Agreed, UNDER SIZED! It's possible that the expander plug is under sized. More likely it's over crimping that is the problem. Make a "Dummy" round or two and pull the boolits. Measure with a MICROMETER, not a caliper. I'll bet you a doughnut that the boolit is to small. Slugging the barrels would be good info also. It MIGHT help you fine tune the loads after you get this problem figured out. Good luck and keep us poasted on your progress.
Matt