Somewhere in one of Bryan's books, he says that really accurate spin rate (hence rifling twist pitch) is a very complex task indeed involving multiple computations and difficult even today with our access to electronic computers. This is still done though fully by military ballisticians for artillery shells where the issues ande their importance step up several gears.
All advised twist rates are based on the Millers Rules formula, and as Bryan explains it takes many shortcuts over the full procedures, but is generally good enough and allows a simple calculation to be done on EXcel instantly.
However, as Steve says we're now seeing bullet shapes on models on general sale that look remarkably similar to those I remember seeing in photographs of prototype ultra low-drag artillery projectiles of some 30 or so years ago. We have moved a long way on from the original VLD rifle bullets that Walt Berger starting producing maybe 25 years ago. Just maybe - I don't know - these sooooper long designs start to need some of the rules rewritten.
In case you doubt this and/or think it some sort of attack on Bryan Litz's designs and work, I'd strongly recommend you go out and buy
Modern Advancements in Long Range Shooting Volume I by one Bryan Litz and published by Applied Ballistics LLC.
The first 5 chapters are:
1 Twist Rate and Muzzle Velocity
2 Stability and Drag - Supersonic
3 Stability and Drag - Transonic
4 Spin Rate Decay
5 Effect of Twist Rate on Precision
All five, some 82 pages and many range tests, graphs, and photographs are basically about getting twist / spin rates right for the job and what compromises may be involved. Ch. 3 looks at transonic debastilisation and increasing twist / spin rates with both theory and range-tested practice done with two relatively short .308 bullets - the elderly 175gn SMK and newer Berger 175gn Tactical OTM, the latter designed by Bryan specifically for short-barrel military semi-autos to retain maximum transonic speed stability. They both benefit in 900 yard plus transonic flight from a move to an 8-twist rifled barrel. Yes that's right 8-twist for something we'd reckon that 12 is optimal for.
There is an excellent set of four photographs of powder fouling edged holes in paper from long-range (977 yards estimated to be close to 1.0 MACH) hits, two with 12 twist, two with 8, showing how the actual hole left in the paper is randomly offset for the slower spun bullets and centralised in those from the 8-twist barrel. In the text and supporting material, this is due to the development of the Yaw Limit Cycle in bullet flight and Bryan says that there is disagreement amongst ballisticians as to the speeds it begins to show with estimates ranging from a high of 1,800 fps down to 1,100 which is what we mere mortals who shoot for recreation tend to believe.
The bullets Bryan uses in this stabilisation chapters are the 185 Juggernaut and these 175s and when we start to talk about Yaw Limit Cycle problems developing for them at 1,000 yards in 10-twist rifled barrels, then you really don't want your new surface to air missile-shaped 3,000gn FTR bullet that is one helluva lot longer than this trio to get anywhere near trans velocities.
Talking this over with one of the UK's top F/TR shooters who was our 2016 F/TR league champion just before Christmas, he'd given a recently introduced super-long / pointy 308 heavy a try in one GB F-Class Assoc league round in his championship year, and that was his one (relatively) poor result. This may have been in the GB FCA league's annual 'Long-Range' F-Class match at Bisley with 1,000, 1,100 and 1,200 yard stages, but I don't remember now. This bullet - I won't name it, but not a Berger - had performed superbly in mid-range club match try-outs but something went wrong with it beyond 800 yards and he returned to the 200gn Berger Hybrid pretty sharpish and his winning ways resumed. He'd wondered too if it was a dynamic stability issue in trans speed zones with a 10-twist barrel.
There is a great big experiment going on right now - funded mostly by the users - re not just heavy for calibre bullets (that was the last generation), but soooper, soooper heavy. I'm struck by how much excitement there is around for such and such a bullet that has a stratospheric BC and people queue up to get ever faster twist barrels, then a year or two later you never hear anymore of anybody using the 'wonder-bullet' and the key long-range matches are still being won with those that are a couple of tenths of inches shorter and 15gn or 20gn lighter. The 2017 F-Class European Championships matches won by Paul Hill with the short, blunt, and low BC (all in relative terms) 7mm 180gn Lapua Scenar L shook a lot of our F-Open shooters to the core, and they really are hoping it was a one-off. BUT .... F/TR shooters have actually gone further recently with super-long / heavy projectiles despite being much more likely to approach transonic speeds in long-range matches. (Well, that was before Sierra introduced its 110gn 6mm, 150gn 6.5 etc - we can watch BR and F-Open competitors make them work ... or maybe not!)
Let's just say we're in very interesting times in LR ballistics and shooting and I for one am keeping my hands firmly in my pockets and letting others splash out and show what works or doesn't in real life matches on real world ranges. I still think there is a very hard truth out there though that Dave Tooley was unkind enough to raise publicly in post #47 above
Stan,
Everyone is looking for a material solution to overcome a training problem.
DaveTooley, Sunday at 2:29 PM Report
#47 Unlike Reply
If only we could buy success in a box of bullets! (However, nobody is going to win any 1,000 yard matches these days with old designs like the 175gn Sierra MK 0.308, fine tolerant design though it is. It has to be about getting the balance right!)