I breech seated full throat dia. cast bullets in my 38-55 TM some years back. I trimmed 2 cases back to fit 1/32" behind that seated bullet with my bench mounted powder dispenser and a card wad.
They shot well enough, as good as I was, in the offhand 100yd. 'shortrange' schuetzen match (normally 200yd., we had reduced targets for 100).
Dont try to just seat the same bullet you would use for fixed ammo, it wont be big around enough to center up and will let gas get by thus cutting it; it has to be throat dia., and unless it is soft lead you will have a hard time trying to breech seat with a pusher (like a short starter for muzzleloaders). If your bullet is even WWt cut 50/50 with soft lead you might not be able to breech seat one that is as little as .0005 too big.
Paper patching is a whole nother art form with its own learning curve beyond cast bullets, and frought with its own demons. You will likely get as good groups from a naked bullet with a lot less trouble.
It is a little fuzty, and the best set up is to have a bench right beside your shooting position, and best done as an offhand exercise (IMHO). You lay out all the bits conveniently at hand. You can take a shot, open the breech and remove the brass. Set the rifle, decap/recap the brass, throw the charge (a fullish case of Trail Boss would be ideal) and top with the precut card to hold the powder in. Bring the rifle up, drop in a bullet, push it in place and follow with the case. Close, aim and shoot; repeat as required.
You can use Black powder, of course, too, again a full case. Other smokeless that is of less volume can be used, though you may want to keep the muzzle elevated and likewise the case while inserting it and closing the action. I do not recommend using a powder positioning wad down upon the powder, the danger of powder ringing the chamber is too great and too well documented.
