I've only crimped one time on some 223. And it was the most accurate round I ever made. I followed the instructions with the die set, that's all I know. Does it create more pressure (cfm) I don't think so? I believe that is generally the powder load. But I can see there being a small, if any changes in ignition. Once the prjo starts moving forward all give in brass should be normal. My thoughts on crimp was to prevent bullet movement from full auto percussion in the mag. Used for mill surp. It does not hurt but it's another step when steps count. I wish I had more time to reload.