More powder,same velocity

[Rocky Raab]

Nope, I'm not talking about those occasional extreme pressure events, such as happen with very reduced charges of very slow powder.

I was referring to the high point of the pressure curve, which I perhaps should have called peak pressure instead of "spike".

That peak can occur before the bullet even clears the case neck, and that's what "hammers" the base of the bullet, causing what you might call over-obturation. All the obturation occurs while the bullet is in the leade or freebore area, and then it is almost immediately swaged back down by the bore and rifling.

Indeed, the rear of the bullet can be still expanding while the front part is being swaged down, depending on the exact geometry of the throat/leade. Once it's fully in the rifled part of the barrel, it cannot expand any more, obviously.

Interesting stuff, huh?