Good Morning Ira Scott,
The question was raised about a black powder load in a .454 Casull and by comparing the case sizes my opinion there would be very little difference. So, I did a somewhat unscientific test to see what could happen.
Using 4 cartridge cases from my brass pile which were a .454 Casull, a Starline .45 Colt, a Western .45 Colt old style button head case and a Peters old style button head case and measured the amount of GOEX FFFg black powder they would hold full to the case mouth.
The results as follows the .454 Casull 46.45 grains, the 45 Colt Starline 39.95 grains, the .45 Colt Western 43.23 grains and the Peters .45 Colt 42.13 grains.
It would be, most likely, a little more boom for the .454, but not all that much. In fact, the old Western case holds more the Starline by a little. These were not compressed loads, just poured into the case until full, then struck level to the case mouth. The average of four was all I did because the numbers did not change by more that one or two tenths.
The .454 is a very high pressure load and the case is just a little longer for safety sake much like the case difference between .38 Special and the .357 Magnum and others.
Respectfully submitted,
Bunk