It won't effect pressure. You're still sealing the powder with the same plastic seal. The crimp and payload will effect pressure. If you're just wanting more spread you're good. But you'll be able to add more shot and that could change things. I've been trying to use 444 Marlin brass for 410 shells and I've been running out of room so I cut the petals just to get the 1/2oz load. With just a OS card glued in I'm not getting enough resistance and a dirty barrel. The shells are bloopers. A buddy has a 45 die what will work. I'm gonna try to swedge the brass in a bit hoping that will help. It's what the military did with their brass case buck shot loads.
