In a .45 Colt Spencer, a 300 grain bullet with about 35 grains Triple 7??? Biggest problem would be the 300 grainers are semi-wadcutters. It would have almost the same thump as the 56-50, but easier cases....
You should allso get better acuracy with the longer bullet
Greetings!
If you're looking for a heavier bullet, try the Lyman #457191; it's a vintage, light-weight design for Winchester Rifles, rated at 294gr. Dad tried them in his Ruger Black Hawk sized to .454"; he said that they shot well. I'd start easy on the powder charge; otherwise, I'd say that it would probably perform well.
Be Well!
M.T.Marfield
3-18-12