Brass thickness . neck lenght. caliper size makes a difference. How close your brass is to the center of the flamematters. It may take a few seconds longer or a few less. You can get templaq to put on the brass it comes in different temperatures and melts when it reaches the temp...
It all depends on the brass formula and what you want from the brass. Some won't use certain makes of brass because it's to soft. The harder you make a metal the more brittle it becomes. The more brittle it is the more it becomes prone to cracking. When i got my 6.5 prc i started with ADG brass...