I have to disagree with some of the comments.
I have found no evidence of distortion whatsoever with my PB dies. Sizing a PB check is effortless, and again does not distort. Results from many of my customers have shown far less leading, and increased accuracy with smaller groups in identical side by side boolit testing.
If you attempt to use to thick of a material for PB checks, boolit alloy that is tempered to hard, and sizing a large diameter reduction then yes- you will have some problems.
1. Use inexpensive AL or copper .004 to .008 gas check material that is not tempered to hard.
2. Size and check your bullets within the same day as casting if you are running harder alloys. I have had no problems with WW air cooled alloy, no matter how old.
3. Don't try to size less that .002 under cast diameter all at once.
4. Lube your boolits with Ballistol, or boolit sizing (case) lube before sizing.
5. Make sure the entry lip on the dies have a smooth transition.