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question about bullet length to twist ratio (Read 36782 times)

Fred Boulton Oldtimer Offline Posts: 801 Joined: Jan 6^th, 2007	Re: question about bullet length to twist ratio Reply #15 - Dec 3^rd, 2011 at 8:45am	Print Post
	Calibre .577, but bullet dia on loading .575". Length of bullet 1.0". Fred


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westerner Frequent Elocutionist Offline deleted posts and threads record holder. Posts: 12334 Location: Why, out West of course Joined: May 29^th, 2006	Re: question about bullet length to twist ratio Reply #16 - Dec 3^rd, 2011 at 3:40pm	Print Post
	Fred, I come up with 43 for the .577X1.00 bullet. I find the GH formula to be REAL forgiving. Theres lots of room to wiggle. I wonder what happened to John Louis? :question Joe.

	A blind squirrel runs into a tree every once in a while.
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westerner Frequent Elocutionist Offline deleted posts and threads record holder. Posts: 12334 Location: Why, out West of course Joined: May 29^th, 2006	Re: question about bullet length to twist ratio Reply #17 - Dec 3^rd, 2011 at 3:42pm	Print Post
	Maybe somewhere........... East of Modesto, can assume that much. Joe.

	A blind squirrel runs into a tree every once in a while.
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frnkeore Frequent Elocutionist Offline Posts: 7653 Location: Central Point, OR 97502 Joined: Jun 16^th, 2010	Re: question about bullet length to twist ratio Reply #18 - Dec 3^rd, 2011 at 4:04pm	Print Post
	I've found that Greenhill will stabilize most bullets down to at least 1000 fps. Better to be over stabilized than tumbling. I mostly use the Don Miller program and I've found it to give more exact results. Before useing the Miller, I modified the constant number in Greenhill to 175 from 150 for the 1400 fps range. Frank

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JLouis Frequent Elocutionist Offline Posts: 10625 Joined: Apr 8^th, 2009	Re: question about bullet length to twist ratio Reply #19 - Dec 3^rd, 2011 at 4:39pm	Print Post
	Frank I agree with you on the accuracy of Don Millers Twist Rate Rule it is the best I have found to date for cast bullets. I played with the one Westerner posted and if one was to order a bullet mould based on the results it provides it would end up being a total waste of hard earned money. J.Louis

	" It Is Better To Now Have Been A Has Been Than A Never Was Or A Wanna Be "
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40_Rod Frequent Elocutionist Offline Extremism in the persuit of accuracy is not a vice Posts: 4285 Location: Knoxville, TN Joined: Apr 20^th, 2004	Re: question about bullet length to twist ratio Reply #20 - Dec 4^th, 2011 at 8:55am	Print Post
	Keep in mind that the Greenhill formula was developed for estimating projectile length for muzzle-loading cannons when they were experimenting using longer projectiles and rifling instead of smooth tubes and balls. It is a happy accident the formulas also worked for the projectile speeds that were developed using black powder or duplex loads used in single shot target rifles of the day. Up until the late 70s or so shooting these rifles was about re-creating those speeds. When it was realised that smokeless powder freed us from those limitations is the same time that we discovered that spitzer bullets could be shot accurately, something that earlier shooters felt was not the case. Spitzers require higher speeds than flat nosed bullets to be accurate. Today if you are not assuming sub-sonic of barley sonic speeds you need a formula that factors in higher speed. 40 Rod


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joeb33050 Frequent Elocutionist Offline Posts: 2613 Location: Marathon, FL Joined: Apr 20^th, 2004	Re: question about bullet length to twist ratio Reply #21 - Dec 4^th, 2011 at 11:33am	Print Post
	40_Rod wrote on Dec 4^th, 2011 at 8:55am: Keep in mind that the Greenhill formula was developed for estimating projectile length for muzzle-loading cannons when they were experimenting using longer projectiles and rifling instead of smooth tubes and balls. It is a happy accident the formulas also worked for the projectile speeds that were developed using black powder or duplex loads used in single shot target rifles of the day. Up until the late 70s or so shooting these rifles was about re-creating those speeds. When it was realised that smokeless powder freed us from those limitations is the same time that we discovered that spitzer bullets could be shot accurately, something that earlier shooters felt was not the case. Spitzers require higher speeds than flat nosed bullets to be accurate. Today if you are not assuming sub-sonic of barley sonic speeds you need a formula that factors in higher speed. I don't understand. Greenhill works for every application I've ever heard of, with the sole exception of LRBP shooting, where Greenhill MAY not give a fast enough twist. What is the question that the alternative formula solves? Minimum twist for a given caliber/twist/velocity? or Maximum bullet length stabilized for a given caliber/twist/velocity? or some other question I can't think of. Clearly, stability is SLIGHTLY affected by velocity, very slightly. I can accurately reliably shoot .30 200 grain bullets to 100 yards at velocities under 800 fps at the muzzle. 40 Rod


