Re: 300 Jazz
Besides the extra velocity improving a case will help with accuracy if done properly.
Here is what I have been taught on improved cartridges. When you blow out a case you have the ability to create the perfect neck/shoulder junction which is the funnel. Use a expander mandrel to put all the inconsistencies on the outside of the neck. Clean up the neck with a quality neck turner. And then fireform the case. Your old neck shoulder junction will be in the body which doesn't hurt anything, and your shoulder/neck junction will be now where you cleaned your neck. Consistent size. I have been at benchrest for about a year and I have cut my average groups in half. Might be worth a look if you are after accuracy.
"Amazing things can happen when preparation meet opportunity" Richard Schatz
"The will to win, compares little with the will to prepare to win" Donovan Moran