.45 S&W blackpowder loads in the modern .45 Colt topbreaks

[kcub]

I have heard .45 Colt BP fouls up real quick, I was just wondering if this would help any (being shorter cases and with less powder).

[Pettifogger]

Nope.  The modern top breaks don't have a gas ring so fouling will be a problem to matter what you load.  Even the subs gum them up.  Best bet is a bullet with lots of sloppy BP lube.  Will help keep the fouling soft so the guns will run better for more shots.