Your description sounds like the normal case expansion ring that occures with any cartridge small enough to fit into a chamber. The is some expansion right at the spot where the solid web of the case gives way to the walls. Chambers have to have larger dimensions than the maximum allowable cartridge. Because of tolerances a maximum cartridge in a minimim chamber will fit nice and snug, but a minimum case in a maximum chamber will cause brass migration at the spot mentioned above. To assure yourself that you aren't making the problem worse, put some marker on the case neck and adjust the die so it doesn't touch the shoulder. If you have new brass that you want to sneak up to size, expand the neck of the case up to the next larger caliber, then resize the neck a bit at a time until the bolt just closes on the tiny shoulder created on the neck.