Twist rate to projectile length (to caliber) ratio?

[Flophound]

I'm very much new to the game here and coming from a muzzleloader background here where a slower twist is optimal for round balls and a faster one for conicals. Now that I'm looking into spitzer bullets and more modern rifles (and thinking back on my time in the service) it seems that the pattern is that the longer the projectile is in general (or perhaps relative to its diameter) the faster the twist rate is. I'm vaguely aware of the terms sectional density and ballistic coefficient, but is there any set formula that gives ballpark determinations for dimensions of projectile:optimal twist rate?
I appreciate the input. You all have been a great fount of friendly knowledge so far. Happy I made an account and no longer just lurk!
Cheers,
Flop

[MUSTANG]

Flophound; in answer to your question


Miller Twist Rate Formula:


https://en.wikipedia.org/wiki/Miller_twist_rule

Greenhill Formula for Optimal Rifling Twist Rate:


https://www.vcalc.com/wiki/AndrewBudd/Greenhill+Formula+for+Optimal+Rifling+Twist+Rate


and


Berger Twist Rate Stability Calculator:


https://bergerbullets.com/twist-rate-calculator/


other sites out there too.

[cstrickland]

I use jm ballistics calculators for a lot of different things. Here is the one for stability. It is based on the miller if I am not mistaken. Just plug in the variables a get results. Not 100% but gives you a great starting point

https://www.jbmballistics.com/cgi-bin/jbmstab-5.1.cgi

[popper]

You are correct, rotational inertia is mass, rotational velocity and radii

[mac1911]

For me I figured out from trial and error for rifles with faster twist you need to take a look at bullet RPM. IIRC larry gibbson touched on this often enough for me to see it first hand.
90% of my cast loads are driven through surplus mil rifles.
Between 1:9 and 1:12
My accuracy falls appart at velocities over 1900 fps with the longest heaviest bullets I shoot shine in that 1300-1700 fps range.

[mdi]

I'm not sure about a "formula" for determining bullet vs twist stats. I believe there are some but for this guy I'm sure they would be pretty complex with many variations that would need to be considered (bullet construction, bearing surface, velocities, type/size of rifling, etc.). I have just gone to the charts available in my reloading manuals...

[Flophound]

Thanks, gents; it's greatly appreciated. Lots of variables to test/tinker with. Fun times lie ahead!

[charlie b]

I'm not sure about a "formula" for determining bullet vs twist stats. I believe there are some but for this guy I'm sure they would be pretty complex with many variations that would need to be considered (bullet construction, bearing surface, velocities, type/size of rifling, etc.). I have just gone to the charts available in my reloading manuals...

Problem is the charts in reloading manuals aren't much better. It is a complex equation and if you want it to be exact it needs specifics about bullet, rifle, velocity and atmospheric conditions at time of shooting.

Fortunately most folks do not ever venture into the world of heavy for caliber bullets. Those of us who do study and test it carefully.

[waksupi]

I'm very much new to the game here and coming from a muzzleloader background here where a slower twist is optimal for round balls and a faster one for conicals. Now that I'm looking into spitzer bullets and more modern rifles (and thinking back on my time in the service) it seems that the pattern is that the longer the projectile is in general (or perhaps relative to its diameter) the faster the twist rate is. I'm vaguely aware of the terms sectional density and ballistic coefficient, but is there any set formula that gives ballpark determinations for dimensions of projectile:optimal twist rate?
I appreciate the input. You all have been a great fount of friendly knowledge so far. Happy I made an account and no longer just lurk!
Cheers,
Flop

I would suggest if you are starting in on cast, forget spitzer point bullets. Okay for fur hunters, but for hunting they don't make the grade. You want a flat nosed bullet. I always recommend a Loverin type for easiest success.

[fcvan]

Boolits are funny things when it comes to rotation. I wish I had a copy of a super 8 movie a friend of my Dad's named Jim had made. Due to the angle of flight and the position of the sun, you could track the boolits in flight. The fun thing was the boolits flew in a spiral pattern which was tight from the muzzle, and opened up at about 50 yards, then closed up at 100 yards. Dad was shooting a 4" 41 magnum and the spiral appeared to be about 3" in diameter at that 50 yards. I few tweaks to the load and it stopped that nonsense. Dad would shoot long range with a shooting strap around his neck and wrapped around the pistol grip. I watched him pile up a running coyote across a friend's farm at 250 yards. First shot was behind, next shot was front shoulder. Dad was a shooter, still was the last time we went to the range.

I think RPMs had something to do with the spiral, and I remember Dad and Jim talking about the rate of twist and the boolit weight and finding the 'sweet spot.' Boolits was from a SAECO Keith Style mold that was supposed to be 220 grains but dropped at 230.

That Keith boolit barely fits in the cylinder of my M57 but was a tad long for the Marlin 1894C lever gun. I switched to a SAECO 22 TC mold and never had a problem feeding. Brother loaded some of those Keith style for me, not knowing about the Marlin. Wow, what a pain. That made my Dad laugh watching me turn the Marlin upside down and wiggle it so the cartridge would finally come out.

My brother just pooped them into his 10" Contender barrel and snickered 'they shoot fine for me.' They do for me as well, I just have to seat them a little deeper and crimp above the crimp groove. I can't shoot them hot in the S&W or they back out, hence the 220 TC mold. The Marlin loves the TC. Oh, and .5 grain more of Unique spun the heavy Keith boolit a little faster and took out the spiral. Loads I shot in my 8 3/8" M57 with Hogue grips were a bit warm for my Dad's 4" with the factory wood grips. Dad liked his a tad milder, which also reduced the spiral.

[popper]

Yes, some call it going to sleep at distance. Rotational inertia takes some time to effect the path, increasing fps increases inertia so the effect goes away. Then you get to a fps where inertia is very large and any imbalance changes the path or the alloy isn't strong enough to hold the bullet together. Poof.