Borderline Stabilization... at long range

[Eutectic]

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

[Good Cheer]

For what it's worth.
My grandfather was trained as a sniper on Long Island prior to going to France in 1918. The German officers acting as instructors explained to him that the bullets scribed an ever changing increasingly larger spiral through the air on their way down range. And, that this was why the bullet drop was in part negated by rise at long range. The energy it takes to create that curve ball type alteration of trajectory must have been lost from the slug as the rifling marks interacted with the atmosphere; i.e., the spin rpm must have decreased...some.
I do not remember how far they were shooting.

[Fixxah]

Interesting read. I can buy the whole theory.

[leftiye]

Depends on how severe the instability is.

Thas (opening spiral) because of air resistance acting against an unstable boolit as the bullet spins. Uneven air resistance against the canted (yawing) boolit planes the boolit and as it rotates this becomes a spiral.

Deflected boolits as in when they have hit a branch fly in an exaggerated cone, spiraling around the outside of the cone.

Uneven air resistance against the canted bullet causes some bullets to stabilize over a (short) time, causing them to go to "sleep". This is with better stabilized bullets than the previous scenarios. Comparable to a top when it settles down and becomes stable. In this case the wobbling of the boolit is mild enough that it "scrubs off."

[NSP64]

Makes sense to me.

[44man]

Long range needs MORE spin to start with and the bullet will rotate around the flight path until it slows enough to "go to sleep." A slow twist is more stable at close range but will lose stability at long range.
I always like a faster twist because you can reduce the load for close shooting but you can't load fast enough for long range with a slow twist.
Faster twists are easier to work with.
I only stray from this with muzzle loaders with a round ball.
Twist is so important for the distance you shoot, it is hard to explain.
A 1 in 22" twist for a .45 is WAY too slow and will never go to sleep. It never spins enough to start with.

[fecmech]

Not in the same class as what you are talking about in the .25 but my .357 Rossi lever gun has a 1 in 30 twist. Shooting the Lee 158 rnfp at 1100 fps I can make 10 shot 50 yd groups of 1 to 1.5" with all bullets hitting point on,no ovals and they will not group inside of 3 feet at 100 yds! Any that manage to hit the target backer are going anything but point on. IMO this is a case of stability at 50 yds and no stability at 100.

[Good Cheer]

Oh wow. Had one of those Rossi levers in '80. It was a tack driver at 100 with Lymans 358156 and just enough powder to coast it in. Total killing machine for head lighting jack rabbits across pastures. But I don't have a clue what the twist was.

[GabbyM]

Right now this evening I think a bullet that is stable will settle down to go to sleep and stay that way. Bullets that are marginal will never go to sleep and in fact open up there spiral. I've never seen a 55gr SBT from a 12 twist 223 go to sleep.

One thing I do know is a M-193 ball round from a 1-12” twist barrel never goes to sleep. They do open up groups at long range and they do it suddenly before you get to a quarter mile. Depending, I've assumed, upon air temperature and pressure. Even on a warm day at five thousand feet they don't make it to a quarter mile before you all of a sudden can't stay on a three inch spotter or prairie dog. This goes for any of the 55 grain spitzer boat tails I've shot. Long or short barrels.

Can't explain the physics behind M-193 going nuts all of a sudden. Only parallel I have is my machine shop. When running tooling it either runs like a charm or when it decides to chatter they go nuts. Not much in between. Chatter marks on the work don't get larger they just stay the same horrible mess. Unless the tool point breaks off. That's mostly harmonic vibration.

[trk]

Eutectic --

What you write is consistant with Dr. Franklin Mann's writings (1920's) in The Bullet's Flight.

Well worth the reading. (If you don't want to buy it, get your library to get it for you - ineter library loan.)

