Ball rotation speed

[missionary5155]

Good morning I have been "front stuffing" since I ETS´ed back in 1974.. my first was a.45 kit... and today well there are more than I can carry. My long time hunter is a Navy Arms Zouave .58 with about a 1-66 twist that launches a .57 RB fantasticly. I have a .54 flinter with a 1-48 twist that does all I can hold out to 100 yards with .53 RB. I have slower twist flinters also...
Some years ago I started experimenting with 12 bore RB smoothbore for LARGE crocadile control (slugs would bounce off the HEAVY neck armor on some Amazon tributaries). Smothbore 12 bore RB is great up close and accuracy out to about 40yd OK for regular hunting.

So the question is... What do you think is the fastest usable (sensible?) twist for high speed RB (1-30mix up to WW at 1500fps) ? Most 12 bore rifled barrels are 1-28 or so.... and only 20 inches long. I would like a 1-70 or slower barrel to work with but there are not going to be any soon. Can you really over spin a large (.685) RB to the detriment of accuracy ?

[mooman76]

The problem is the faster the twist, the slower you are going to have to go because the ball is not going to follow the rifling if it is pushed too fast and accuracy will suffer, especially with those big bores. The bigger the ball, the less twist you need., this I am sure you already know. A really tight patch will help but only to a certain degree.

[northmn]

Twist rate is also related to bore size. A 1-48 inch twist in a 54 is faster than a 1-48 in a 40 due to the angle of rifling. The 12 ga. riled barrels are likely designed to stabilize a longer sabot. As a general rule the slower twists with roundball are less fussy or handle a wider range of powder charges than a faster one. About the time you state that someone will tell you how their 1-48 54 shoots 1 inch groups at 100 yards with 140 grains of powder. The factors that go into stabilizing a round ball are as mentioned, the patching, but also depth of rifling. Some of the production rifles with 1-48 inch twists are also shallower rifled as a comprimise between slugs and ball. Ideally I would think a 12 ga. would want a 1-60 or slower with a round ball, but that is a S.W.A.G. Heavy loads in 12 ga guns hurt.

Northmn

[longbow]

I recently shot some 0.735" round balls through a Remington 870 with 1:38 twist. Accuracy was pretty good ~ results are posted in Casting for Shotguns.

I didn't chronograph the load so don't have a measured velocity but Precision Rifle listed 36 grs. of blue Dot under a 610 gr. slug at 1260 FPS so this RB should be at about 1300+ FPS.

I recovered one RB and it showed good clear rifling ~ no sign of stripping. I kind of expected stripping or really poor accuracy as Greenhills says 1:110 for 0.73" RB. While I agree that a slower twist is proably better this did seem to work okay.

Longbow

[missionary5155]

My Thanks to each of you !
I can see I need to study up on rifling , twist shape and depth for large size RB. There certainly are a bunch of factors involved to consider... But as was stated at least with RB there is not much need for alot of vicious spinning to inpart.

[waksupi]

Lt. James Forsyth, in his book, "The Sporting Rifle and It's Projectiles, recommended a 1-120 twist for large bore rifles. The slower the twist, the more powder is required for good performance. But, the ball will stabilize better at longer ranges.
I built a .62 flinter about 30 years ago, with a 1-72 twist in it. I had to put 170 gr. of 2FF in before it began to perform well. As this was built as a mountain rifle, weighing only 7 1/2#, it killed at both ends. Traded it for a banjo, and carried on wide spread torture with it, until some kindly soul gave me a M29 S&W, and $600 for it. All lived happily ever after.

[missionary5155]

Thanks Waksupi.. How long was the barrel on the .62 ? And do you think barrel length would affect the stabalizing if it was lets say 6 inches less or 12 inches less ? I understand length affects velocity but does it affect rotation ? Or is just 6 inches of rifling enough ? Were the parodox guns just rifled out at the end portion ?..

[waksupi]

I believe my old one was around 24", but it has been a lot of years since I've seen that rifle. Length of barrel shouldn't affect rotation. I can't answer your question on paradox rifles, but believe your description is correct.

