What seems to be an avarage caliber?
Average caliber for what? Breech seating? Start from about .25 caliber on up.
Guys with original Stevens target guns, chambered in calibers that are almost impossible to get cases for, like 25-25 Stevens, will shoot entire seasons on the same single case, either by using a breech seater, or a case filler of the appropriate length (wood dowel, brass rod). No reason that rolled paper would not work, but it would probably not be very durable in the long run.
Deprime the case, reprime, throw a powder charge, seat a bullet, insert case with powder, take rifle off rack, aim, shoot, repeat.
Some actions, like the Stevens 44 , and the Ballard single shots, had a strong camming action, and could be used to seat the 'pusher' case. others, like the Remington Hepburns and Winchester 1885's had a little less (or none) but were handy to fit with a lever action seater.
There are a couple books out there on the subject of Scheutzen or Stutzen shooting, that cover the matter really well, and a fair bit of info online.
Some current cartridges used for the discipline are pretty much a breech seat only affair, having no real neck to hold a bullet in. IIRC the Miller short and long cartridges are of that bunch.
If you really want to get a headache start reading through the writings of the shooters from the turn of the last century, over the relative merits of breech seating vs. false muzzle muzzle loading for target work. Then start reading about the merits of black, smokeless, or duplex loads (some smokeless, rest black, in the case), and the relative merits of various other minor details...
Cheers
Trev