I can't see how displacing that much lead is going to be a GOOD thing! Also, how are you controlling the depth you place the boolit into the puller? In other words, how consistent are you? Reloading is all about consistency, each bullet the same as the last.
Pat marlin over on the cast boolit forum makes a plain base gas check maker. The gas checks are made from aluminum soda cans, or any sheet aluminum, or even copper. They are OVER size inside diameter, so they slip on the base of a boolit, then are sized in the size die.
I was skeptical about their use, so a member here sent me a few to try out. He advised me to size the boolit first, then put the check on and size again. IT WORKS! They look great, I haven't loaded any yet, or for that fact even shot any of the un-checked boolits for a reference point. These were for .357's.
I'll let him chime in here, then he can explain how the die works, and deal with the requests for some to try out!