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 Post subject: Barrel's Twist Rate | Few questions

Standard Shot

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Joined: Sun Dec 12, 2010 11:53 am
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Location: Peshawar
Hello Respected Members,

For long i have been observing discussions related to twist rate and its effect on accuracy and type of ammo used in it. I have few question regarding twist rate which i would like the senior and experienced members to clarify:

a) What is a twist rate and what does it quantify?
b) What does the twist rate numeric ratio quantify i.e 1:12 or 1:9 e.t.c
c) What is the aspect of twist rate in handgun and in rifle?
d) How is the accuracy affected by twist rate?

I would like all members to simplify my confusion about it.
Regards


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 Post subject: Re: Questions regarding barrel's twist rate>

Sniper

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Twist rate identifies the twist of the grooves that are twisting in a given length of barrel. i.e. 1:12 means 1 twist in 12 inches. In rifles the bullets can be of heavier weight and lighter fired from the same barrel such as 55 grain to 250 grain in 30-06 rifle. For a heavier bullet you need more twist rate to stabilize it gyroscopic twisting. When the bullet is pushed in the barrel it twists and when it exits the barrel attains couple of thousands of twists in a second. the lighter bullets such as 125 grain go good in 1:12 twist and as you approach the 150 grain level you need to surpass 1:12 to 1:11 and preferably 175 grain to 210 grained bullets need 1:10 at least.

However you need to remember that firing a lighter bullet such as 55 grain in 1:10 at 3000 fps is going to make the bullet vanish in thin air possibly, mainly because too much of rotation pulls apart the bullet and it vanishes in thin air.

The bullets are specifically designed for a specific twist rate of barrels as far as rifles are concerned, in pistols the rate is usually 1:10 i think. Anyways when you are into the 1000 yard shooting that is when this matters a LOT. lesser than 500 yards you do not have much factors except that heavier bullet fly straighter on their bore axis when shot from higher twist rated barrels

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 Post subject: Re: Questions regarding barrel's twist rate>

Standard Shot

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Brilliantly written. Thanx a lot brother atiq. Learnt a lot


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 Post subject: Re: Questions regarding barrel's twist rate>

Master at Arms

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Well explained by Atiq sb. One thing that is to be kept in mind is that twist rate is caliber specific. That means a 1:11 twist rate in .308 Win is not be compared with 1:11 twist rate in 270 Win or 243 Win etc. Basic thing to understand is that all small arms bullets are spin stabilized. ie, spin on the bullet stabilizes them during their aerodynamic performance. Without spin, the bullet will have an unstable aerodynamic performance, might tumble and might not hit the target at its nose which is desirable. For comparison sake, arrows and rockets are fin stabilized.

Manufacturers have worked out the ideal amount of spin that is to be given to a bullet of a particular caliber to keep it stabilized aerodynamically. The effect of spinning the bullet is achieved through rifling of the barrel (lands and grooves). The distance in which the bullet completes one 360 degree revolution inside the barrel is the twist rate and as Atiq sb explained, 1:12 means the bullet completes one revolution in 12 inches. There are many factors including length, dia, weight of projectile, length of the barrel, muzzle velocity etc that contribute towards ideal twist rate in a given weapon of a given caliber. Generally speaking, bullets of bigger dia would need a lower twist rate to stabilize them gyroscopically as compared to low dia bullet which would need a higher twist rate. And therefore, generally, a 220 grain bullet will need a lower twist rate (eg, 1:12) as compared to a 140 grain bullet of the same caliber which might need a higher twist rate like 1:11 or 1:10. But as I said, twist rate is to be considered within one caliber. Comparing the twist rates of different calibers serves no purpose and is confusing.

Twist rate effects the accuracy of a weapon (with a given ammo). If the spin is too low, bullet will remain destabilised gyroscopically and would therefore be inaccurate. Too high spinning rate will cause a drift towards the direction of spin on longer ranges, besides wearing out the barrel quickly. Therefore, spin rate has to be ideal and a correct twist rate helps in achieving the ideal spin stabilization thus a better aerodynamic performance and thus higher levels of accuracy.

Oh boy, I had to recall some of the ballistics lesson from my studies around 28 years back. Hope it helps. :mrgreen:

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 Post subject: Re: Questions regarding barrel's twist rate>

Standard Shot

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It really helped a lot KBW sb


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 Post subject: Re: Barrel's Twist Rate | Few questions

Master at Arms

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Really nicely explained. Now I unserstand. Thank you KBW Sb.


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 Post subject: Re: Barrel's Twist Rate | Few questions

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Thanks for explaining KBW and AM.

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 Post subject: Re: Barrel's Twist Rate | Few questions

Sharp Shooter

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Excellent explanation. Very easy to understand for a new gun enthusiast like myself.

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 Post subject: Re: Barrel's Twist Rate | Few questions

Regional Ambassador - Multan Chapter

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Basic question of twist rate has been explained in the best manner by the best.

I think for the sake of our curiosity, lets hear from the seniors something about,, BARREL LENGTH and THICKNESS and of course FREE FLOATING and TRIGGER PULL will come afterwards. :mrgreen:

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 Post subject: Re: Barrel's Twist Rate | Few questions

Sharp Shooter

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Mustanir wrote:
Basic question of twist rate has been explained in the best manner by the best.

