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Let me look, DB. I'm positive that it will be included in Ken Howell's new book, if it ever sees the light of day (he's very ill and he self-publishes his books...).
He and I have discussed this phenomenon at length, and we agree on it. The basics are well known.
ALL bullets obturate on firing. The pressure applied to the base of the bullet has to act against the bullet's inertia, plus the retarding effect of engaging the rifling. The result is a shortening of the bullet, accompanied by the resultant "fattening" effect.
This is normally a good thing. That's what seals the bore and keeps the powder gases behind the bullet.
The harder the bullet is pushed (i.e., the higher the initial pressure spike) the more obturation that occurs. In rifles with long (or worn) throats and rough or eroded rifling, the problem is worse.
So, with extreme pressure, the bullet can obturate several thousandths larger than bore diameter in the time it moves from the case neck until it hits the rifling. It then has to be swaged back down by the rifling. The energy that is required to do this can be extreme. Coupled with the drastically increased friction, and the net effect can be BOTH reduced velocity and much higher pressures.
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