[Digital Dan]

M5155, Round balls do not require spin for gyroscopic stability, but benefit from rotation for distribution of aerodynamic loads in a consistent manner. That is aeroballistic fact. It's a fairly easy concept to understand....think of a knuckle ball versus one that spins. Some will suggest that bullets from fast twist guns will kill better and that is terminal ballistic superstition.

Typical and accepted twist rates of 100-120 times caliber have been used successfully for centuries. I know of a target shooter from many years back that used a 113" twist in a .50 caliber target rifle to good effect. The Gentlemen had built like bore barrels with a range of 48-113" twist and found the only great difference between them was powder granulation preference. The slower twisted barrels preferred faster powder, but not for reasons of stability. On my own part I have a 66" twisted .45 caliber barrel that shoots just fine and that's somewhere around a 1:146 ratio, using a .440 ball w/.015" patch and 50 gr. of Goex fffg. Fine to me looks like this at 50 yards when I'm on my hind legs:

http://img.photobucket.com/albums/v298/muddler/107.jpg

The high shot on the left target was 100% my fault, not the twist of the barrel.:mrgreen:

As mentioned above, the problem with round ball guns and fast twists might be found in their minimal bearing surface and propensity for stripping under heavy loads. As a matter of opinion, I think most factors affecting accuracy in RB rifles are found in details not related to twist rates. On the matter of the Greenhill formula, it is not and never was pertinent to round ball twist rates. The intended purpose of the formula was to provide for gyroscopic stability in conical bullets and that is all.

[Rattus58]

On the matter of the Greenhill formula, it is not and never was pertinent to round ball twist rates. The intended purpose of the formula was to provide for gyroscopic stability in conical bullets and that is all.

Not quite true... I don't even remember now the name of the gentleman/ballistician I called who was discussing greenhill and asked a bunch of questions about the formula and what he told me makes sense TO ME.

If speeds are around 1500 to 1800 or less, then use 120 as your multiplier of the square of diameter.

Bullet diameter 0.72
Bullet lenth 0.72
Speed = <1500 120
Twist 86.4

If you're in the typical firearms range of 1500 to 2800 then use 150 as your multiplier.

Bullet diameter 0.72
Bullet lenth 0.72
Speed 1500-2800 150
Twist 108

If you are shooting hypervelocity bullets, then use 180 above 2800.

Bullet diameter 0.72
Bullet lenth 0.72
Speed 2800 Plus 180
Twist 129.6

When folks have actually twisted barrels and then checked the twist versus length versus caliber questions and you can find the data, you'll find that these greenhill estimates are real dang close.

For example my .451 caliber at 1400 fps with a 1.1" bullet shoots out a 20" twist in one gun, and 22" in another. My friend had a barrel from a 45-70 he made for his rigby replica had a 1-18 twist that he took to Africa and used the same bullet I do... here's the formula and the result...

Bullet diameter 0.451
Bullet lenth 1.1
Speed = <1500 120
Twist 22.1892

The greenhill with the 150 would have suggested

Bullet diameter 0.451
Bullet lenth 1.1
Speed 1500-2800 150
Twist 27.7365

Most of us realize that you can get away with a slightly longer twist when velocities are up there to impart spin, but this is a boundry twist that I don't think would work at all. Whitworth and Rigby both used twists of 1-18 to 1-20 to stabilize their 500 grain bullets derived from experimentations, and these are replicated by the greenhill with a 120 for slower than 1500 fps value.