[mrbill2]

I was at the range yesterday and shot this load out of my 308 with a1-12 twist. The bullet
was a Lyman 311334, 200 gr. cast from Lyman #2 and water cooled. The powder charge
was 20/A 5744, velocity I would guess around 1650 - 1700 fps. let's say. Light winds. I was
getting 5 shot groups from 1.250 to 1.5 at 100yds. I was happy happy. So I thought lets
shoot this puppy at 200 and see how things pan out. I realize 200 is not long range. Well to
say things went south would be a under statement. Groups were 6" or larger.
Two Questions. What made the groups become so large ? What can I do to get this bullet to
group better at 200 ?

[Ford SD]

I was at the range yesterday and shot this load out of my 308 with a1-12 twist. The bullet
was a Lyman 311334, 200 gr. cast from Lyman #2 and water cooled. The powder charge
was 20/A 5744, velocity I would guess around 1650 - 1700 fps. let's say. Light winds. I was
getting 5 shot groups from 1.250 to 1.5 at 100yds. I was happy happy. So I thought lets
shoot this puppy at 200 and see how things pan out. I realize 200 is not long range. Well to
say things went south would be a under statement. Groups were 6" or larger.
Two Questions. What made the groups become so large ? What can I do to get this bullet to
group better at 200 ?

not a expert but givein the number of people on this site you will get several excelent answers to your problem

your bullet has a bc of .340 length App 1.200
your vel at 200y is 1250-1300 +- and i think is border line stable (at 200 yds) only good bullet shape is keeping it from going in to the target side ways
i think you need a higher velocity even a extra 50- 100fps would help and may decrese group at 200

still good shooting for a first try at 200 yd seen a lot worse

[GabbyM]

I think what Ford SD is saying is your bullet may have begun the dreaded transition to sub sonic flight.
Most bullets do a little dance as they transition to sub sonic. Maybe all bullets?

[Doc Highwall]

mrbill2, good shooting. I am shooting something similar you can check out my post under reloading equipment titled Modifying Dies For Cast Bullet Shooting. Things get magnified when the shooting distance gets longer such as what was the wind doing at the firing line vs. at the target also what kind of rest are you using, do you have the parallax adjusted correctly if the scope even has it. They are even more like where you rest the rifle on the rest and do you place it the same shot to shot.

[Shiloh]

I think what Ford SD is saying is your bullet may have begun the dreaded transition to sub sonic flight.
Most bullets do a little dance as they transition to sub sonic. Maybe all bullets?

I buy the subsonic transition that will cause a wobble. Then there are the grooves in the boolit to add to the complication. What about trying it with a paper patched boolit to eliminate some of the rifling variable??

SHiloh

[Larry Gibson]

Eutectic is essentially correct except that his down range loss of rotation is exaggerated as over 530 yards the rotational speed decays very little. The rotational speed (RPM) does aversely affect the flight of the bullet as mentioned by the "spiraling" effect. The bullet is traveling point on BTW. At some point the velocity becomes low enough that the centrifugal force on the imbalances of the bullet cause complete loss of stability. That loss of stability can happen in a short distance or it can happen at very long range depending on the bullets suitability for the RPM.

Let's take a look at Palma shooters for an excellent example. The Palma match is shot at 800, 900 and 1000 yards in strings of 15 shots at each range. Originally when the Palma matches were restarted in the 50's the requirement was for the host nation to provide their service rifles and 7.62 NATO spec ammunition with a 145 - 155 gr fmj service fmj bullet. After a match or two the rules changed because the "home Team" always had the advantage of using their own rifles. The 1st change was to allow the teams to bring their own service rifles in 7.62 NATO and the Host nation still provided the ammo. That didn't work out well enough so they allowed "match rifles" to be used. It was quickly found that the service grade ammo didn't shoot well in 10" and 11" twists. It shot better at the longer ranges in 12" twist. It also shot better with longer barreled 12" twist.

