I measure and mark how much I need to trim, slide a piece of Pex water pipe inside the shell and cut with a razor knife. Pex fits in the 20ga hull perfectly. I lay it on the edge of my work table with the base of the shell off the edge and roll as I cut. Kind of crude, but make a nice cut for roll crimps.. of course you can always buy a shell trimmer, but I'm just a tight wad..
Randy