I think for the sake of our curiosity, lets hear from the seniors something about,, BARREL LENGTH and THICKNESS and of course FREE FLOATING and TRIGGER PULL will come afterwards. :mrgreen:


Further to above questions, I also wanted to know the main difference between Rotating Barrel and Fixed Barrel.

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 Post subject: Re: Barrel's Twist Rate | Few questions

Sniper

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Mustanir wrote:
Basic question of twist rate has been explained in the best manner by the best.

I think for the sake of our curiosity, lets hear from the seniors something about,, BARREL LENGTH and THICKNESS and of course FREE FLOATING and TRIGGER PULL will come afterwards. :mrgreen:


This is what I understand and KBW plz set me straight on this plz.

Barrel length, thickness and free floating are all related to each other.. it is called harmonic vibration effects and bore axis shift caused by temperature of barrel flexing the steel in different ambient temperatures (air temp around barrel) Though this is insignificant in most cases in temperate climate but in extreme eventualities it will have a drastic effect on the harmonics of a barrel.

First is the "Free Floating"; This is done so that NO external pressure is applied on the barrel on any of 4 sides i.e. left right up and down. IF there is anything touching the barrel it will almost always shift the bore axis differently after each and every shot. Next time you shoot it will still have a different effect and precise placement of bore axis is next to impossible. So the way out of this phenomenon is completely scrape the under side of the barrel channel so that nothing touches the barrel and let the barrel free float so that the temperature takes its own course and bend and warp according to its own wishes. The temperature of different propellants i.e. burning rate and temperature and pressure have effects on this to some extent and the warping may be different when different weight bullet is used with different propellant. Free floating should be enough that at least THREE notes can slide under the channel upto the barrel shank tapering near the action.

Next is thickness, The thicker the barrel the minute the temperature will have warping effect as thicker barrel will heat slower; except when it is continuously being fired one round after another in short span of time. Thick barrels when coming from good barrel makers such as lilja or shillen have a very very good quality 4140 chrome moly or 416 stainless steel billet turned and center holed with pilot going though the center hole all the way. When a good steel comprising of same grained steel from tip to end is used it warps same manner and does same angle warping all the time. IF there is uneven grain mixture in steel that anomaly mixture in some portion will heat differently and will ultimately warp the barrel differently. The thicker barrels have a tendency of being rigid, see the barrels and rifles of F-Class shooters at almost 9 kg in weight for 308. THICK barrels composite stock and what not. To give you an idea of what a barrel starts with is the Contour on a scale from 1 to 8 and unturned contour with thickness of 1.25 inches and weighing in at 9 labs straight, the contour 8 stands at 1.25 inch at the chamber area and tapered slowly to 1 inch at the muzzle end weighing in at 7 lbs for just the barrel only. The Front is thinner because if front is heavy the weight will ultimately push the barrel down due to gravity and temperature flexing with time; tapering to front thinning ensures that it doesn't happen as well as front heavy of the barrel doesn't add any help to stopping warping in any manner what so ever. Contours explained here: http://www.brownells.com/.aspx/lid=1118 ... urs#Shilen

Now the barrel length: As KBW brother rightly explained that twist rate coupled with barrel length added with in muzzle velocity decides the spin rate after the exit of the bullet. There are some barrel makers that increase the barrel twist rate in the barrel with some difference from the rate at chamber (1:12) to muzzle (1:8). The more the length of the barrel the lesser twist rate can work fine, shorter barrels need more twist rate; as same amount of spin is to be taken out from shorter length of barrel with the bullet touching the lands for a lesser time hence needs more twist rate. Shorter the barrel the more stout it is and lesser prone to warping but sacrificing the Muzzle velocity.

The formula for calculating the best twist rate for a rifle is
Image
taken from http://www.chuckhawks.com/rifling_twist_rate.htm

This is a vast subject and the harmonics are all combined into one matter when it comes to barrels and is a world of its own.

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I can explain it to you, but I can't understand it for you.

Graffiti in ruins of Pompeii (in the basilica): The one who buggers a fire burns his p....


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 Post subject: Re: Barrel's Twist Rate | Few questions

Standard Shot

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great explanation Massan Sb. very well done.


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 Post subject: Re: Barrel's Twist Rate | Few questions

Standard Shot

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so basically the bullet weight and diameter is the major deciding factor for barrel's twist rate. For comparison purpose what will be the barrel twist rate, say for example in a psg-1 sniper rifle and g3 assault rifle, both employing the same 7.62 NATO??????


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 Post subject: Re: Barrel's Twist Rate | Few questions

Sniper

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Ok both the rifles fire same ammo. Soi don't think there are any differences unless the psg is made intentionally with tighter rate

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Graffiti in ruins of Pompeii (in the basilica): The one who buggers a fire burns his p....


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 Post subject: Re: Barrel's Twist Rate | Few questions

Regional Ambassador - Multan Chapter

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PSG has tighter tolerance and Polygonal rifling (grooves), along with free floated and thicker barrel as compared to G3,,, these features change the complete ball game,,, and one thing more same calibers may have different twist rates according to the preferences of bullet weight

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