SW said:
Randy,
What twist do you believe would be required to stabilize the 285 Spitfire? I certainly didn't give an adequate test but do believe it must be something faster than 36. The speed range I think will be in the 2330-2375'/sec range(I get a consistent 2335'/sec with 300SST/70 2015).
How about a good, old-fashioned but honest I really don't know? I really don't know.
What I am comfortable with saying is that there are only two primary factors in stabilizing a bullet: rate of twist and muzzle velocity. That's about all there is in producing revolutions per second.
We know that a .45 cal 1:20 rate of twist barrel stabilizes 495 gr. conicals well, deriving the RPS more from twist rate than from MV which is typically 1300 fps or so (defined in Doc White's book, by Sir James Whitworth, and others.)
We know that spire points are harder to stabilize than more blunt ogives, we know that boat tails also require more RPS to stabilize than flat bases, all well documented by the Aberdeen Proving Grounds testing via Hatcher, Whelen, et al.
As published by Barnes bullets, a 1:28 may not be adequate to stabilize a 300 gr. MZ-Expander-- though that observation came from BP / Pyrodex loadings, not from Triple 7 or three pellet loads. With adequate velocity, 1:28 is not a problem.
A "gentler" powder does not make a great deal of sense except for heat of explosions values. I can't imagine acceleration from zero to 2300 fps in under 1.4 milliseconds being "gentle" no matter how it accomplished.
It seems that a 1:28 certainly has done the job for good 200 - 300 yard accuracy with the 285 gr. Spitfire; Knight has well-documented that with their own testing.
Whether 1:30, 1:32, might be the optimal 2300 fps rate of twist, I don't know. That would be a guess, but again-- I don't know.
A looser rate of twist may be a bit easier on sabots, but how much easier as a percentage? Interesting, but again I don't know.
I have to give credence to MMP's generalization that 1:22 is about the limit for reliable sabot function, and even a 1:22 is too tight if moving at high velocity.
Savage's 1:24 is quite good for 300 gr. MZ-Expanders at "moderate" velocities, 2050 fps or so as can be expected from 5744. It has done the same for most .45-70 bullets from 300 to 350 gr. as well.
But, if I were to go anywhere with it-- I agree it would be a looser twist of perhaps 1:26 - 1:30 plenty for low velocity 250 gr. XTP's, yet enough spin for those who want use 300 grainers with some giddy-up and go.
A long way to go to say I don't know.
I'll guess 1:30 will do it, but that is just a SWAG.