0.2435 inches or even a fraction more. What is my Border barrel? I haven’t a clue, not being even aware of this issue six years ago when it was fitted but it gives slightly better results with the ‘fatter’ Berger VLDs than Scenars .........
So, twist rates? You can have anything you fancy these days if you’re prepared to wait a while for a less popular choice to be produced by the barrel shop. By ‘anything’ I mean twists from 1-7 or 7.5, needed for super-long 115gn weight projectiles out to a really slow 1-14 or 1-15 if you’re only going to load stubby varmint or short-range match bullets in the 55-75gn weight bracket.
faster twist rate lets them use bullets that extend the rifle’s effective range to 600 yards or more.
The twist rate is only one part of the mix though, the other being throat configuration, or more precisely the amount of ‘freebore’ built in, which in turn determines the bullet’s position in the case. It’s no good having a 1-7 twist that will stabilise the 1.365 inch long Berger 115gn VLD if the barrel has been chambered with a super short throat that requires a 2.265 inch COAL. The base of the seated bullet will be deep inside the case – much closer to the case-head than the neck and far too deep for optimal performance or accuracy. (In fact, it’s unlikely the case would grip the bullet, the 0.243 inch diameter shank being pushed below the bottom of the neck!.)
This is an extreme example of course but the 6mm BR user often has a choice of a ‘compromise’ chamber or one optimised for a particular bullet that sees its base located above the ‘case-body to shoulder’ junction, ideally at the base of the neck. Well, this applies to any cartridge in a custom rifle, you’ll say – and you would be right. However, the range of 6mm bullets and the 6mmBR design’s short body with its limited powder capacity make the choices starker to some extent.
Heavier match bullets that need a 1-9” or 1-8” twist barrel. Left to right: 95gn Berger VLD, 105gn A-Max,
105gn Lapua Silver Scenar, 107gn Sierra MK.
The CIP spec for BR Norma is 1-8, this needed to stabilise 105-108gn match bullets and this twist rate is also just enough for the 115gn Tubb/Sierra DTAC – if you can drive ‘em fast enough, although designer David Tubb advises 1-7.5. The point here as always is that going for a bit faster twist than the planned bullet(s) need will give you more flexibility should you subsequently decide to load something heavier. For instance, a 1-10 twist barrel lets you shoot 87 and 90gn VLDs and will still perform superbly with 55gn varmint bullets, while choosing a 1-14 means you are irrevocably stuck with short, light pills.
But ... surely, 1-14 is the optimal twist for wringing the smallest groups out of flat-base 62-68gn benchrest quality bullets? Yes, and if you’re going to compete in 100/200 yard benchrest matches, the marginal reduction in group sizes may be vital in a sport where the margin between top performers and also-rans is measured in thousandths of an inch but I doubt if even ‘picky’ 300 yard prone shooters or same distance ‘varmint hunters’ would notice any difference and a
100yd groups shot by the author with the 70gn Sierra MK from a 1-8” twist ‘Norma chamber’ rifle.
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