Left-

Identifying

combinations

may seem impossible given the potential combinations of weights, shapes and MVs. This quartet represents four very different options. Left to right: 155gn Lapua Scenar (mild secant ogive); 175gn Sierra MK (traditional form match bullet with 7-calibre radius tangent ogive); 175gn Berger VLD (aggressive

secant

ogive); 210gn Sierra MK (not only a heavy- weight, but Sierra’s only full-blown VLD design with aggressive secant ogive nose section).

for that matter higher, than the exact ME equivalent to an existing load. Naturally, the rifling twist rate has to

be suitable too.

Right, we have a chronographed load and likely MVs for other bullet weights, so how does weight affect BC, again stressing all other things are equal? To find this out we need to look at the BC’s constituent parts, and find it is the ratio of SD (sectional density) to ‘form factor’, that is divide the former by the latter and you get the BC. That tells us not a lot, so let’s dig further. SD is the ratio of bullet calibre (diameter) and weight, specifically weight (measured in pounds) divided by the square of the diameter in inches. With 7,000gn to 1lb, a 155gn bullet weighs 0.022142 lbs and a 210gn example 0.0300 lbs. Both are 0.3080” diameter, square this figure and you get 0.094864. Divide our weights by this figure and we get SD values of 0.233 and 0.316 respectively – note that any such weight thirty calibre bullets have these SDs whether shaped like ICBMs or a brick-built public conveniences. The aerodynamic efficiency bit of the equation

Left - Berger’s two 0.308” 185s, BT Long- Range on the left with long tangent nose and VLD on the right with a 14.7-calibres radius secant ogive nose on the right. The tangent BT L-R model has the marginally higher G7 BC at 0.283 v 0.281, and is much easier to ‘tune’ in handloads for most rifles.

Target Shooter 53

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