Dixie Gunworks sells a kit for making the Sharps Cartridge, to be honest I've not heard the mention of the cartridge sliding forward being an issue. What size are your finished cartridges? It sounds like the diameter is a bit small.
A lot depends on how you are making your cartridges. When I was making them for friends (several years ago) I bulk purchased empty tea bags, soaked those in a nitrate solution then wrapped & poured in my powder. IIRC I was putting in 50 grains FFFG for my blanks ad about 60 grains for live fire (which I never did enough of) and the cartridges worked well. I don't recall any issue w/ my method. A friend of mine who shoots a lot of sharps rifle rounds down range (he has the Shiloh M1859) goes to his local meat locker and buys up intestine for making sausage. I'm not certain how he does it but the end result is perfect. As he only live fires I'm not sure if blanks could be made using his process. I'll drop him a line this week and ask him if he can pass any pointers.
You really might want to check out
www.civilwargunstalk.com as there are many more qualified than I there who should be able to answer your question.