As to the 6.5x284 of which I have owned quite a few, I have seen more variation in barrel bore/throating characteristics effect on velocity then I have with a 2" difference in barrel length. I have never had an issue getting velocities in the 3000FPS range using 140 class bullets with my 24" 6.5x284's. Additionally, a 24" barrel will get a complete burn with powders as slow as R26/Retumbo. I would certainly not be neurotic about a 2" difference in barrel length.