Yep, doing a chamber cast with sulfur is easy to do. Plug the bore ahead of the chamber, melt the sulfur in a double boiler, warm up the gun with a propane torch, and pour your cast. When hard, push it out from the muzzle with a rod. Sulfur is good to get you in the ballpark and identify what caliber you have, but it does shrink more than cerrosafe so I wouldn't try to have a die made from a sulfur cast. Also, if you're interested in the throat to fit cast bullets you're better off with an impact cast. You'll need the accuracy that gives to fit your bullets properly to the throat.
