So here is a question that I have found little evidence to point me in the right direction. I'm currently building a precision ar-10 and I am having a hard time deciding what barrel length to go with. My plan is to use either a 11 or 12 twist however what i need to know is how many rotations should the bullet have inside the barrel to be properly stabilized. So for example if I go for the 12 twist would it be worth having a 24" barrel to allow the bullet 2 full roations in the bore or is 1.whatever roations fine enough? Im not concerned with the overall length of the rifle this is going to be a range toy more or less, but should the bullet have more roation time in the barrel or am I over thinking this? Thanks for any input...
Barrel length vs Stability?
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Bob Wright
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Spin is spin. Time in the barrel is not what I would look at. It's velocity at the muzzle and twist rate to arrive at some rotation number.
Go to Berger website and use the stability calculator to give you an idea of what parameters are typically used to determine how a bullet is stabilized as there are many variables.
Go to Berger website and use the stability calculator to give you an idea of what parameters are typically used to determine how a bullet is stabilized as there are many variables.
sable tireur
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You're overthinking this particular aspect. The number of complete rotations in a barrel length is not necessary for your project. How do you know the twist rate you need before deciding on the bullets first. It's the length of the bullet which determines the twist rate.
Depending on your cartridge of choice, decide on the type, style and length of the bullet. Usually, many of the bullet makers write the twist rate on the boxes of bullets nowadays or they state it in their ads or on their website. If not email or call them to get that information.
If you know the direction you want to head in, let us in on your choices and we can maybe lend a hand.
If you use my app
Eagleballistics it does this for you you can play with velocities and twist until you find what you like it even gives you rpm's
Www.eagleballistics.com
Eagleballistics it does this for you you can play with velocities and twist until you find what you like it even gives you rpm's
Www.eagleballistics.com
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HNDLDR
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Welcome to the forum.
What caliber are you building?
What caliber are you building?
Thank for your input, so this project is also somewhat of an experiment. Cartridge is 308win. The experiment is going to be what variety of bullets can I shoot and their accuracy. Most of my bolt action 308's are 10 twist and my best accuracy comes from 175gr shooting a little over a half inch at 100yds. Now i have tried lighter rounds in them and obviously got fairly poor accuracy. Going off of Bergers calculators and other charts it seems that 12twist barrels seem to provide the proper stability for rounds as low as 147 and up to 175, so the experiment is going to be what groups I can get with a lighter twisted barrel and also if cheaper 147gr rounds can shoot just as accurately if, according to charts, as the heavier bullets in a tighter twisted barrel...You're overthinking this particular aspect. The number of complete rotations in a barrel length is not necessary for your project. How do you know the twist rate you need before deciding on the bullets first. It's the length of the bullet which determines the twist rate.
Depending on your cartridge of choice, decide on the type, style and length of the bullet. Usually, many of the bullet makers write the twist rate on the boxes of bullets nowadays or they state it in their ads or on their website. If not email or call them to get that information.
If you know the direction you want to head in, let us in on your choices and we can maybe lend a hand.
308Win. Not my first 308 I shoot plenty of bolt guns, this is my first ar10, built plenty 5.56's and very was too much of a fan of factory ar10s so this is my experiment.
The way I have come to answer this question is based on the intended target, or purpose, then desired range. Twist must be sufficient to stabilize, some say you can't over stabilize, I don't believe that's true unless your bullet is perfectly balanced, like a monolithic solid. Velocity will determine energy at a given range, you need enough energy to have the correct results on the target. Once you work through that you can figure out the type rifling, direction of rotation and if gain twist is helpful.
(Now who is over thinking?)
It is fun though, enjoy your journey!
sable tireur
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Experimentation is a large part of why so many of us get into reloading.
The .308 Win. Has been standard in the field of accuracy for a very long time. So there is a ton of information already available which might prove useful to you before determining how you want to proceed.
Palma shooters were at one time limited to Sierra produced .308 Win. ammunition loaded with the 155 gr. Palma bullets. These being shot to 1,000 yards using open sights.
The AR10 platform is not new to this endeavor at all, although mostly the .223 or 5.56. My experience rose out of the need to shoot movers in the PRS competition field. It has now sprouted into a category of its own in PRS except the cartridge has changed significantly.
I suggest doing some searching for background data first then jumping into the pot and working on your own theories.
Remember, cheap bullets are cheap for a reason. They will usually not perform up to our standards of accuracy.
Enjoy the process!
The .308 Win. Has been standard in the field of accuracy for a very long time. So there is a ton of information already available which might prove useful to you before determining how you want to proceed.
Palma shooters were at one time limited to Sierra produced .308 Win. ammunition loaded with the 155 gr. Palma bullets. These being shot to 1,000 yards using open sights.
The AR10 platform is not new to this endeavor at all, although mostly the .223 or 5.56. My experience rose out of the need to shoot movers in the PRS competition field. It has now sprouted into a category of its own in PRS except the cartridge has changed significantly.
I suggest doing some searching for background data first then jumping into the pot and working on your own theories.
Remember, cheap bullets are cheap for a reason. They will usually not perform up to our standards of accuracy.
Enjoy the process!
lancetkenyon
Well-Known Member
Try and keep RPMs to around 300k-320k max if using cup & core bullets.
(Velocity x 720) ÷ twist rate.
Thin-skinned copper jackets can start to let loose above that 320-330k RPM rate.
Monos do fine on fast twist.
Heavier bullets need faster twist to stabilize properly.
I have a bunch of 6.5mms w. 1:7.5" twists. The 130s do just fine in them.
Fastest RPM round I shoot is a 22GT w. 1:7". 77TMKs @ 3200 are at 329k RPM. They are wicked accurate still, and absolutely crush coyotes with the terminal velocity on impact.
If a 1:10" .308 won't shoot a 140gr well @ 3000fps, I highly doubt the twist rate is to blame. That is only going to be in the 216k RPM range. Pretty slow spin rate actually.
(Velocity x 720) ÷ twist rate.
Thin-skinned copper jackets can start to let loose above that 320-330k RPM rate.
Monos do fine on fast twist.
Heavier bullets need faster twist to stabilize properly.
I have a bunch of 6.5mms w. 1:7.5" twists. The 130s do just fine in them.
Fastest RPM round I shoot is a 22GT w. 1:7". 77TMKs @ 3200 are at 329k RPM. They are wicked accurate still, and absolutely crush coyotes with the terminal velocity on impact.
If a 1:10" .308 won't shoot a 140gr well @ 3000fps, I highly doubt the twist rate is to blame. That is only going to be in the 216k RPM range. Pretty slow spin rate actually.
VarmintBob
Well-Known Member
barrel length is not relevant to spin stability other than the velocity it generates. Go to the JBM online twist stability calculator. That one handles plastic tipped as well as all metal bullets.
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While not necessarily the most accurate firearms, alot of old muzzle loaders had 1:66-1:72 twist. Probably 1/4-1/3 rotation of the projectile and some were pretty darn accurate.
Not apples to apples, but illustrates the point.
Not apples to apples, but illustrates the point.
Bob Wright
Well-Known Member
With a patched round ball, that is the ideal twist.
