You skipped the whole chapter on understanding pressure. The more you seat the bullet, the more pressure you add without adding powder.
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True IF you are talking about using the same projectile!
projectile weight, point style, material the projectile is made from all determines if shortening a round increases pressure.
what does increase pressure is reducing the case capacity, but If using a lighter say all copper round then using the same OAL will increase pressure because the projectile sits in the case deeper, (by a lot usually) even though it is a lighter weight.
same with if using a SWC and switch to a regular point projectile the pressure will drop a good bit as more of the projectile is outside the case and case capacity goes up.
this is what one needs to be concerned with is actual case capacity once the projectile is seated, and get away from only looking at projectile weight as to OAL.