And the .577 at 1-48

Bullet diameter 0.577
Bullet lenth 0.85
Speed = <1500 120
Twist 47.00174118

and with a 1-66 twist... :)

Bullet diameter .575
Bullet lenth .575
Speed = <1500 120
Twist 69.000000




Aloha... :cool:

[missionary5155]

Thanks again to everyone !
If I am understanding correctly the problems with Fast twist in 12 guage (say about 1:28 -32) and using patched RB is the problem of the ball slipping in the rifling with "more velocity" applied. That make sense.
So is it reasonable to say there is not a problem with "Overspinning or Overstabalization" of a Large round ball ? If a ball is spun very fast or "at an average spin speed" ( 1:70 lets say) then the results (accuracy) at the same velocity should be generaly same ?
I am mainly asking all this because when I have access to my Mossberg Rifled barrel again I want to continue experimenting with a .685 ball in a Win shot cup and see just what I can achieve with that barrel. With the above info I can see some potential "difficulties" and can possible have an idea what I am dealing with.
Again THANKS ! This is great having access to your previous experiments and knowledge.

[Rattus58]

Hey... Miss... :)

If you are using a shot cup, stripping is not a problem, I don't think. The problem with stripping comes with too tight, too deep usually. My Zoave has very shallow grooves with a gain twist I'm told. That means it starts out with a relatively lengthy twist and tightens up at the last second... not enough to make a difference probably... and with a pure long twist, not a problem either...

But a .685 diameter bullets only needs an 82" twist.. 28" is like 3 times tighter and just might cause your bullet to "walk over in the direction of twist". I think it will leave the bore in good shape though.

Aloha... :cool:

[Digital Dan]

Rattus, Greenhill's formula was developed in the 19th century to assist in determination of required twist rates for rifled artillery and conical projectiles. It does not have anything to do with round ball exterior ballistics or stability.

Your twist numbers will work, but so will about anything else for round balls when they are fired at appropriate velocities. "Walk over in the direction of the twist" sounds a carry over from Gen. Julian Hatcher's thought that bullets walk on air as he indicated in one of his early books. They don't. Over stabilized conical bullets have a pronounced yaw of repose and are deflected by resultant offset of the axis of rotation. It does not happen to round balls.

[Rattus58]

Hi Dan... :)

I understand how and why greenhill was developed. However I haven't been able to come up with anything else that references what twist I would like to have for my 410 caliber long gun shooting a 420 grain bullet at 1450 fps. Care to give it a shot?

As for no applicaton today, i completely disagree with you. In my 50 caliber 1/48 twist T/C shooting a round ball with 100grains of powder produces very poor accuracy, as did my 54 CVA Hawken with the round ball compared to either caliber in a 1-66 or 1-60 twist. True, barrels are different, guns were different and that could have everything to do with the result, but I'm tending to go with my gut and say, the longer twists work for the round ball and seem to peg what is chosen.

My 58 uses a 1-70 twist... and with the green hill formula...

Bullet diameter 0.58
Bullet lenth 0.58
Speed = <1500 120
Twist 69.6

Ummmmm so, whether you agree or we agree to disagree, I'm sticking with the modified formula and I'm sticking with it especially with conicals, because it hits the optimums that have been proven by trial rather than computer right on.

As for yaw of repose, I've downloaded the how bullets fly discussion that helped me a lot with theory of bullet flight.

As for the round ball, I don't shoot hyper velocity bullets, but I do and have shot REAL bullets that in my White over 70 grains of powder push these bullets consistently to the right, so I'm at a loss myself as to why this happens. I've read that with a cross wind from the right, the spin of a ball to the right will drift the bullet/ball further to the left than if the barrel was spun left...?? Now I'm totally confused.... :D :D :D

Aloha... :cool:

[Digital Dan]

Dunno if we agree or not, doesn't much matter. Your T/C twist is more suitable for conicals and I'm guessing you were stripping the ball with a 100 grain charge. Had one of those myself and it was quite accurate with a 50 grain charge of FFG. The T/C barrels do not have a particularly deep groove so I'd guess you were stripping the ball or blowing patches or both.

You'll need to give it your own shot as you don't provide bullet length regarding your .41 w/ 420 grain bullet. That's a right smart velocity for a muzzle loader using conicals, or is it a cartridge gun? Regardless, Greenhill will work for that application with your constant. I'd guess maybe something in the 18-20" twist will do the trick...just a guess. All twist rate calculators use the Greenhill Formula or variations of that theme. One can alter the formula but not the physics involved.

