This comparison, of two, “real” bullets – that is, yes, I have made both – is posted for your perusal – no further comment from me for a day or, two . . . well, except for the following explanation! 😉 Oh, this blog will go on for a while – I’ll date each update.
Both are .264 caliber, and of equal length, flat point (.260″ me’plat diameter), of nearly cylindrical construction (almost no ogive/nose, or, in other words, “wad-cutter” style), flat base bullets, but of differing weights. The right column features a bullet which is, “full” of lead, while the left column is filled to the level of a conventionally pointed bullet, but left as a hollow-point wad-cutter: both bullets feature identical geometry – only the mass is different.
Note the far right hand column (bold, black) SG, which is the specific gravity of construction: 7.45 vs. 10.32. These were arrived at by making the actual bullet weights/lengths.
The data displayed in red are self evident: the last figure 4 (below ballistic coefficient), is the twist rate recommended to achieve a 1.5 gyroscopic stability factor at sea-level and standard atmosphere, times 1.018 – this is my, “pulled from a dark place” safety-net, or, denser than normal air. . .
Bold, far right column: BD=bullet diameter; BL=bullet length; MD=me’plat diameter; BTL= boat-tail length; BTA=boat-tail angle; ED= end diameter; QR= ogive type (tangent/secant); RC= ogive (nose) radius; SG=specific garvity of construction; V= muzzle velocity; AD= air density.
Below is a link to a recent & related thread on the AccurateShooter.com/6mmBR.com forum:
1/10/18 – So, what’s the point?
Contrary to common belief, for equal stability, the heavier bullet, of the same geometry, requires less spin – in this comparison, fully 1.0″ slower twist rate! There are several reasons: the dominant being that, for a given form, mass makes for a [more] centrally located center-of-gravity (CG), shortening the arm between the CG and the center-of-pressure, thus reducing the overturning overturning moment. Even for conventional nose designs, this will hold true! Forget everything you ever believed about heavier bullets needing faster twist rates – they may, or, they may not: stability is far more dependent upon length than mass. Selecting twist rate, based upon bullet weight, can result in poor performance, as this example demonstrates: most people would presume less twist requirement for the lighter bullet.
1/11/18 – Before continuing, I want to point out my reason for initiating this diatribe: merely, to help assure that unsuspecting, first-time buyers of custom barrels do not mistakenly opt for too slow a twist rate for their anticipated pride-and-joy”! This is especially important since the advent of the polymer tipped bullets! Being much longer than their traditional siblings, of equal weight, the polymer tipped bullet, across the caliber spectrum, from the various mass producers, always require almost 2″ faster twist rate than the, “old style” (hollow-point/soft-point) bullet offered by the same manufacturer.
While I do not stand to profit from this (I have no ties to any of the mass producers of bullets, nor do I offer polymer-tipped projectiles), I feel that I can assist people in selecting a barrel/twist combination which will optimize their enjoyment – indeed, possibly prevent disappointment. Surely, we can all recall our first venture into the expectation of precision! Further, for those of us in the advanced class, often, there is, either, “money left on the table”, or, as in the Clint Eastwood/ Lee Marvin musical, PAINT YOUR WAGON (ooops there goes my age giveaway), “[gold] dust under the floor“! There for the taking! 😉
1/12/18 Note: before proceeding, I want to make clear – I am neither exterior ballistics professional, nor mathematician, but, rather a self educated layman, who has dedicated a fair amount of time and energy in real-world exploration/conformation of hypotheses of those who are. or, were (as in, “The Late Mr.?). The works of Robert McCoy, William C. Davis Jr., and Harold Vaughn have all opened doors, which have provided me with advantage(s) over my fellow bench-rest competitors. How else could I have garnered yardage and Grand Aggregate victories (collectively, totaling nine) @ NBRSA National Championship [score shooting] events? 😉 The preceding GURUS made some pretty good cookies . . . now, the fun can begin! 😉