Ball rotation speed

[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.

[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...

[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.

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.

[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.

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

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...