Perhaps a spare o-ring, in case you have one fail in the middle of a hunt. I've never had this happen, but it could. And also remember to install the barrel seal activator if the loads you are shooting are too light to cycle the action. My 1187 supermag cycles everything 3 1/4 dram-1 1/8 oz. or heavier without the activator, so I've never used it YMMV.