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bullet weight/twist rate?


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#1 Brada2

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Posted 14 July 2009 - 12:55 AM

i just bought some 223 45 gr JHP and on the box it says that the load was designed of 1:9 twist rates or slower....can i shoot this out of my 1:7 AR and still maintain decent coyote destroyin' accuracy (2MOA)

#2 CA Desert Dog

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Posted 14 July 2009 - 06:13 AM

This article may help you decide."The Greenhill Formula for most standard cartridges is T=150(d/r), where T is the twist rate, d is the bullet diameter, and r is the bullet length to diameter ratio (bullet length divided by its diameter). For cartridges with a muzzle velocity of more than 2,800 fps, substitute 180 for 150.If we were to do the Greenhill Formula on Sierra's .308 168-grain MatchKing, we would come up with an ideal twist rate of 1:11.76. That's close enough to 1:12, which is a common twist rate for .308 and one that does very well with 168-grain bullets.The numbers on the 175-grain MatchKing, which has a diameter ratio of 4.081 (1.257 inches divided by .308), look like this: T=150 x .308/4.081, which gives us an optimum twist rate of 1:11.32. That 0.44-inch difference may not seem like a big deal, but it is. As an example, take a look at just about any custom .308 tactical rifle these days, and you are almost certain to see a barrel with a twist rate in the 1:10 to 1:11.25 range. That's because the classic 1:12 twist usually shoots lighter projectiles--155-grain Palma bullets and 168-grain MatchKings--just fine, but that twist rate rarely shoots the more popular 175-grain bullets well at all. Speeding up the twist a bit has no adverse effect on lighter bullets, but it ensures stability with heavier projectiles. And that's a plus in anyone's book.If you're a hand loader, take note of velocities if your barrel barely meets the recommended twist rates. I've seen starting loads shoot fair to terrible, while the same bullet over a bit more powder shot the lights out. Clearly, in that situation, the rate of twist was just right; it will get the job done, but a slightly faster twist wouldn't hurt. Over-stabilization as a result of a too-fast twist rate can occur, resulting in a bullet that travels along its downward arc with its tip pointing skyward, exacerbating wind drift and hastening velocity loss. Ideally, the tip of a properly stabilized bullet should tip downward as the bullet begins its downward arc. Clearly, an over-stabilized bullet is not conducive to accuracy, but it is only noticeable in extreme cases of over-stabilization and at very long range. You'd never notice it at hunting distances."
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#3 Shoot-it

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Posted 14 July 2009 - 02:08 PM

desert dog do you have a shorter answer? :smiley-outta-here:

#4 CA Desert Dog

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Posted 14 July 2009 - 04:26 PM

LOL.... Ya got me Shoot-it.....Truth is, I have never seen a 1:7 AR, most are 1:8 and 1:9 that I have come in contact with. My RRA coyote rifle upper is a 1:9 and it will shoot 45 to 60 grainers accurately.
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#5 A17Shooter

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Posted 14 July 2009 - 04:49 PM

do you have a shorter answer? :smiley-outta-here:

I'll take a 'shot' at a short answer. Shoot the ammo and find out if it works for you. end of answerShort explanation:The faster twist might over spin the bullet and cause it to disintegrate on the way to the target. I personally don't think the 223 has much chance of doing that.

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#6 Frank

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Posted 14 July 2009 - 05:08 PM

Shoot the ammo and find out if it works

Yep, Exactly!What certain rates of twists "should" do & what they "actually" do can be miles apart. Often Times!With that said, I would be semi-surprised if a 7" twist will shoot a 45gr bullet very well. Then Again? You are stating 2 MOA, so it just may do that... Maybe!Good Luck and let us know what happensFrank

#7 Brada2

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Posted 15 July 2009 - 01:07 PM

i have a kel tec with a 1:9 and i've shot 75 gr @ 100yds and there was no keyhole entry....but, i herd somewhere (i can't remember) that too fast of a twist rate can cause more wear and less accuracy. the 45 gr load i was talking about is fast (3600 fps) so the overall balistics should be close to a 55-60gr 22-250...nice and flat...i guess i dont want to screw up my new AR so im over precautious....

#8 Desert Fox

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Posted 15 July 2009 - 08:55 PM

Here's my response on some discussion I've had on another forum related to barrel twist.

Banshee wrote: Do the formulas for calculating twist rates factor in the polymer tip on modern bullets? It appears to be old when polymer tips are "modern". The tip weighs little to nothing but is a significant length component, should the calculations be made for the length of the lead/copper only?

It does have a bearing when using the old Greenhills formula, although probably insignificant. The bullet make-up, their density and their specific gravity can have more impact when choosing the right barrel twist for a particular bullet. Greenhills though as old as the formula was, like the G1 drag model, still works or at least can be use as a base line for todays modern projectiles. For those interested, I borrowed these from Robert Rinkers "Understanding Ballistics" good book by the way. I recommended everyone to read it - Very informative! Greenhills formula is base on this role, "The twist required in calibers equals 150 divided by the lenght of bullet in calibers". This is fairly limited in it's application for todays modern bullet since it was based on a jacketed bullet with fairly blunt nose and has a specific gravity of 10.9, Remember Greenhill came up with this formula at the turn of the century. It's funny, what a coincedence this thread would be. I happened to look at this same questions when I switched bullet with my 300 Weatherby, from 180 grains Barnes TSX to 180 TTSX. I noticed that their is a significant difference in bullet lenght when you compared the two. I was wondering at the time if this will affect the stability hence the accuracy of my rifle. Barnes bullet, being pure copper were already built longer than lead core bullet of the same weight as it is. So I was wondering, if this will have an effect on the stability of the bullet for the 300, which has the standard 10 twist rifling. Being an accuracy freak, I left nothing to chance so I did my own little researched. One phone call to Barnes would have answered my questions but my curiosity always got the best or worst of me . So here's what I got: Bullet lenght: 1.473" TTSX Applying Greenhills formula 1.473 divided by.308 caliber = 4.78 calibers long Now devide 150 by 4.78 = 31.38 caliber Multiply the bullet diameter, in this case .308 to convert it into inches 31.38 X .308 = 9.66" this will be your barrel twist Compared to the 180 grain TSX using the same formula, I came up with 10.26" twist. To sum it up, the 10 twist barrel on my 300 Weatherby is just about right for the TSX but a little slow for the TTSX. Can I see the difference in accuracy? Don't know yet but I suspect it won't matter much!
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#9 clampdaddy

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Posted 16 July 2009 - 03:27 PM

I wonder why none of these formulas incorporate muzzle velocity. These formulas were thought up back when 2500fps was really fast. A bullet that is unstable with a certain twist rate will often stabilize when driven at a higher velocity. Some math wiz should figure out a formula based on how many revolutions per second a given bullet needs.
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#10 Stan

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Posted 16 July 2009 - 11:07 PM

When you have time call Sierra Bullets http://www.sierrabullets.com/800-223-8799




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