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DarylSModerator
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Shortform for Greenwell Formula& the .50/70 Shaprs
      #50071 - 16/02/06 05:33 AM

In looking over my .50/70 Sharps data, I came accross the Formula for determining minimum rifling twists.
: As my 1859 Sharps relined barrel has the slow 42" twist bullet mould modification was required to maintain resonable weight, yet shoot accurately in the slow twist.
: FORMUAL: bullet length in inches divided by the diameter in inches = 'X' which is divided by 150 = 'X' which is multiplied by the diameter in inches = twist in inches required. As stated earlier, this "Twist" should be reduced by 1" or 2" to get an even more stable bullet.
; Expamples: - Lyman mould # 515141 casts the original type bullet for both the .50/70 and the .50/90 Sharps. It is a pointed, grooved lubricated bullet with square grease grooves, with a diameter of .515" and is .950" long. According to the formula, it needs a twist of 40.3" or SHARPER for best accuracy.
: As my rifle's twist is 42", a bit slow, by at least 2", I shortened the mould by boring the nose and hard silver soldering in a rouded top plug. The mould now casts a cup-point bullet of 420gr. weight fo 1/2:1/2 Wheel Weights and pure lead fo a length of .830" which by the formula requires a twist of 47.4", well within the capabilities of this rifle.
: Subsequent testing of this bullet shows full BP loads, as well as Herco smokeless loads with buffer material, to shoot into less than 1.3" at 50yds, just a ragged hole for 5 shots and 1.45" at 100yards, also a single hole group, hand-held on the bags. The shortened bullet was a definite success in this rather slow rifling.
: The full length bullet shot resonably accurately, however as we've seen with the 48" twist buttoned TC barrels, it may not be stable once it strikes a large game animal. My best accuracy with the pointy original bullet ran 6 shots in 3.26"X 3.12" at 100meters, with 4 of those in a snug 1.565". This was with the original battle sights, blade front and "V" notch adjustable rear. The barrel is 22" long. Straight from the mold 520gr. Lyman bullets were tipping at 25yds, and whistled down-range when shot at the further bunker.
: The heavy bullet Lyman #515142 mould, casts a bullet of 520gr. and requires a minimul twist of 36.0" for stability.
: My only testing with BlackMag3 powder showed a load of 75gr. weight gave a velocity of 1,400fps with the 420gr. cup-point bullet. The recoil was quite heavy in this light-weight carbine.
: Other testing with Herco lists 21.0gr. and Kapok filler as giving 1,260fps, which should do for a nice shooting hunting load. It went 1.1" for a 3 shot group at 100meters.
: 44.0gr. 3031 and the 420gr. listed at 1,307fps.
: All of the above were shot in blown-out .348Winchester brass. It's only detriment is the extractor has to be manually held in, so the slightly smaller rim won't slip by. Extraction is desisive, just as if using Dixie .50/70 brass.
: A final thought, is that most bullet will shoot accurately and maintain stability with very much faster twists, but the reverse doesn't work. In other words, the full sized 450gr. Lyman bullet for the .50 will shoot very accurately down to about a 20" twist, but at 42", becomes unstable, as shown by slightly oblong bullet holes in a 100 meter target.


--------------------
Daryl


"a gun without hammers is like a Spaniel without ears" King George V

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JohnTheGreek
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Re: Shortform for Greenwell Formula& the .50/70 Shaprs [Re: DarylS]
      #50251 - 18/02/06 08:32 PM

For the algebraically inept (me), here is a web-site that will do the math for you.

http://www.shortmags.org/shortmags/ref_data/TwistRateCalc.asp

Also, it is important to note that bullet material may have an influence on the efficacy of Greenhill's numbers. A discussion of this can be found at G.S. Custom Bullet's web site. G.S. Custom

Gerard notes that....

"Gyroscopic stability is what makes a bullet fly in a stable manner. If the variety of conditions governing gyroscopic stability culminate in a gyroscopic stability value of less than one, the bullet is unstable and will fly funny. No amount of lecturing the offending bullet will make it straighten up and fly right. Something has to be changed to bring the gyroscopic stability value to more than one.

Gyroscopic stability is a rather complicated subject and one often hears of the Greenhill formula being used to calculate whether a bullet is stable or not. Using the Greenhill formula is better than nothing, like some makes of chronograph, but for more exact calculations and a true picture, one must turn to the work of one R.L.McCoy. Using the Greenhill formula results in an inconsistent effect on the gyroscopic stability of a bullet if sectional density is changed. The equation does not ask for the right information to accurately take into account varying material densities and forms.

Using McCoy’s method requires inputting the specific gravity of the bullet material as well as a number of form variations that will affect gyroscopic stability. Specific gravity denotes how dense a material is. If specific gravity changes, density changes. This must therefore lead to a change in sectional density! Eureka! A connection! Now we must investigate more thoroughly by making some comparisons to prove the connection.

Still using McCoy’s method, we find that two bullets, identical in form but made from different materials, have different gyroscopic stability values as well as different sectional density values. Changing the sectional density therefore changes the gyroscopic stability. The connection seems to stand. As expected, two identical bullets, made from the same material have identical gyroscopic stability and sectional density values. Not changing the sectional density, leads to no change in the gyroscopic stability and we have two out of two. Now we are cooking! Now we compare two bullets, made from the same material, with identical sectional density values but with different forms, one a semi-wadcutter and the other a spitser boat tail. Alas, they have very different gyroscopic stability values and disprove once again what seemed to be a possible use for sectional density. It is form, rate of twist, diameter and speed that are the big hitters when gyroscopic stability is calculated. As usual, sectional density just tagged along as a coincidental by product of the important stuff."