It was during this period of matches and practices that it was realized that the increased velocity from the longer barrels was keeping the bullets (145-155 gr service FMJBTs do not have a really high BC) transonic to 1000 yards. It was also realized the RPM of the twists was causing a large accuracy reducing spiral at lng range with the imbalanced service grade NATO spec bullets (we all know milsurp bullets are not as well made, read that as “balanced”, as commercial bullets and are not as accurate because of this). Thus the slow twist barrels developed. It became standard to use 13 or 14” twist 27.5 – 32” barrels for Palma matches. This was to shoot the issue 7.62 NATO spec ammunition with the most accuracy.


The rules of Palma Matches again were changed to allow the teams to supply their own ammunition and reloaded ammunition was allowed with match grade bullets. Then the rules were again changed to allow any cartridge/caliber as long as the bullet weighed in at the NATO spec of 145 – 155 gr. Thus we see the 6.5 caliber pretty much rules the matches today. Many of us lament the rule changes to suit the high power shooters. The rules should have at least stuck with service type rifles and the current or past NATO cartridges. Today it has lost its “romance” and is now just another long range belly match with single shot rifles.

It all developed that way because slowing down the RPM and using a longer barrel to increase velocity was the way to get the best accuracy with the imbalanced bullets and velocity of the issue service grade ammunition. The same principles apply to cast bullets but they apply earlier because of the greater imbalances in cast bullets. However, the more balanced we can cast the bullet, load it and keep it balanced through acceleration the more accurate it will be. I have promoted the RPM threshold concept here for some time but that concept did not originated with me. It originated with numerous cast bullets shooters and their writing many years ago as the lessons from the Palma rifle development was applied to cast bullets. The 1st time I read about was when a fellow asked;

Why is it I can load a 311359 to 40,000 psi in a .30 carbine and get excellent accuracy and yet when I load the same bullet to the same psi in a ’06 the accuracy is non existent?

He proceeded to answer the question by explaining that the real difference in accuracy was not based on psi but simply that the .30 carbine had a 18” twist and the ’06 had a 10” twist. It was the increased RPM of the ’06 load that caused the inaccuracy. I have conducted many experiments with varied twists and have found that to be the case. Most all of you have also found that to be the case but many do not realize it yet.

But the point here is that with cast bullets in particular we must control the RPM for best accuracy with a specific bullet. That is done, other things being equal, by adjusting the velocity. If we want to get into the realm of accuracy at high velocity then we must understand the adverse affect that RPM has on the imbalances of cast bullets, particularly above a certain level of RPM. The easiest way to consistent accuracy at high velocity with cast bullets is to use as slow a twist as will initially stabilize the chosen bullet at the expected velocity. The yaw and pitch will smooth out and the bullet will “go to sleep” at longer range with better accuracy because the RPM increases relative to the bullets loss of velocity at longer ranges as Eutectic mentions.

In the case of the 250 with the 10” twist the long RCBS 120 gr cast bullet as it was cast and loaded was being pushed to too high a velocity/RPM and inaccuracy resulted.

Larry Gibson

[sagacious]

My first question: "Why is stability less at 300 yards than at 100 yards?"

I am slightly hesitant to join in, but let's see how it goes. You may wish to define your interpretation of 'stability' to move forward with that question. There are several different ways in which a bullet can be 'stable.' I suspect that you mean gyroscopic stability along the course of the trajectory.

A bullet may be more stable, or less stable, at longer ranges dependent on a host of variables.

I would not forget his comment 'go to sleep'....... But what is it????

Hard to know what was meant exactly, as it relates to what was happening physically, but probably a diminishment of yaw relative to trajectory.

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.

No, the second rpm figure is not correct for 530yds. The rpm at muzzle is not "locked in", it's simply the rate with which it begins. Calculating rpm based on at-distance velocity is not accurate. Rpm and forward velocity are two properties that do not diminish in sync, as easch is subject to distinctly different forces.

Note that there seems to be a slight confusion of terms: Rate-of-twist (rot), rate-of-spin (ros), and rpm are not synonymous. Unlike rpm, rot does indeed start as an invariant factor. Rpm will vary as a function of initial velocity and decrease with time-of-flight, and ros will decrease as angular momentum decreases. Rot does not change, because rot is simply the rifling pitch rate, and that can't change unless the gun is rebarrelled. Rot = ros at muzzle, but not at distance. This fact is at the very heart of your question.

