See these kinds of posts fairly often and wonder why you're not building a PPC. Having said that, I would build it with a 13.5 or 14 twist and a .040 or a .045 throat 1.5 degree. Your idea is pretty much old school looking at the whole combination. Might help you to go to the Super Shoot or a registered SR match and look at what you'd be competing against and build something like they're shooting, it won't be a BR and most of the guys will be shooting BT bullets. I've watched guys try to keep a BR in tune on a hot day that were pretty good hands with a PPC and they were never succesful. Unless you're the second coming of Tony Boyer don't try this.
Looking to shoot 64-70gr fb bullets in 100-300yd benchrest. Based on stability calculators, it looks like I should be using 1:12, but so many shoot 1:14. What do I need? Why? Is the Berger stability calculator wrong? Are best groups below stable?
See these kinds of posts fairly often and wonder why you're not building a PPC.
Looking to shoot 64-70gr fb bullets in 100-300yd benchrest. Based on stability calculators, it looks like I should be using 1:12, but so many shoot 1:14. What do I need? Why? Is the Berger stability calculator wrong? Are best groups below stable?
IMHO, go 8 twist, zero freebore and start out with the lightest bullets you can get your hands on. The throat will go away faster than you think so with the zero freebore, it's easier to go to a heavier bullet and still get closer to the lands.
Nothing worse than finding your load with the bullet you chose and after a short time, that load won't shoot anymore cause the throat has moved and you can't get close to the lands. Plan ahead.
60-70 grain 6mm bullet??Don't even try it!!
8 twist is even too fast for ANY 6mm bullet.
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Well, I’ve found a Hart 1:14 that I couldn’t pass up.
I'm going to sidestep the BR vs PPC stuff and get right to the twist part of your question.
It's very important to look at bullet jacket length rather than weight when you decide on twist rate. 6mm bullets for 100-300 BR are normally done on the .825 or the .790 long jackets...the finished weight is barely relevant to the twist requirements in this scenario.
.825 jackets: 1:13
.790 jackets: 1:14
My experience with very good PPC's and 6BR's is that as you get closer to 'just stabilized', things show up on target that look like tune related issues when in fact, they aren't. I believe that a bit of extra stabilization (to a point, obviously) can help with non-tune-related 'on target' issues, as well as giving making the tune up not as tempermental as the day goes on or from day to day.
Ducking my head for the flying bricks up here..... -Al