The key to forming Gibbs brass, I think, is to make sure that the case fits snugly in the chamber. I've found with my 8mm that if I punch out 30-06 brass with a 338 neck expander, I can feed the cases in the 8mm Gibbs with a good crush fit. I load them with Red Dot and Toilet paper. After firing I get cases that are about 95% formed. Everything is proportioned right, the shoulder is just a touch rounded.
