THE HANDLOADING BENCH
308 Winchester Rides Again Part 3 By Laurie Holland
200, 220 and 240gn MatchKing (SMK) models; the others date from the last five years or so, some only months old. Sticking with the geriatric Sierras, they’re short and blunt for their weights compared to their youthful rivals and have correspondingly high (poor) G7 ‘form factors’. The metric for this attribute is ‘i7’, a measure of the bullet shape’s ballistic efficiency compared to that of the G7 ‘standard’ or ‘reference bullet’ shape with its i7 value of 1.000. As this is a drag related metric, low values are better than high and we’re really looking for 1.000 or lower here. It is easier for the bullet designer to create a long streamlined shape in a heavy model than in a necessarily shorter 150 or 155 while retaining other essential design features and the overall balance. Despite this, our baseline 155.5gn Berger BT FULLBORE has an exceptionally low 0.988 value, so that’s the target we’re looking to equal or better with our heavy designs. The heaviest (240gn) of the old Sierra quartet comes in at a very poor 1.092, which allied to other issues and problems in using this design, has led me to exclude it from the comparison; the other three have relatively high i7 values exceeding unity, but have compensating features. (There is a fifth, modern and more efficient heavy SMK, the 210-grainer with a 1.000 i7 form factor value.)
Pros
But, to go back to first principles, why should we even consider using heavies in the cartridge? ...... and what penalties do we pay in using them? Pros and cons in other words. Assuming 155 and 210gn models group equally well in a given rifle specification, and assuming they’re equally well designed (same or similar i7 values), the heavier model moves less in the wind at long ranges despite starting out at a lower muzzle velocity. This is easily demonstrated as it happens that two bullets with very different weights share a 0.988 form factor – the aforementioned Berger 155.5gn BT and the same company’s 210gn BT Long- Range model. Let’s assume we drive the 155.5 at 3,000 fps. This produces 3,108 ft/lb of muzzle energy, and a load and barrel throat optimised for the 210 should be capable of producing the same amount
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What we’re trying to avoid by using heavy, high-BC bullets – a bullet blown to the right barely in the ‘Three-ring’ by a wind change. The competitor’s elevation isn’t too hot either being a full MOA high!
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