Knox gelatin is used. This topic was discussed recently, so it may be worth your while to do a search.
The key points to remember is not to add the gelatin to boiling water. I can't remember the exact temperature you're not supposed to exceed. Once made, the gelatin is kept refrigerated at 4 deg Celcius, and should be refrigerated until used. Temperature has a significant effect on bullet penetration/performance, so it should be used quickly once removed. It is calibrated with a steel BB. I keep threatening to make some and finally put to rest the whole question of 7.62X39 HP and SP expansion question (for myself, anyway). Dr. Martin Fackler developed the exact recipe in one of his articles that I have buried on my desk somewhere. If you want the info, IM me and I'll look it up if you can't find it with a search or if someone else doesn't post it/a link to it here.