for a cartridge gun it just seems to be a solution for a problem that doesn't exist!
An interesting note on smokeless loads and big cases, the 45/70 is almost the oldest BP cartridge out there with just a handful being older and it has been factory loaded with smokeless for many years. I don't know what powder they use today but I assume Winchester still uses the same thing they did in the 70's when I pulled one down out of curiosity and the intent to reuse the components for a heavier load for my then brand new 1895 Marlin. It was surprising to me then at just how little actual powder was contained in that case under that 405 grain bullet, lot's of air space in that case and it obviously has not been a problem but still with the right powders most cases can be loaded nearly full. This is NOT to advocate using smokeless instead of BP but rather IF real honest-to-goodness Black is not to be used then why use something that's only similarity to BP is being corrosive (or even more so) and cleaning critical for both firearm and cases?