Re: Gain twist
Rafael,
You are correct about the idea of a high-gain twist reducing "pressure" being conceptually faulty. It does not have any measurable effect.
Augustus is on the right track, however, with a gas-seal concern. What he means by the bands "failing", is that the primary (single) rear band opens up a swaged gap on the idling side of the land/band interface. If this happens, hot gases will rapidly vaporize the copper band, and all sorts of problems are initiated from that point.
The objective is to maintain a seal, but with the minimum amount of friction. A 6.0 caliber 338 in a constant 9.0" twist <span style="font-style: italic"> </span> barely <span style="font-style: italic"> </span> meets this requirement. A longer projectile would fail to seal instantly, and the design paradox is that the longer projectiles (6.5 & 7.0 caliber) need an even tighter twist.
There are other potential, or real, advantages having to do with shooter comfort (which is established), and barrel harmonic amplitudes (which is not proven).
Best,
Noel