Using Sulphur is also easy, just do not get it too hot, of course, you do not want to get cerrocast too hot either.
Put a plug in the barrel beyond the chamber, pour the melted Sulphur in the chamber, after it cools, insert a wood dowel fom the muzzle and push the Sulphur out.
Unlike cerrocast, Sulphur does not change dimensions over time, you can measure it as soon as it is at room temperature and several days later and get the same results.
Sulphur is brittle, so do not drop it or hit it.
Don