If the Matrix bullets share the same shape as the Berger VLDs could the difference between the 168g and 180g BCs be extrapolated to the 190g Matrix bullets. BC difference between 168/180g, 0.73, add that to the 180g Berger = 0.779. I guess pretty close to the 0.801 listed BC for the bullet. [Kyreloader]
BC is the ratio of Form Factor (a measure of bullet shape efficiency and as it's drag based, the lower the figure the better) and SD. AS SD is a ratio of calibre and weight, it's determined solely by the bulletr's weight in the calibre, so any 7mm 168 is 0.2976, and any 190 is 0.3365.
To get the G1 based form factor or factors (i1) that Matrix is using, we divide the two bullets' SD values by the claimed BCs and we get a single i1 value for both bullet weights of 0.417, ie they are deemed to have equally effcient forms or shapes. That is a little suspicious in that same basic design but different weight (hence length) bullets should have similar but not identical i1 values. Bryan Litz comes up with 0.483 and 0.486 for the 168 and 180gn Berger VLDs. Anyway, let's not nitpick, the important thing is that Matrix is apparently using an identical front and rear-end design on its two 7mm VLDs and varyies the length of the bit in the middle to change the weight - fairly common practice when you have an inherently good shape that works but you want different lengths/weights.
So the key issue is that 0.417 i1 value that is the key determinant of the BC for any given 7mm bullet weight. Is it reasonable?
Here's Bryan L's
average i1 values for similar designs from other companies:
Berger 168/180 - 0.483/486
Sierra 168 MK (tangent ogive design) - 0.528
Sierra 175 MK (secant ogive design) - 0.486
Hornady 162 A-Max (secant ogive) - 0.480
JLK 180 VLD (secant ogive) - 0.495
Looking through Bryan's book, I cannot find an i1 value for any bullet in any calibre that comes anywhere near the 0.417 that the Matrix bullets would need to obtain those claimed g1 BCs. There is nothing below 0.480 in 6.5 and 7mm, and looking at 6mm and .308 takes to you to values above 0.500.
To be fair, i1 form factors and hence g1 BCs vary significantly throughout their flights according to their velocity - which is why the G1 form is a poor measure for long-range shooters - and we don't know what particular velocity Matrix has used in what is most probably a computer generated BC. Bryan's i1 and G1 BC values are
averages for a 1,000yd flight starting at around 3,000 fps MV.
In terms of the bullet nose shape (its length and shape as determined by the radius in calibres), the Matrix design is close to the 7mm 175gn Sierra MK so it would be reasonable to assume that the 168's BC is a bit lower than that bullet's, while the 190gn will be a fair bit higher. In either event, having looked at the figures again, I don't believe the claimed BCs, not unless the company has reinvented the laws of aerodynamics!
Laurie,
Yortk, England