OK... A tougher question.... At 530 yards does our bullet still travel 14" to make one full (360 degree) revolution?

No, it travels slightly further than 14" to complete a full revolution, because the ros at distance has slowed from the rot at muzzle.

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!!!!!

If the rpm slowed by 20%, then the bullet would have to travel 20% further to complete a revolution, not 20% shorter. Also, taking for granted your rpm figure, still, the ros calculation at 530yds is not correct. Your math shows the bullet revolving faster than 1:14" at distance. Where did it's ros get the energy to increase from it's initial rot? Of course, the ros cannot speed up unless energy is being added. Rot is always >or= ros.

Just hoping to clarify, and respond to the questions given. Hope this discussion stays friendly. Good luck, and thanks for posting your thoughts. Good stuff!

Regards, and good shooting!

[44man]

Much good stuff here. It always comes down to the bullet or boolit, the length and weight when looking for the proper twist.
Been a long time from my long range varmint days but the .220 Swift was always better with heavier bullets then the .22-250 because of the faster twist. That made the Swift much better at long ranges to well beyond 600 yards. But the Swift sucked at close range because of over spin so I did not bother to sight in anywhere except 350 yards.
That means you need to also look at the distance you want to shoot.
Now that most of my work is with revolvers, I found the faster twist rates of BFR revolvers has made it so easy to find accuracy to 500 meters while making tiny groups at 50 yards. My love of the longer barrels also figures into it because velocity achieved is important for stability. I argue time and time again that a 4" barrel with the same TR as a 7-1/2" is not as stable because you can't match the velocity unless you use fast powders that ruin boolits. The spin from a short barrel is less then from a longer barrel UNLESS you match velocity. I feel that the shorter the barrel, the faster the twist needs to be so it matches the velocity.
I see it with a friends Marlin .44 that has the same twist of 1 in 38" as mine. My barrel is short. He gets better accuracy with his longer barrel then I can with my short barrel so I need more powder and pressure.
Going to sleep is when an over spun bullet loses both spin and velocity so it settles into smoother flight. A boolit spun too slow will not go to sleep but will get unstable at long range from lack of spin and the transition from supersonic to subsonic will affect it much more. Spun correctly and the transition has almost no affect. Keep the boolit or ball subsonic from the muzzle out and stability can still be good and so can keeping the bullet supersonic to the target.
Ask me what twist your rifle needs and you will get a blank expression because I don't know the bullet, the velocity or the distance.

[Eutectic]

Eutectic --

What you write is consistant with Dr. Franklin Mann's writings (1920's) in The Bullet's Flight.

Well worth the reading. (If you don't want to buy it, get your library to get it for you - ineter library loan.)

trk,

Thanks for the info. I believe my good friend has this Mann book. I will borrow it while this discussion is fresh!

Eutectic

[mrbill2]

" mrbill2, good shooting. I am shooting something similar you can check out my post under reloading equipment titled Modifying Dies For Cast Bullet Shooting. Things get magnified when the shooting distance gets longer such as what was the wind doing at the firing line vs. at the target also what kind of rest are you using, do you have the parallax adjusted correctly if the scope even has it. They are even more like where you rest the rifle on the rest and do you place it the same shot to shot. "

I'm using a Caldwell rifle rest with a position stop and a Weaver T36 with parallax adjusted. I think I'm doing everything right as far as shooting the gun I just need to follow the instructions from Ford SD and Gabby M and speed the bullet up. I'll give that a try next time I get some loaded.

There is one thing that I could never understand with this RPM stuff. Why the math formular includes x 60 to convert the answer to RPM's. It would seem the me that RPS (per second) would be more appropriate as no bullet has a flight time anywhere near 1 minute. But I'll leave that stuff up to the experts.

Thanks to all for the advice.