CTanM,
Will re-visit my thought process for the Redline, as it looks like you're asking some if the same question I wanted to answer, myself...
I have a 1:8 22-243AI that'll push a 75 about 100fps less than your 22-6mmAI. If you reference an ROM calculator, you'll see that, even at 3600, a 75 is spinning 324,000rpm. At your 3,700, it's at 333,000 rpm. I.e. That's ALOT to ask a bullet to endure and still fly/perform...
You speculation about dropping to a 1:10rpm launch a 75AMAX, I fear will not work. Given my AO near sea level, and the velocity capability of the REDLINE, even a 1:9 didn't work for the 75AMAX in this cold, dense air. Perhaps, an "out west" shooter in thinner, mountain air could spin a 75AMAX with a 1:9 REDLINE, or even a .22-6mmAI, but not here in my AO. That said, dropping even more twist to a 1:10 in hopes of stabilizing that same bullet might very well prove to be a fool's errand? Least I know it'd be so, for my AO...
As for Mr. Majestic' .22-284, IIRC it's a 1:8 with a 30" barrel pushing an 80 AMAX @ 3750. And via, RL-33. Please correct if inaccurate?
That is some SERIOUS performance! Mr. Majestic's setup was actually one of the 'measuring sticks' I've kept in mind to use against the REDLINE. Dunno how fast his .22-284 can push a 75, posssibly 3900+? I wouldn't doubt it, from a 30" barrel, but there again the RPM 'dilemma' factors back in...to the tune of 351,000 RPM...YIKES!
So, if the REDLINE can cover that, from a 1:9, with 3" LESS barrel length, then you can see how pleased I am with how my little baby is shaking out!
Please know, I'm not 'dissing' Mr. Maj, his .22-284, or any other cartridge. Just explaining that it isn't any wonder that a more capacious case pushes the same bullet, faster. It's the same logic that a 6.5SAUM outperforms a 6.5-284, a .300 Norma beats a .300 WinMag, a .17-204 beats a .17Remington, .22-250 beats a .223, etc.
Simply stated, given the constants of bullet and twist rate, the larger case always wins. I didn't make that up, it's just how it is. All I did when dreaming up my REDLINE was to apply that logic to the .224 caliber, and did it using components I already had. That said, I really don't give a 'floppy-flop' about who did what, when and why, in the past...other than to use that information as a gauge to compare against...
Back to twist/RPM. At a scorching 3950 fps (already established), a 75 from the 1:9 REDLINE is spinnin' 316,000 RPM, which is well below where my .22-243AI is churning from a 1:8. So, I have a good idea that the bullet is 'safe' at that RPM...
HOW IT REACTS ON GAME REMAINS TO BE SEEN! But, we will all have some indication of that whence the first groundhogs meet their demise...
LASTLY...
I did think about a .240 REDLINE, and already have reamer specs to make it. But, the reasons why I dropped to .224 are basically twofold:
1. .224 is PLENTY enough to kill coyotes, and quite capable on deer. This, I've proven myself, to my satisfaction...as "Any centerfire" is big game legal in my AO
2. G1 b.c. of a .224 caliber 75 JLK is .425
....G1 b.c. of a .243 caliber 75VMAX is .330
Given the same launch speed, it is common knowledge that the smaller caliber, longer bullet, with the higher b.c. is going to shoot flatter, buck wind better, and retain more energy downrange.
FACT.
Here's pic of a 75 JLK / .220 REDLINE, next to a .243 CATBIRD / 77 Lapua:
-----------------~71grains case capacity------------~70grains case capacity
Guess which one is faster???
Guess which one has the .425 b.c., and which the .315 b.c. ???
That's about as close I can get to comparing a .220 REDLINE to a .240 REDLINE, without actually building it.
Does anyone else have something tangible/ comparable to reference?
If so...BY ALL MEANS, PLEASE SHARE!!!