I shot several round balls once at a chronied average of 2400 fps. I didn't hit the target. Miracle I didn't hit the Chrony.

On twists and winds, the operative word is conical bullet, not ball:

A right hand twist with a crosswind from the right will cause a bullet to strike high as well as left. Conversely, a left to right crosswind will cause a bullet to strike low and right. Left hand twists will do the opposite in corresponding winds. The vertical change in POI is caused by aerodynamic jump, the lateral displacement by deflection. If you have a variable wind speed with a constant direction, and very precise rifle you can calculate the gyroscopic stability factor of your twist/bullet combination by measuring the slope of the impact points of a string of shots. They will make a diagonal line, the highest being so because it was fired with the strongest crosswind and the lowest....well, you get it. Aerodynamic jump is a phenomena wherein the bullet responds to an abrupt force (cross wind) as it exits the muzzle. Because it is upset, but gyroscopically stabilized it will wobble a little bit (nutate) and eventually reorient itself on a new axis of rotation relative to the flight path. There are multiple upsetting forces on a bullet as it exits the muzzle: wind, gravity, bullet imbalance to name but a few. Each imparts it's own destabilizing influence and what comes out of the process is called a resultant precession. If one has a right hand twist and a right to left crosswind, the bullet will impact high, all else being equal. To put it in perspective, a B-17 waist gunner shooting at a FW190 from the left side at 500 yards with his M2 would have to hold about 16 feet low to hit his target. The right waist gunner would necessarily shoot 16' high, all of this predicated on an airspeed of about 200 mph....all because the M2 has a right hand twist. I dunno if you've ever seen bullet glimmer or not, but it's a circumstance where you can see the sun's light reflected on the heel of a copper jacketed bullet when the sun is behind you and the background of your field of view is shaded by clouds. It's an interesting thing to see what happens to a bullet in that circumstance in mountainous terrain...the wind currents being a random mix of direction and strength. Is a wonder we can hit anything at long range. Bullets certainly do not travel in a simple vertical trajectory in such environs.

On the matter of Yaw of Repose, it affects all conical bullets and the deflection will be in the direction of twist. Its effects are a function of gyroscopic stability factor(Sg) and range, and it is a deflection. The effects are proportional to Sg. Its component is very small in rifles shooting lead bullets, especially inside of 1,000 yards. The common form of such bullets creates a small difference between Center of Pressure(CP) and Center of Gravity(CG) in the bullet, so fast twists are unnecessary....and impractical. If your White is going right at modest ranges there is something else going on there.

In this discussion it is of great significance that you differentiate between round balls and conicals. They are from different universes when it comes to exterior ballistics. Your mention of "Real Bullets"...maybe I know what you mean or not, not certain of that. You mean the bullets that severed our ties to the Crown or those that defeated the Axis in WWII? Yeah, I'm being a wiseazz but I'm not certain what you refer to there. To the point: Conicals require Sg, round balls do not. It all falls back to the point of displacement between CG and CP. The distance with conicals is a matter of form and materials. The distance with round balls is "0", regardless of caliber, velocity or phase of the moon. Thus there is no overturning moment in round ball exterior ballistics calculation. The requirement for shooting round balls with consistent accuracy goes only to imparting a predictable rate of rotation that will avoid the knuckle ball syndrome. Spheres are not perfect and they present imperfect aerodynamics, which imparts random forces in unpredictable directions. By putting a slow spin on the ball the result is the elimination of their random nature.

[mooman76]

The 1/48 twist is more of a compromise twist between RBs and conicals. It generally can shoot both good in around the 45 and 50 cal range but only with moderate loads.

[Larry Gibson]

Missionary5155

I'm not going to quote theory and formulas. I'm just going to tell you what I found by actual testing.

