THE HANDLOADING BENCH
308 Winchester Rides Again Part 4 By Laurie Holland
Anyway, moving on, let’s look at loading ‘heavies’ in the 308 Winchester but – there’s always a BUT, isn’t there – we must address the related issues of rifling twist rate and chamber throat configuration too.
Everybody knows a ‘faster’ (shorter- pitch) rifling twist rate is needed to stabilise heavy bullets but many will be surprised by how relatively ‘slow’ the required rates are for all but the longest of bullet designs. (Table 1).
Note I say ‘longest’, not ‘heaviest’ as it’s the bullet length in relation to weight and calibre that determines how fast it must be rotated for stable flight – a heavy but short bullet actually needs a slightly ‘slower’ twist than a same length lighter model when fired at equivalent velocities (that produce an identical muzzle energy value).
For example, the relatively short-for-weight 190 and 200gn Sierra MatchKings’ requirements bracket the 1-12 rate, not 1-10 as most people assume. I’ll go further - by noting that until the exceptionally long 215 and 230gn Berger Hybrids appeared, 1-11 was as ‘fast’ as you’d need for everything other than the 240gn Sierra and this is the preferred twist for many Match Rifle competitors who load Berger and Sierra 210gn VLD-form bullets for 1200-yard shooting.
The five ‘heavies’ mainly tried in the 2010/11 barrel. Left to right: 155gn Sierra Palma MK (for comparison); 200gn SMK; 208gn Hornady A-Max; 210gn SMK, 210gn Berger BT; 210gn Berger VLD.
I’ve used the Miller’s Twist Rule spreadsheet allied to the bullet lengths as measured by Bryan Litz for his excellent book Applied Ballistics for Long Range Shooting 2nd edition to calculate the twist needed to produce a Stability Coefficient (Sg) value of 1.4 at the MVs I’d worked out for the different bullet weights (Table 1).
Although the normally recommended minimum value, 1.4 is actually ‘belt and braces’ and a lower value normally suffices for our shooting conditions in the British Isles, so a half or even full inch slower twist rate is usually fine. The benchmark 155.5gn Berger BT shoots very happily in a 1-13 twist barrel for instance (Sg = 1.29, higher still in thin warm summer temperature air, while any value above 1.00 theoretically produces stability).
Even so, many readers will look at the optimal twist values for the first two bullets in the table and reckon I’ve got something wrong. The 155.5 Berger and 190gn Sierra surely merit a much greater differentiation in their rates? I’ve checked the calculations and that’s what the formula says – it’s thanks to their mix of calibre, weight, and length: the Berger is exceptionally long for its 155gn at 1.25 inches; the SMK adds just over a tenth of an inch length (8%) over the Berger’s 1.25 inches while increasing the weight proportionately more by 34.5gn (22%). So, if you have a rifle with a 1-12 twist barrel, you can likely shoot much heavier models than you’d supposed.
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