Excessive RPM -> Poor Accuracy?

[Black Jaque Janaviac]

w30wcf

Black Jaque Janaviac,
Information I have read on bullet spin decay indicates that it decreases much slower than the velocity of the bullet. If that were not true, there would not be any long range target shooting.

OK. That's what I was told. But that doesn't mesh with the bullet-only-turns-so-many-times per distance traveld idea. Just to simplify the equation let's hold spin decay as null.

If a rifle is pitched 1:12" the rifling extrapolated from muzzle to the 100 yard mark should make for 300 turns over the distance regardless of velocity, or so I understand your early post.

If a bullet starts out at 3000 fps @ muzzle the spin is 3000 RPS @ the muzzle. By the time the same bullet passes the 100 yard mark, the forward velocity has decayed to 2000 fps while the spin remains at 3000 RPS. If I average the speed at muzzle and at 100 yards to estimate the flight time I get an average velocity of 2500 fps and a flight time of 0.12 seconds.

3000fps+2000fps/2 = 2500fps
and
300ft/2500fps = 0.12 seconds

So if the bullet takes 0.12 seconds to cover 100 yards and it is spinning at 3000 RPS the # of spins it made on its way to the target is actually 360.

3000 Rev/sec * 0.12 sec = 360 Rev.

Is my math wrong? Or is there an error in my assumptions?

[leftiye]

BJ
You're right, boolits with poorer BCs will turn a few more times while reaching a given target because they slow down more rapidly. Also, the "Same number of turns to target regardless of speed" theory is wrong for the same reason - faster boolits slow down more.

[Larry Gibson]

RPM of a bullet is the rotational speed of the bullet. It is established by the twist of the barrel and the MUZZLE velocity. It has noting to do with how much the bullet slows down by 100 yards. It's rotational velocity is very near what it was as it left the barrel. It is pretty close to that at much longer range also. The rotational speed of the bullet does indeed decay very slowly.

Larry Gibson

[floodgate]

' "The rotational speed of the bullet does indeed decay very slowly." Larry Gibson'

Indeed: There are reports of high-angle (near 90*) antiaircraft shells coming back down base-first and still spinning as they sat in their impact craters.

floodgate

[Junior1942]

Hummmm. . . . I guess the slow rotational decay is due to Newton's Law--an object in motion tends to stay in motion.

[Scrounger]

And don't forget that gravity is messing with it too. The gravity rate is constant but as the projectile loses velocity, it has more time to work on it and to more effect.

[44man]

All of this makes my head spin. Too many questions nobody can answer.
What is the optimum spin to keep any individual boolit stable at all ranges?
What happens when a boolit is underspun and transitions back through the sound barrier?
Does an underspun boolit start to wobble when it gets way down range like the top?
What is the relationship between foreward velocity and spin effect as the boolit slows?
Exactly how much does spin slow over distance? There is no doubt at all that it WILL slow unless in a vacuum. Air resistance is a tough customer.
If started out with a slow spin and it loses just a little spin over distance, at what point does the boolit go unstable?
Does an underspun boolit lose spin faster then a boolit with the proper spin? (Because of instability, speed of sound transition or wobble?)
All of the math and arguing only shows how much spin a boolit has when it leaves the muzzle but not what the boolit needed to stay stable.
So put the math to work and tell me exactly what every weight boolit for the .44 magnum needs as far as spin and rate of twist to stay stable and accurate to 500 meters. The velocity for the equations can be the book max velocities. The math work needs to be proven with actual field results.
I will stick by my guns and say if you underspin a boolit in a cartridge that will NOT support an increase in velocity, all is lost.
If you have a faster twist, the velocity can be slowly reduced to find the stability. To shoot VERY long range it is best to overspin a boolit and put up with less accuracy at short range before the boolit/bullet goes to sleep.
To apply what a BR shooter needs for an underspun bullet at 100 yd's to keep out yaw will not apply at 300 to 500 yd's. They want the bullet to be asleep soon after the muzzle. If you spin the top real hard, it will walk around until the spin reaches the perfect rate, then when it slows, it will start to wobble all over the place. The fast spin has yaw until it goes to sleep. But not enough spin creates wobble. Bullets do the same thing.
So all of the math has not answered a single question! I don't care if a boolit spins 350 or 450 times to the target, I want to know WHAT the boolit ACTUALLY needs.
Tell me how to get a Marlin 1 in 38" twist to shoot better then my revolvers at 100 yd's! (No, I don't want to shoot 150 gr boolits!) The piece of crap needs Gyro-Jet bullets or fin stabilized bullets! My stinking arrows are more stable.

[Larry Gibson]

Hummmm. . . . I guess the slow rotational decay is due to Newton's Law--an object in motion tends to stay in motion.

That would be the phenomenon.

Larry Gibson

[Larry Gibson]

I was running some military, factory and handloads through two .308s and two 30-06s yesterday over the M43 yesterday. I had the down range (100 yards) screens set up so along with the chamber pressures I was measuring muzzle velocity, down range velocity, time of flight and the BC. The test of some M118 Special ball through the two .308s was interesting and related to this topic.