Starting with a .50 cal ML with 1-28" twist (TC BLK MTN Magnum to be exact) I began working up RB (180 gr) load (1-40 alloy) with a tight patch over increasing charges of 777 fffg. I was testing at 50 yards and chronographed every shot using my Oehler M35P. I started at 60 grV (volume) and increased by 5 grV shooting a 5 shot group with each charge weight. I had gone up to 90 grV with the 5 shot groups holding at 1-2" at 50 yards. The average velocity of the 5 90 grV shots was 1891 fps. At 95 grV the group opened and at 100 grV the group was 6" at 50 yards. I figured I was good to go with the 90 grV load so I moved the target to 100 yards to get a good zero. The 5 shot group at 100 yards was 16"!!! I figured I made a mistake and the 50 yard group was a fluke. I put the target back at 50 yards and proceeded to put 5 shots of the 90 grV load into 1.5"!!! Somewhere between 50 and 100 yards the RBs were losing it! I put the target back at 100 yards and then began "loading in reverse", i.e. reducing the load by 5 grV until the group tightened. I quickly found that 80 grV of 777 fffg under the 180 gr RB would put 5 of them into 3" at 100 yards. The velocity averaged 1785 fps and that was good enough for me. That has been my standard RB load in that rifle and it always shoots well to at least 100 yards...I've not shot it farther.

Now, with reference to an RB in a 12 ga rifled shotgun. I friend gave me 5 of his loads to test in my M1100 Remington with a rifled barrel (full length rifling/Hastings barrel) to see if they would funtion the action which they did not. I do not know what his load is but it is a RB in a plastic wad in AA hulls. He shoots them in his M870 pump Remington. Velocity is right at 1600 fps out of his M870 over his PACT chronograph. The group I shot was 6" at 100 yards but as I said the action did not funtion enough to eject the shell. It did partially function and it is not known if that adversely affected accuracy. I also shot 5 of his slug loads with a cast Lee slug and they went into 3 3/4" at 100 yards. My M1100 rifled barrel has a post front sight and a Williams open rear sight. He uses Lee's data for loads and the action of the M1100 functioned perfectly.

That's what I know so perhaps it will help.

Larry Gibson

[northmn]

In muzzleloaders with round ball, no one has really determined the "best" twist that I know of for a particular caliber. 1-48" twists have been used successfully in bench matches in 54's and 50's as have 1-70's. They commonly use pretty heavy charges. There are matters of rifling depth and fouling to contend with that also enter in. Slower twists are known to have less problems with fouling, hence the development of the gain twists which reduce fouling at the breech but still give adequate spin at the muzzle. Winchester used slower twists for their lever actions to get their "Express" bpc's to work with lighter bullets of 300 grains or so in their 45's. Faster twists were used in the singleshots for heavier bullets. But a 325 Grain Gould shoots very well in a 1-22" twist. Its possible that the slow twists worked with the lighter bullets with less fouling and not for reasons of better accuracy. 1-82" sounds about right for a twist in a 12 ga. round ball, and with proper rifling would work, as would a 1-70 or a 1-120. Matter of matching the bore to the ball and patch thickness. As to Larry's comments about accuracy going to pieces after 50 yards. Sounds like a typical smoothbore. They can shoot beautifully at 50 yards but open up rapidly after that. Yet he was using a 1-28 inch twist. TC's have been known to shoot beautifully with round ball and I saw one kick butt at a match one time against custom guns. They have also been critisized for haveing a "comprimise" twist and rifling depth. Generally, they do not shoot as well with heavy charges and RB's. Save the formulas for bullets, I have had too much experience on the range to believe that a "optimum" round ball twist exists.

Northmn

[missionary5155]

Again THANKS everyone !
This all is a help.. I may have to look into a custon twist RB (1:70) for my Mossberg....
THANK you !!!!

[Rattus58]

Hi Dan,

The bullets for the .410 are around an inch or a litle more if I remember, quite long anyways... The twist in my .410 right now is 1-18" and pushes a non gas check 410655 flat nosed bullet at an average 1315 fps. A gas check conical with a much reduced meplate at 420 grains averages just over 1400, both with 80 grains of Pyrodex P.

