I have opened several lyman dies up that cast undersize bullets by charging soft cast (from the mold) bullet. Do two or three at a time. It helps to have a lathe but it can be done with a drill press as well. Drill a hole slightly smaller than a 8-32 bolt. Put the 3 bullets between to plates of steel, no big deal as to type, I have 2 1x4x1/4 pieces. Smear some diamond lapping compound on the plates and with some pressure roll the bullets between the plates.
Take and insert an 8-32 screw with the head cut off and chuck it in a cordless drill on low speed. Put the bullet into the mold and turn it for 30 secs or so slowly. After doing all three, clean the mold very well, cast 3 more, measure and repeat if necessary.