I have banged my head against the wall with this issue as well. In one regard the bullet can be pressing back into the case increasing pressure OR the crimp can increase neck tension and raise pressure. I called Sierra and Redding to ask about this issue and they said yes, crimping can cause an increase in pressure, but it's consistent. The problem with not crimping is that not every round will press back into the case. Depending on how the tip strikes that feed ramp. They said this is a less consistent variable compared to crimping. I don't put a real heavy crimp on mine.