It was some ballistician I was reading an article from a couple of years ago concerning the Greenhill formula that caused me to call him about muzzleloaders. What he told me was that for sub 1500 to 1800 velocities, use a lower multiplier, like 120, and to use a multiplier like 180 for hyper velocity bullets. So on that point, I'm just a parrot.

I understand spin causing lower pressure on the upwind side, but I don't understand the lift while at the same time moving downwind. Something is missing in my cognitive ability.

The Rifling Engraved At Loading (REAL) bullets I have in 320 grain are a tapered bullet with an oversized coller as the top ring. This was supposed to allow easy loading in almost any bore. I used them in teaching people to shoot a muzzleloader when we were pressed for time. At light loads, they shot ok in my 1-20 and 1-24" twists till I got around 70 grains or above. Then strangely these bullets (and also the Lee improved minie even with a heavy skirt) seem to drift right at heavier loads. With a 1-48 twist these bullets seem to shoot just fine up to much higher charges than I could.

I'm a backyard muzzleloader guy. Did some really strange things to make my bullets shoot well, made many mistakes along the way, and experimented an awful lot with different things. Some of my results I sort of adopted as gospel, only to find out that when I was introduced to some very longtime muzzleloader shooters and some slug gun shooters, I then realized that what worked in my guns might be an abomination to another. However, I did learn some basics that apply to all guns and for that I'm grateful.

One thing I did learn about round balls along the way, and I'm not disparaging anyone elses OPINION here, as all of this is MY OPINION, but the larger the ball the longer the twist. In none of my 1-48's, for example, and I've 5 of them, does a 58 round ball shoot well at above 85 grains of powder. Some are left to shoot at 70 even. Reason? Blown patches mostly, so I started using a card wad under... This helps the blown patch part, but back up to 85 grains cuts the patches. I've used .570, .575, .580 balls in my guns and with the 1-48's 85 grains is the limit. However, in a 1-70 twist, I'm all of a sudden able to shoot 110 grains (max recommended by Green Mountain). Now some gentleman made the comment that any ball could survive with any twist, and in my OPINION, that aint happening. I-48 will easily work with small calibers, but not necessarily as easy with larger, where on the ohter hand I am convinced that he is right on the long twist side of things... I think you can get a 45 to shoot just as well with a 1-70 to maybe even 1-120, as I think you could maybe get my 58's to do the same, but you may need higher charges to do it, but then really so what, better for what they were intended... game.

I'm speculating here a little too on the pick for a 1-70 twist for the 58's during 1800's initially in that I thnk that some of the shorter conicals (minies) actually shot not bad from the 1-70 out to 50-75 yards with quite decent accuracy. I know I have a Lee "new" minie that shoots decently to those ranges and god help anything it hits in the field. Trash Cans shoot really well from my 1-70 as well and work for game at close range, but not my choice for hunting.

I truly wish I could understand the theories around bullet flight like you do.

Aloha... Tom :cool:

[John Taylor]

A lot of big bores twist rate depends on how much powder you want to burn. Very slow twist rates of 1 in 140 using around 200 grains of powder are very effective for flat shooting and lots of knock down. Lower velocity works better with a faster twist. This guy has some good info on Forsyth rifling .http://underhammers.blogspot.com/ The T/C barrel is a compromise for the round ball and bullet, it will shoot good with powder charges of 50 to 70 with a round ball in 50 caliber but the maxi may require 90 plus. It would seem to take a certain rate of spin to get a round ball to shoot well. If the rate of twist in the barrel is slow it will need to be push to higher velocities. If it is a fast twist barrel it may not shoot well at higher velocity.
http://1.bp.blogspot.com/_j7PMWDJke08/RrdgOocVKRI/AAAAAAAAACU/PpSgweo2vUw/s1600/Forsyth.jpg

[Rattus58]

If I'm not mistaken, my zoave has similar rifling as the forsythe design.. That's an interesting site you sent me too. I've always been intriqued by the underhammer and one of my friends went an built one from a kit and is quite successful with it. There's little to not like about the underhammer and they all look like greyhounds to me where many ohters look a little less svelte.... :)

Aloha... :cool:

[northmn]

