Pressure in a shotgun shell has a lot to do with the final crimp. Military brass shells have a slight crimp. We don't do that and one reason other than not having the equipment to do it is after a number of times the end of the shell would go bad. So we use a OS card and glue it in. When I loaded Magtechs with BP they shot fine, but when using nitro powders I always got bloopers. Charlie said he put pressure on the OS card as it was glued and it worked for him. Fiber OP and cushion wads don't seal as good as plastic wads so there'll be less pressure. There's also a guy on the reloading forum over at
www.shotgunworld.com who loads RMC shells all the time for hunting. Go over there and post the question. Let us know how it works out.