Ken, as you can see, your observations match those of most others - I believe that this (the similar velocity regardless of case capacity/bullet weight) is closely related to effeciency: the relative powder burning rates and the expansion ratio provided by the thirty caliber bore diameter. The phenomena of certain velocities delivering the highest degree of precision has been known but, I'd suggest, not entirely understood for quite some time: as far back as I can recall, 3200 FPS (MV) was the "magic number" for the 22 Calibers.
This carried over to the 6mms - untill with newer powders, MVs in excess of 3300 FPS became obtainable. Some of this (possibly ALL of it, but it's over my head) is explainable via the harmonic theory; however, except by coincidence, harmonics do not explain why these velocity / precision 'windows' open within specific velocity ranges - REGARDLESS of barrel LENGTH!
My latest example is that with the same bullet, both my LV and HV thirty BR barrels (respectively, 21.75" and 24") deliver nearly the same velocity AND precision level via identical powder charge weight: 3025 and 3040 FPS via 34.8 Gr. of H-4198(Extreme)! I have discussed this topic at length with competitors over the last several years - as yet, I have heard no really good explanation . . but your observations have been common to many thirty caliber competitors. Often, for me, the most difficult aspect, especially where excessive pressure IS NOT and issue, has been to back off the throttle and return to that 2940-2980 FPS SWEET SPOT! I suspect that others also fall prey to the same desire for MORE SPEED! ;)
With regard to twist rates: the best potential for precision will be provided with a twist which imparts a gyroscopic stability factor(Sg) ranging between 1.4 and 1.9 - in theory, anything faster or slower is "leaving MONEY on the Table"! For example, a bullet based upon a .925" long jacket (weight between 1110 and 123 Gr.) exits the muzzle of a 1:17" twist BBL with a Sg of 1.9! Via the 1:18" twist, the 1.00" long bullets are blessed with a Sg right at 1.5! So, for the short bullets, the 17 &18 twist BBLs are "JUST RIGHT"! ;) Keep 'em ON the X! R.G.