Many of the original guns were made with a variety of twists. some pretty fas by todays standards. The famed Hawken were generally made with 1-48 to 1-60 inch twists. They made some for Picket bullets or slugs with faster twists. One thing to note on original BP use. One guideline was to put a ball in the palm of your had and cover it with powder to get a load. With a 50 this works out to about 50 grains of powder. Most rifles were likely loaded lighter than we load today. A 58 with 85 grains of powder is still more formidible than many modern shooters want to believe. A .570 hole is a pretty big hole. I firmly believe that a slower twist is less fussy and more versatile with heavier loads, but also have seen some pretty fantastic groups shot with faster twists. The standards (give or take) for modern barrel blanks, such as Green Mountain, for RB's are 1-48 for 40 cal and under, 1-60 or less for 45's and 1-70 for larger bores. Colerain makes 54's in 1-56 but uses .12 inch rifling. These are probably the best guidelines one can go by.

Northmn

[missionary5155]

Thanks again to all...
John Taylor.. thanks for that websight info.. Very helpful !
Northmn... My .58 (Navy Arms Zouve) is my first choice Capper... I use a .57 with 85 grains FF and it just goes clean through...

[Digital Dan]

Rattus, didn't understand the significance of the capital R-E-A-L in your post. OK, well here's some odds 'n ends on the subject of twists and Greenhill constants.

Firstly, you can do the necessary math for maximum bullet length or required twist using the formula, it's not that difficult. The specific length of the bullet is necessary, not "about" or "almost"....put your mic on it to 3 decimal places.

twist rate = (120 * bullet diameter squared) / bullet length
bullet length = (120 * bullet diameter squared) / twist rate

The first will provide the required twist for a given bullet length at black powder velocities. The second will give the Maximum bullet length for a given twist.

The constant, in this case 120, is an adjustment used to address variations in overturning moments generated at different velocities. They are greatest at velocities in the transonic realm, or about Mach .7 - 1.3. Another issue as bullets transit this velocity realm is shifting of CP due to changes in flow field. It causes the bullets to nutate or wobble a bit, therefore they are less tolerant of marginal Sg. As velocities are increased substantially, the constant is modified as your acquaintance suggested, because the moments are lower. This component of the science is not applicable to black powder firearms.


I understand spin causing lower pressure on the upwind side, but I don't understand the lift while at the same time moving downwind. Something is missing in my cognitive ability.

It is a deflection component resulting from from the interplay of crosswind, direction of rotation and gravity. It is difficult to visualize this in whole, but easier in specific elements. Gravity causes one precession, crosswind another. They result in ballistic deflections. It is significant that each force generates it's own destabilizing influence yet the resultant is what influences bullet path, or what you see on paper. Analyze the components, view the results as a whole. The list of destabilizing forces acting on a bullet is quite long. Better to keep it simple for the moment before we wind up with a quadricyclic precession and no vaccine.:shock:

I think you're looking at this, perhaps from the perspective of magnus force....which is not the cause of aerodynamic jump. Remember, the forces are applied at different points....aerodynamic moments to the CP and other forces to CG, such as gravity. The resultant precession will present in relation to that reference. A point of consideration regarding aerodynamic jump is simply that the front of the bullet is entering an asymmetric flow field with a cross wind condition and is influenced before the bullet has completely cleared the muzzle. It should not be confused with ballistic jump which results in random dispersion. One is caused by bullet imbalance, the other by aerodynamic forces.

When a force is applied to a gyroscope it precesses (moves/deflects) 90* to the axis of rotation, perpendicular to the applied force, in the direction of the gyro's rotation. Your bullet is a gyroscope.....

If ever you should get very bored, try these books:

"Rifle Accuracy Facts" by Harold Vaughn
"Modern Exterior Ballistics" by Robert McCoy

If you do so, read them in order presented. The second was written to levels appropriate for graduate level studies and is very heavy on math. It is also clearly explained and a great deal can be learned from it if one has the time to focus on it without distractions.

