I hear comments on this forum all the time about slower rifling twists for cast bullets... Let's call them 'Borderline Twists'... It is an old philosophy with cast bullets, and many 'old masters', even Harry Pope, believed this way.
We have quite a few 'new masters' on this forum. I say this honestly and truthfully and I would ask all readers to please not let my direct nature of communication to come across the wrong way..... A certain 'opinion' come up here all the time it seems. An opinion I disagree with... An opinion, that for years, has proven opposite for me; many of my experiences supporting this! Truthfully my goal here is to find out if I have been wrong all these years. My goal is to find out WHY my findings are different. A discussion of why your opinion may be different than my findings......
The 'opinion' I would like to discuss usually reads something like this:
"If 300 yards is the maximum distance you intend to shoot this bullet and twist, and the boolit remains point on and accurate at that distance then your twist is fast enough."
Usually, if not always, the load/twist combination has been already shot at a closer distance, say like 100 yards.
My first question: "Why is stability less at 300 yards than at 100 yards?"
Perhaps the best way to get answers is to write down how I see it and why. Then I'll let your expertise show me if, where, when I went wrong.
In the 40's I got an old trapdoor Springfield. Black powder was fairly uncommon at that time period with few using it. But my father had me loading 70grs of Hercules Fg behind Lyman's #457125. Boy, they kicked! At 50 or 100 yards the bullet holes were oval. Not keyholes... just oval. I learned to call this yaw... The barrel was decent and the twist is 1 in 22" I believe.. I figured if they were bad at 100 yards, they must really get bad out at 300, 500 yards. So I asked my father...
"Nope..... they'll get better way out there. The bullet has time 'to go to sleep'."
I would not forget his comment 'go to sleep'....... But what is it?????
A current thread talks about 120gr RCBS gaschecked boolits in the .250 Savage with poor results.... So let me choose the ol' .250 Savage in a hypothetical situation if I may....
We have an older .250-3000 with a 1 in 14" twist. We have a Lyman #257325 bullet mold that casts a .25-35 round nose boolit at about 112grs. We finally work up a load at 2000fps muzzle velocity that shoots OK, but the bullet holes display 'yaw' at the 100 yard target.... Wow! I wonder what would happen at 300 yards? I wonder what would happen at 530 yards???
Why 530 yards? I hear someone saying....... I'll answer that. The retaining velocity at 530 yards is 1000fps or 50% of our muzzle velocity.
RPM's..... Several here talk a lot about boolit RPM's... Let me put a little different 'spin' (sorry) on RPM......
If we calculate 2000fps in a 14" twist I get 102,857 RPM at the muzzle....
If we calculate 1000fps in a 14" twist I get 51,429 RPM...
Is this the correct RPM at 530 yards for our example load above then?
The boolit in our example load above leaves the muzzle of our rifle 'locked in' at departure turning one 360 degree revolution in 14" of travel.
OK... A tougher question.... At 530 yards does our bullet still travel 14" to make one full (360 degree) revolution?
I believe most believe this twist distance is always 'locked' in as it is at the moment the boolit leaves the rifling....
I say NO!!!! I'll say that we experience a downrange phenomena I'll coin a 'compressed helix'.!!!! I'll say that rotational spin and forward travel become independent of each other at departure from the muzzle; each having their own set of variables!!...
Noted Ballisticians have made comments that gyroscopic or rotational speed slows down much less than our boolit's forward travel fighting air resistance head-on. Some Ballisticians say RPM slows very little... I agree with this but cannot prove it. But in our hypothetical example above lets pick a generous amount of rotational, or RPM lost... Let's say we lose 20% of our RPM over 530 yards or more easily looked at we have 80% of our original RPM that we had at the muzzle. That figure is 82,286 RPM. I think it could be higher... we are only talking 1.175 second of flight time here.
So if you allow me the 82,286 RPM figure and we calculate this against our 530 yard retaining velocity of 1000fps to get a theoretical 'twist' for these numbers the twist would then be the equivalent of 1 turn in 8 7/8" for our boolit!!!!!
This is 'going to sleep' !...... This is a 'compressed helix'.....
I say a borderline stabilized at departure boolit (balanced - with no flaws) becomes more stable, not less stable, at longer ranges!
Where have I gone wrong gentlemen?
Eutectic