Best,

John

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DarylSModerator
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Re: Shortform for Greenwell Formula& the .50/70 Shaprs [Re: JohnTheGreek]
      #50279 - 19/02/06 04:26 AM

Thanks for the URL that does the calculations and John's notes.
: My own findings, and beliefs are that the old-style bullets benefited from higher SD numbers, which seems directly co-related to their penetrative power. This seems to be true, whether they were lead, an alloy of lead, or jacketed in nickle or some other alloy with a soft lead core. Bullets were pretty much round nosed soft points, or flat noses and preformed pretty much the same in regards to any differences in compostion. Greenhills formula seemed to work and was adopted. Was it perfect, not precisely, but it worked and is just fine today, with a simple adjustement.
: Newer bullets, the Premium Bullets if you will, have changed the way bullets penetrate and hold together. Even simple bonding has changed the way bullets penetrate, but composition to a great extent, determines their length, which directly effects stability.
: Stability in the air, has literally nothing to do with S.Density, and all with length & it's rate of spin - always has. With the old style bullets, SD was mainly directed at how a bullet penetrates, not with it's gyscopic stability.
: The the ctgs. as noted above, pure lead or an alloy is used, and the Greenhill formula works pretty good, that is, if you subtract an inch or two, it's precise enough.

--------------------
Daryl


"a gun without hammers is like a Spaniel without ears" King George V

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Dphariss
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Re: Shortform for Greenwell Formula& the .50/70 Shaprs [Re: DarylS]
      #55422 - 22/04/06 02:11 AM

I never really thought about the twist needed to stabilize the 450 gr Lyman. Intersetin g it comes out at 36".
The Sharps made 50s (and Shilohs to at least #10000) had a twist of 1:36. The 50-2 1/2" shot a 473 gr bullet but it was usually PP and these stabilze a little easier. However, none of the 50s were noted for very good accuracy. Due I am sure to the marginal twist. I believe Sharps stated in print that the 44-2 5/8" shot better at 1000 than the 50 2 1/2" did at 600. I beleive Wincheser used a 60 inch twist for the 50-95 Express and something similar for the 50-110 version.
I always considered the 50s to be hunting rounds with accuracy being good enough for that purpose. This is based on my experience with Shiloh's.

Modern reproduction SS with faster twists, 1:22 or maybe 1:24 seem to shoot very well, as they should. I have often wondered at the gov'ts slow twist and why Sharps did not increase the twist somewhat to maybe 25 or even 30.
It just occured to me that perhaps the rifling machines used by the gov't for the job, certainly the same ones used to do the Minie ball muskets, may have needed a major rework to get faster than 42"??? Seems silly in this day and age but it could have been as simple as that. Or somebody goofed when doing the twist calculations.

Dan

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bonanza
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Re: Shortform for Greenwell Formula& the .50/70 Shaprs [Re: Dphariss]
      #55430 - 22/04/06 03:22 AM

Has anyone taken into account bullet velocity and/or barrel length?

How much of a revolution should the bullet make in the barrel, at least 1?

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DarylSModerator
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Re: Shortform for Greenwell Formula& the .50/70 Shaprs [Re: bonanza]
      #55432 - 22/04/06 04:05 AM

bonanza - I don't for a minute think a bullet has to be in a barrel for 1 turn, or even for 3/4 turn or carbines and handguns wouldn't shoot well at all. Even my 50/70 Sharps shoots well with it's 42" twist and 22" barrel, with my modified mould. I am certain that if the barrel was 2" shorter, it would shoot just as well, even thouth the bullet turned less than 1/2 turn inside the barrel.
; Dan may be onto something with the rifling being handled with available equipment, except the .50/70 Sharps had 2 main twists, 42" and 24", but then, depending on which armoury made them, so Dan's theory may hold weight.
: I was merely unlucky in getting a re-lined 1859 barrel with a 42" twist, but agin, it does shoot well with bullets that require only a 38" twist according to the rifling formula. That barrel has the longer, 1.9" chamber, not 1 3/4" normally associated with .50/70's. I expect tollerances with the first centrefires weren't very narrow.
: With McGowen 24" twist .50 cal. barrels, Taylor and I managed 10" to 12" groups at 450 meters with our Rolling Block and Sharps rifles, both with Mcgowen barrels and chambered for the .50 Alaskan. We used straight BP loads with OxYoke wads and 550gr.RCBS and 525gr.Lyman bullets. We were shooting off the hood of my truck. Our sights consisted of Globed pin-head front and tang mounted aperature sights. Rifle weight was identical at 11 1/2 pounds. My RB routinely ran 5 shot, 1 1/2" groups at both 100 and 200 meters using 550gr. RCBS bullets. Taylor's rifle, iwth 520gr. Lyman cast bullets wasn't as accurate close up, but at 450yards, it matched my Rolling Block for accuracy.


--------------------
Daryl


"a gun without hammers is like a Spaniel without ears" King George V

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Dphariss
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Reged: 18/04/06
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Re: Shortform for Greenwell Formula& the .50/70 Shaprs [Re: DarylS]
      #55471 - 22/04/06 02:07 PM

I rebarreled a #1 Shiloh to a 17+- pound 50- 2.5" for a customer before I stopped doing such things using a Badger with a 24" (IIRC) twist and he REALLY likes this gun. Shoots really good about as far as anyone would want and recoil is not a problem.

Dan

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