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John Boy Ex Member	Re: question about bullet length to twist ratio Reply #22 - Dec 4^th, 2011 at 8:31pm	Print Post
	Quote: ... with the sole exception of LRBP shooting, where Greenhill MAY not give a fast enough twist. Joe, I am not a big fan of the Greenhill formula, primarily because I am a 'LRBP' shooter. But if one must use it for BP, change the factor from 150 to 125. For BP, Don Miller did an excellent article for the Spring 2007 issue of the Black Powder Cartridge news entitled … *A New Twist Rule Applied to Black Powder Rifles.* I am mentally challenged past 2+2, but he calculated that the best 38 caliber BP bullet for a 1:18 twist was the old Ideal 375166 with a factor of 17.65. That was enough for me and started a nearly 2 year quest to have this bullet because there at that time 2 known molds of which one was owned by an unknown. Ok, fast forward ... there is now a remake of the that mold. I put the elements for the 375166 into the Powley Bullet Stability Calculator and it was dead nuts on with a velocity between 1050 and 1150 fps. It is my favorite 38-55 BPCR bullet and will klang 500 meter Rams if the trigger man does his part. Now, if I had the mental capability to convert Don Miller's formula into an Excel equation, I would use nothing but it and the Powley calculator. Another neat Powley calculator is his Interior Ballistic Calculator that computes for nitro powders: ESTIMATED CHARGE WEIGHT POWDER SELECTION INDEX ESTIMATED VELOCITY Yes I am a fan of Powley and haven't used Greenhill's in a long time
	« Last Edit: Dec 4^th, 2011 at 8:37pm by »

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JLouis Frequent Elocutionist Offline Posts: 10625 Joined: Apr 8^th, 2009	Re: question about bullet length to twist ratio Reply #23 - Dec 4^th, 2011 at 8:40pm	Print Post
	John Boy & Joe Don Millers Twist Rate Rule is an excel equation and I would be more than happy to send you a copy just send me a PM with your e-mail address and I will get a copy off to you. J.Louis

	" It Is Better To Now Have Been A Has Been Than A Never Was Or A Wanna Be "
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John Boy Ex Member	Re: question about bullet length to twist ratio Reply #24 - Dec 4^th, 2011 at 9:54pm	Print Post
	John, PM on the way ... when I read Don's article, I sent him a mail if he had his formula in Excel and never got a reply. Much Thanks


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JLouis Frequent Elocutionist Offline Posts: 10625 Joined: Apr 8^th, 2009	Re: question about bullet length to twist ratio Reply #25 - Dec 4^th, 2011 at 10:08pm	Print Post
	John it just went out and is headed your way. Please let me know if you have any questions and I would be more than happy to address them for you. J.Louis

	" It Is Better To Now Have Been A Has Been Than A Never Was Or A Wanna Be "
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JLouis Frequent Elocutionist Offline Posts: 10625 Joined: Apr 8^th, 2009	Re: question about bullet length to twist ratio Reply #26 - Dec 5^th, 2011 at 3:46pm	Print Post
	Gentleman I received quite few requests for Don Miller's Twist Rate Rule this morning and if anyone else would like a copy of it just send me a PM with your e-mail address and I would be more than happy to e-mail it to you. J.Louis

	" It Is Better To Now Have Been A Has Been Than A Never Was Or A Wanna Be "
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joeb33050 Frequent Elocutionist Offline Posts: 2613 Location: Marathon, FL Joined: Apr 20^th, 2004	Re: question about bullet length to twist ratio Reply #27 - Dec 6^th, 2011 at 10:52am	Print Post
	J. Louis sent me the Don Miller twist excel worksheet. I don't know that Don wrote it-can't make the connection. The worksheet estimates Sg, stability factor. The formula is unnecessarily complex, easily simplified, almost looks like somebody is trying to BS the reader with math. I rewrote the formula, and algebrad it to show TWIST on a second page. Enter the data, get TWIST. I don't know what TWIST means in this formula. Greenhill TWIST and BULLET LENGTH are threshold values-slower twist or longer bullet and stability goes. I don't know where the formula comes from-how it was constructed. I can't see a benefit over Greenhill. LRBP?? The revised workbook is ready and will be sent on request. joe b.


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JLouis Frequent Elocutionist Offline Posts: 10625 Joined: Apr 8^th, 2009	Re: question about bullet length to twist ratio Reply #28 - Dec 6^th, 2011 at 1:52pm	Print Post
	Joe send me a copy at johnlouis123@att.net I am not sure why you are trying to fix something that is not broke and I would like to review the changes you made. Thank You J.Louis

	" It Is Better To Now Have Been A Has Been Than A Never Was Or A Wanna Be "
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frnkeore Frequent Elocutionist Offline Posts: 7653 Location: Central Point, OR 97502 Joined: Jun 16^th, 2010	Re: question about bullet length to twist ratio Reply #29 - Dec 6^th, 2011 at 2:08pm	Print Post
	I don't shoot beyond 200 yards with single shots so, if the bullet tumbles at 201 yards, it really doesn't matter to me. I know nothing about LRBP shooting. The Greenhill formula doesn't take temperature or bullet weight ( SD ) into account, nor velocity (it's not a variable in the formula) in any real terms. I've used Greenhill since the early 80's, first in round balls and then when I got into single shots in '85. At 200 yards, Greenhill over stabilizes bullets! If a bullet is rotating faster than it takes to stabilize it, the centrifugal force is acting on any imbalance within the bullet and magnifying it above what needs to be. I first started on this path with Ron Long in '85 when I bought my first bullet mold from him. He called it "sleeping", when the bullet settled down to it minimum stability. Here are real world results and independently, they correlate with the DM program that I use from JBM. That program uses 1.3 as there lower end for stability. Greenhill min. bullet length for a 16 twist .321 groove barrel is .960. My Douglas 16 twist stabilizes a 1.12 lg bullet down to 1400 fps. Greenhill says that it takes a 13 twist for a 1.165 lg bullet, my 15 twist Douglas stabilizes a 1.165 lg bullet. It will also stabilizes a 1.23 long 323471 bullet at 2157 fps. Greenhill says it takes a 12 twist to stabilize a .308 bullet that's 1.165 long. My Douglas 14 twist barrel stabilizes a 1.165 lg bullet and will stabilize a 1.21 at 2060 fps. Again, these results are at 200 yards and my results where independent from any stability program (just my experiments) but, they correlate with the DM program at 1.3 or above. Frank

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