“A 250gr .338 bullet is just not long enough to maintain stability of those distances. As you said, once it drops below the sound barrier in velocity, it wobbles off course.”
You apparently failed to read my entire post. The Lapua 250-gr lockbase bullet is unperturbed by transonic transition. Corporal of Horse Craig Harrison’s 2707-yard shots — using the 250-gr lockbase — were ~300 fps below the speed of sound on impact. And he was using an identical rifle to the one Vickers and Wilkinson were shooting in the linked article. So, no, the 250-grain bullet IS NOT too short for transonic stability.
And according to Bryan Litz, length is not controlling of transonic stability. Litz (of Berger Bullets) is (was) an honest-to-goodness rocket scientist, formerly a rocket design engineer for the US Air Force.
I don’t know where he ranks among the world’s living ballisticians but I will tell you without fear of contradiction that he’s the best one on the planet who routinely schmoozes with us common schlubs on the Internet. He maintains a presence in several shooting forums and gladly answers any reasonably intelligent question from the peanut gallery. And thank god he does because he manifestly has increased the body of fact-based ballistics knowledge available to Joe Sixpack. His first book, Applied Ballistics for Long Range Shooting, belongs in every serious shooter’s bookcase. Best of all, it’s written so simply that a reasonably intelligent sixth-grader could understand it.
Anyway, Litz’s research shows that boattail length and angle are the key factors in transonic stability. Not bullet length. Which isn’t to say bullet length and BC aren’t connected, which they obviously are. But that’s a separate issue from transonic stability.
FWIW I checked the specs on the Accuracy International AX in .338 Lapua (same rifle as the British Army’s L115A3) and the factory barrel has a 1:11 twist. Berger and Sierra both recommend at least a 1:10 for their 300-gr .338 bullets. So the mystery of why Vickers and Wilkinson only were shooting 250-grain bullets is solved. Stability from the 1:11 barrel they were shooting would have been shaky at best.
Thank you for the edifying comments.
I now understand why they limited themselves to 250gr.
Of course the 2707yrd shots may have reached their target supersonic if he could have used the Nosler 300gr with a .72 BC.
And yes, those long bullets need to spin.
Kreiger will cut you a barrel of any weight, caliber or twist rate and then cryogenically treat it for temperature stability at first shot.