The two rifles were my M70 target with 12" twist 26" barrel and my M98 Palma rifle with 14" twist 27.6" barrel. The test consisted of 10 shot strings through each rifle. I will list the actual numbers with the M70 first then the M98, e.g.; (M70/M98). The average velocities were within 14 fps of each other (2644/2630); note the difference of 14 fps. The ES and SDs (12,42/25,84) equal to what is normally got with this lot of M118. The down range velocity ES and SD was still consistant (11,40 - actually less ES and SD down range) with the M70 but the SD and ES had dramatically increased (27,91) with the M98s down range velocity loss which was greater than the M70s 173,199). The BC for the M70 was .517 but the BC for the M98 was .448. The average peak psi(M43) was 49,600 for the M70 and 48,200 for the M98. Group size for each 10 shot string was 1.1" for the M70 and 1.2" for the M98.

So what does this tell us? If judging by the 100 yard groups I'd have to say both were equally as accurate. Note; I have shot better groups with this M118 out of the M98 vs the M70 on other days. Obviously the 174 gr M118 bullets were having stability issues with the 14" twist in the M98 vs the 12" twist of the M70. This is evidenced by the bullets higher velocity loss, greater ES/SD down range than at muzzle and much lower BC when fired in the 14" twist M98 vs the 12" twist M70.

Obviously this explains the noticeable accuracy difference at 300 yards between the two rifles with this ammunition (1.5 MOA for the M70 and 2.75 MOA for the M98). Thus if just judging "accuracy" from 100 yards groups we would have been wrong to assume both as accurate as each other. Moral to the story is; testing at the longest range we intend to shoot will tell us what the accuracy accually is. Testing at close range can easily lead us to false assumptions.

Larry Gibson

BTW; I've a new barrel coming with a 10" twist so when I get it assembeld onto an action I will begin the tests of cast bullets in 10, 12 and 14" twist barrels in .308 Winchester. All tests will include group size at 100 yards, chamber pressure, muzzle velocity, down range velocity, BC and computed RPM.

[Ricochet]

Does anyone have Dr. Mann's The Bullet's Flight? All of 44man's questions sound like the kinds of things he would've investigated.

[Black Jaque Janaviac]

I've seen wind drift tables, trajectory tables, KE tables, even programs that will calculate all this stuff out for you - all for free.

But what 44man is asking for, "exactly what every weight boolit for the .44 magnum needs as far as spin and rate of twist to stay stable and accurate to 500 meters" just doesn't exist.

Perhaps such a compilation could be made, but I'm not willing to do it for nothing.

[44man]

Very true and answers to most of my questions do not exist. That is the point! To dwell on how fast a boolit is spinning doesn't answer what each boolit needs as far as twist and velocity. We all know that the longer a boolit gets, the faster the twist has to be. There is no magic twist that will shoot all boolits.
That is why the .244 Rem got shot down by the .243. The .243 shot heavy bullets better and the .244 was made for light varmint bullets. The .244 is a better round but the twist was wrong for deer bullets. The 22-250 is harder to get long range accuracy then the .220 Swift because light bullets have to be used.
It is also why my 45-70 BFR with the 1 in 14" twist outshoots every rifle I ever shot in that caliber.
To just tell me what RPM's my boolits are going will not tell me if it is right or wrong.
Just about every factory uses too slow of a twist for a lot of calibers but on others they get it right.
For revolvers, the BFR's have the most perfect twist rates I have ever seen. Each is a perfect match for the caliber. They did not depend on some pencil pusher to determine what the guns need.
I have a list published long ago telling the best twist rates for a lot of calibers and almost all are wrong, some so far out you need a round ball.

[Larry Gibson]

44man

I'm not sure what you mean by; "To just tell me what RPM's my boolits are going will not tell me if it is right or wrong" but the RPM threshold is not about stability of the bullet. The RPM threshold is about where the RPM adversely effects the rotational stability of a cast bullet causing decreased accuracy. It is assumed that the twist is sufficient to stablize the bullet normally.

Larry Gibson

[44man]

I understand and did get off track a lot.
What I was thinking about was that if the maximum rate of spin is reached for a cast boolit and it starts to come apart, hasn't the accuracy point for that boolit been long exceeded?
I feel each boolit design when matched to the twist has a very short span for a sweet spot and to shoot under or over that gives the same result of poor accuracy at different ranges.
An under spun boolit is usually pretty accurate at close range.
Are we really sure we reach the stability limit of lead when a boolit is spun too fast? Or was yaw introduced so the boolit rotates around the flight path until it goes to sleep? That boolit might shoot tighter groups at 300 yd's then it does at 100.
I have done it, seen it happen and even watched boolits/bullets in a spotting scope.
Can we really say the spin rate is too fast when only one distance is shot?
Anyway, that was my point, no argument intended, just trying to get things clear in my feeble mind and understand the math.
I don't feel I want to push things to the point of having a lead cloud out in front of a gun!
Which brings me to a question sticking with me; What would the velocity limit be with a cast boolit from my 6.5 Swede that has a 1 in 8" twist? Reading everything here has me wondering, I don't have a mold yet and never tried it. Should I be scared off by the twist rate?
So you see, I have ulterior motives!

