Big, a chamber cast is really easy to do and especially on a break action firearm where you can remove the barrels.
For the last 25-30 years I've always used Cerrosafe but prior to that had used sulphur and it worked very well except that the cast is quite brittle. Easy to handle but bump or drop it and it's in pieces. Cheap drug store sulphur works fine and as with a crayon, wax or Cerrosafe can be repeatedly used....anyway...
Pull the barrels off the receiver. I use a piece of newspaper but anything would work. Shove it in the chamber and deep enough to get a couple inches of rifling, tamping the newspaper it so it is sort of solid. Melt a small amount of whatever it is you're going to use, obviously enough to fill the chamber. I use a little rectangular tin that the General Food International coffees used to come in, it's easy to bend a spout in one corner and control your pour. Pour it into the chamber until full enough that you can get a good casting of the rim depth, let cool and gently tap it out from the muzzle with a cleaning rod. Voila'! There's your cartridge.
