I barely chamfer cases and shoot flat base bullets quite a bit. Makes me think the dies may be wacked. Measure the expander ball, and a necked case. It could be to tight.

By the way, I'm curious about your little project there. I wanted a bit heavier bullet out of an AR15, but so far the 5.56 with 64gr have been ok. I've been told the 77gr OTM is even better, so I balked at the Sharps.

“You never need fear a man, no matter what his size. When danger threatens, call on me, and I will equalize.”
Samuel Colt.

�Common sense is genius dressed up in work clothes.� - Ralph Waldo Emerson