John,
two issues here - G1 v G7 and manufacturer quoted BC v Bryan Litz's data based on actual testing over the best part of 1,000yd.
BCs comprise two elements: sectional density which is determined solely by bullet weight and calibre, so all 140gn 6.5s have the same SD, while the 139 Lapua is marginally down and the 142gn Sierra is marginally up. The other element is bullet shape and how much drag that generates. This is quantified by a 'form factor' that gives the drag in relation to the numerical value 1.000 which is generated by a 'standard projectile' shape. As this is drag related, the lower the value the better. BC is the ratio of form factor to SD, ie the higher the SD and the lower the form factor, the higher the BC.
The above applies to G1 and G7 BCs. The difference is the shape of the standard bullet. The G1 reference standard uses a projectile shape that looks like a 180gn .30 round-nose soft-point; the G7 uses a long and sharp-nosed boat-tail projectile that looks rather like a modern hollow-point match bullet.
So we get two different sets of BC values that are not directly related, that is you can't say that a BC of 0.5 automatically becomes 0.3 or anything, and you have to use appropriate ballistics programs or tables.
The 'problem' with G1 BCs is that they are very velocity dependant. This isn't a problem for short-range shooting, and G1 programs like Sierra Infinity don't give that much difference in remaining velocities, drop and wind drift at 600yd. But, it's a different story beyond that range and especially at 1,000. Anybody who shoots .308W at long range knows that there must be something not right with most sets of ballistics tables as bullets that are supposed to be comfortably supersonic at 1,000yd at a particular MV, often aren't. Hence, Fullbore / Target Rifle or F/TR shooters using 155s tell you to ensure a minimum MV of 2,950 fps, better still 3,000 fps or more for 1,000yd.
Sierra has partly got around this problem by quoting three, and for some recently introduced models, five BC values in velocity bands. Infinity applies those values selectively in its compuations. Most companies quote a single value, and they don't tell you at what velocity. Usually it's the highest the bullet will ever fly at because that gives the highest ('best') BC. The trouble with that is your 155gn thirty might not be given 3,200 fps in your rifle, and it loses speed as soon as it exits the barrel, so that value only applies for a few inches of flight anyway.
While the 'real' G1 BC value might vary by 15 or so percent along a 1,000yd flight as it slows, the G7 version is far less velocity affected and only varies by two or three percent. So, you can sensibly quote an average BC and it'll work at 1,000yd almost as well as at 500yd or 300yd. So, G7 BCs are much better for the long-range target shooter.
But .............. if the quoted BC values, of whatever type, are accurate and calculated in the same manner, their order in terms of efficiency should more or less apply to the other method too. That assumes there is a fair difference in those values as the relative ranking of two very similar designs might reverse as G7 compared to G1, say. But if the 139 Scenar's BC is outstandingly better than the others as a G1, it should be at or near the top of the pile as a G7 too.
This is where the other and equally useful bit of Bryan Litz's work comes in - the long-range testing. We don't know how the manufacturers calculate them. Some use short-range velocity loss tests, some use a computer generated BC based on the bullet shape and SD. Sierra's BCs are conservative compared to other makers as well as being banded, so the average of the several G1 BCs quoted for its new 0.308" Palma MK #2156 come out almost identical to Bryan's experimentally derived long-range G1 BC.
All Bryan's results are based on the same method using sectional flight times over equally spaced audio sensors that use the supersonic 'crack' to record the bullet passing over, and which measure the times (which translates to velocities) over a long range. He also quotes G1 average BCs and the 139gn Scenar comes out at an average of 0.557 compared to 0.588 for the 142gn SMK, and VLD values over 0.6 (.612 Berger and .639 JLK).
Anybody interested in this subject, or who shoots long-range should buy a copy of his book 'Applied Ballistics for Long Range Shooting'. It not only explains external ballistics better than any other book IMHO, but it is full of practical stuff as to how to make use of the information.
Laurie
