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A cannelure on bullets for semi-autos is also there to keep the bullets from being jammed INTO the case during feeding. When or if the bullet hits the feed ramp, it can be jammed deeper into the case - a little or a lot. That sure upsets your precise "off the lands" distance, but it can also raise pressures.
The worst part is that you never know if it's happening. You can't inspect the round that's in the chamber unless you eject it and measure it. But then you have to feed it in again, and you can't tell AGAIN.
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