50Driver, a couple of questions:

What action are you using?

Single shot or magazine?

Your numbers look comparable for the .257 and 6.5, bullet weights heavy for the .257, normal for the 6.5. What twists for the 150/160 in the .257AM? I always wanted a 140 gr bullet for the 1/4 bore, but 160 must be looking like a torpedo.

Bullet weights for all three are essentially the same, don't quite follow that. Why not take the same path with the 6.5 and well, and why the .270? I'd be looking at the 7mm just for a little more spread on calibers. Both the 6.5 and 7mm have a large supply of bullets available in the marketplace that are suitable for the velocities you project. The .257 does too, but I'm guessing you will need a fast twist for yours and you might be rolling the dice a bit with the .257 bore though I sincerely wish you the best with that aspect of the endeavor. It is the red headed step child of modern ballistics.

You may be aware that Warren Jensen of Lost River Ballistics claims to use a different alloy for the brass of his .408 Chey-Tac. He says it is sufficiently strong and durable for 70 kpsi, or maybe 75, forget which.



Sounds like a leap, good luck to you.