There will not be any definite step from chamber to bore. Don't forget the considerable leverage the gun has to push that shell into the chamber, you may not feel the resistance. The best way would be for a gunsmith with the proper tools to measure the chamber. Another practical option is to take a 6" machinists scale ruler and slide it into the chamber. Usually you can feel where the forcing cone starts. Mark with your thumbnail and read the depth.
I admit that the 2 3/4" plastic shell used in what is probably a 2 5/8" chamber is not going to be a big deal. The plastic is thinner and slicker than the paper hulls of the past. Especially if you are going to stick to black powder. One drawback of the plastic hull will be deposits of melted plastic which may accumulate at the chamber mouth due to black (or Goex's) higher burning temps.
Post some pictures if you can of the gun and it's proofmarks. I am sure someone will have more info on your gun than I do.