[Larry Gibson]

44man

"What I was thinking about was that if the maximum rate of spin is reached for a cast boolit and it starts to come apart, hasn't the accuracy point for that boolit been long exceeded?"

It does appear the accuracy is long gone. However I've got to say I have not had a cast bullet "come apart" in flight due to RPM, but then I've not tried to push them that fast. I know that a Hornady SX bullet with its soft lead core and thin jacket will spin apart somewhere above 180,000 RPM. So will some other jacketed varmint type bullets. Since cast bullet alloys are somewhat harder than lead they may not spin apart as easily. Again I don't know because I've not tried. Fact is accuracy is long gone before we'll spin one apart using our normal cast bullet alloys.


"I feel each boolit design when matched to the twist has a very short span for a sweet spot and to shoot under or over that gives the same result of poor accuracy at different ranges."

I think you may be correct for the most part but there are exceptions or at least cases where the sweet spot actually has a large span.

"An under spun boolit is usually pretty accurate at close range."

Exactly the point I was trying to get across with the test. The accuracy at 100 yards was comparable but the BC of the bullet (same box of M118) fired in the 14" twist was noticeably lower than the bullets fired in the 12" twist. This is a very good indication of stability problems with the bullet. There is a noticeable accuracy difference at 300 yards as pointed out.

"Are we really sure we reach the stability limit of lead when a boolit is spun too fast? Or was yaw introduced so the boolit rotates around the flight path until it goes to sleep? That boolit might shoot tighter groups at 300 yd's then it does at 100. I have done it, seen it happen and even watched boolits/bullets in a spotting scope."

I to have seen that but not with enough frequency to bet on it. Most often a bullet that is inaccurate at short range because of stability problems will be even more inaccurate at longer ranges.


"Can we really say the spin rate is too fast when only one distance is shot?"

No we can't.

"Anyway, that was my point, no argument intended, just trying to get things clear in my feeble mind and understand the math. "

Point well taken.

"I don't feel I want to push things to the point of having a lead cloud out in front of a gun! "

Me neither, I always noticed that when an SX, Blitz or other varmint bullet blew up out in front of the rifle the target walked away unscathed! Not exactly what I had in mind....

"Which brings me to a question sticking with me; What would the velocity limit be with a cast boolit from my 6.5 Swede that has a 1 in 8" twist? Reading everything here has me wondering, I don't have a mold yet and never tried it. Should I be scared off by the twist rate?"

I don't let the twist of the Swede 6.5 scare me off. You just need to understand what it is doing RPM wise is all. I have played with several Swedes and several different cast bullets. I'm of the opinion that the Lyman (Lovern) 266455 is the best regular cast bullet design for the military chamber/throats. Accuracy with most cast bullets will be best in the 1400-1500 fp. With a bullet that fits the throat as well as 266455 does and with some tweeking you can boost this up into the 1700 fps range. Again the swede makes for a pretty decent cast bullet shooter if you understand the effect RPM wil have on the bullet. 45 2.1 and others claim higher velocity with accuracy but most of us haven't seen it, even when using their loading techniques. Who knows, maybe you succede where most of us haven't.

"So you see, I have ulterior motives! "

It has been said here that trying cast bullets in the Swede can make a man drink! I drink anyways so it doesn't matter to me. Cheers!

Larry Gibson

[44man]

Thanks Larry, I have to look at that boolit. I have been putting it off because my barrel is pitted some but shoots great with jacketed. I don't shoot it as much as I would like because of the price of bullets.
And I can fall back on the bottle too!

[Black Jaque Janaviac]

It has been said here that trying cast bullets in the Swede can make a man drink!

Well then . . . all the more reason to give it a try!!!

Part of the problem with trying to speak with any wisdom regarding the affects of spin on a cast bullet is that I know of no one who has done testing that controls for the effects of internal ballistics.

So often "too much twist" means stripping the rifling, and it really has nothing to do with the external ballistics of the bullet in flight.

One of my favorite guns is an Early Virginia flintlock with a 42" barrel. The long barrel really doesn't seem to set me back at all when in the deer woods. So I've often wondered what could be done if a guy played with a long 40+ inch barrel and slower smokeless powders. Could you get better cast bullet velocities if you accelerated the bullet over 40 inches instead of 22 inches?

[44man]

That sounds a lot more sane then shooting 150 gr's of powder from a 24" barrel with an inline!

[leftiye]

B.J. It would fer sure lower exit pressures at the muzzle. Might make high velicities with lower pressures too. Problem is that slower powders don't burn well at low pressures. Maybe we should outlaw using the expression RPM on this board? Label it a swear word.

[Black Jaque Janaviac]

RPM is better suited to vinyl discs and ... maybe ... the .45-70. But the bullets I shoot are best measured in RPS!