Lapua brass has drilled flash holes they don't need de burred. I think you changed the angle of the flash hole thus giving a different ignition flash to the powder column.
Lupua is outstanding brass, no prep needed, just load and shoot. We all learn from our mistakes don't get bothered by it learn from it and keep chasing them little bitty groups.
If you want to play with brass prep get some winchester, rem, even federal lol or some mil spec and play with brass prep. Keep some as factory, and change one thing at a time. When deburring flash holes the weight of the tool should be enough to remove burr from cases with punched flash holes. It doesn't take much, and any altering to the depth, angle etc will diminish accuracy.
edit to add= mark or trash the brass that you altered so far with flash hole tool, and make the mark permanent. Can you imagine chasing fliers from a few of those dropped in a pile of brass ? A fellor would be cussing up a storm !!
--------------------------------------------------------------------------------
Last edited by Clint M; Yesterday at 11:01 PM.