Hope this makes sense after a fashion. It's late here in Hurricane Alley and this subject makes my head hurt...a little.:mrgreen:

[405]

"Rattus, didn't understand the significance of the capital R-E-A-L in your post. OK, well here's some odds 'n ends on the subject of twists and Greenhill constants."


DD,
The REAL is a bullet designed and marketed by Lee to compete with the TC Maxi Ball conical when muzzleloading gained new popularity late 60s- early 70s. I think Rattus was using the REAL bullet as a short conical comparison bullet for the twist rate discussion.

[Fleataxi]

DD: I'm assuming your formulas for maximum bullet size and recommended twist are treating bullet density as a constant?

The difference in density between pure lead and alloys might vary the weight of equal sized bullets by up to 10%. Also, velocity is never a constant in BP shooting since even the most careful shooter could vary their load as much as 10% if they load in field conditions at the range. Most BP shooters use a volumetric measure to estimate grain charges of powder, and rarely if ever weigh their charges.

JMHO

Fleataxi

[Digital Dan]

Fleataxi, the formula and applicable constant deal with a range, not a specific velocity. The weight or density of a bullet is of no significance to the calculation, bullet length is what matters. Also, I'm the messenger, not the creator of the formula. It's been around for over a century. To repeat, the necessity for spin stabilization of conical projectiles is found in the difference in location of CP and CG in the bullet. The CP can be altered due to bullet form (shape) as well as velocity. Round balls are not so affected for the CG and CP are co-located. Their need for stability is found in aerodynamic variables which impart random influence unless the ball is spun on a consistent axis. This is the essential difference in accuracy performance found between smooth bores and rifled bores. There are a great many other factors weighing on accuracy of round balls and when one sees displacement of POI with a change in load or components, that is where you need to look for solutions, not twist rates.

I add for the record, I am not expert in this subject. The world of spin stabilized projectiles is complex and I have a simple mind. I have studied this specific subject at some length but there is much I've not yet digested over the last 8 years. The authors mentioned above are legend on the subject and worked at Sandia National Labs and Aberdeen. They had access to analytical processes beyond what any of us will ever know and in some regards I accept their teachings without full understanding. I try to present what little I know about this in short written form, w/o the math or writing another book. It is sometimes difficult to do so....sometimes I fail miserably.

One may also find information on this in "Understanding Firearms Ballistics" by Robert Rinker, and it is a far simpler read. Art Pejsa has published a lot on the practical side of the science. His manner of writing makes my brain ache a little for some reason, but he knows what of he speaks. There are others as well, no slight intended by omission. All in all, it is more important to enjoy shooting first and foremost. Counting angels on a pinhead is tedious work, and there are a LOT of angels to be found within this subject.

[Baron von Trollwhack]

If you are to wrangle round ball, R-E-A-L, Minie' , Maxi, or bore/groove riding slug you must neccessarily do so with consideration of GROOVE depth as a factor as important to accuracy as twist and charge .

Patched round ball stripping and micro-groove accuracy shows us that truth.

BvT

[Fleataxi]

DD: Thanks for splaining to me!

Sometimes something I read triggers a "what if" question out of left field in my mind, and this was one of those cases. I've seen some pretty bizzarely over-simplified theories just because the scientist doesn't want to deal with the math, and makes basic logic errors by declaring something a constant when it's not.

From what little I've read/studied in Physics, very few objects moving in 3-dimensional space can be accurately described by simple formulas.

For instance, we know a bullet with a higher sectional density isn't as affected by wind as a bullet with a lower SD. The mathematics to prove it fill several pages, and give ME a headache too. :lol:

Ballistics is a great field for people that think beating their heads against the wall is fun! :lol:

Fleataxi

[Underclocked]

Greenhill is just general guidance and seems to me to be wrong about as often as not. Far too many variables.

[Rattus58]

You may be absolutely correct there Uncle... :) all I know, is that with muzzleloaders, substituting 120 to the 150 value of greenhill, and 180 to velocities above 2800 seem to pan out... but what do I know? Nada... I'm just a parrot... :)

Aloha... :cool: