I have never tried BP in a cartridge. Up until now I have always shot smokeless 45LC in a gun made for those cartridges I.E. Colt SAA or Ruger BH.
When I reload with smokeless I measure the charge and seat the bullet to the correct cartridge length. Air space is always there and no problem.
How do you insure there is no air space with BP? Do you just push the bullet down as far as possible like when loading cap and ball even if the cartridge ends up shorter than a standard 45LC? Do you still crimp the case? I just bought a cylinder for my Remington and would like to use BP FFG with a either a 200 grain flat nose or a 230 grain round nose. I have lots of those bullets.
"Peace is that brief glorious moment in history when everybody stands around reloading" - Thomas Jefferson