twist rate question?

[bootsmcguire]

I am looking into building another 243AI for shooting bullets 75gr and less. Primarily 58gr V-Max's but also 55gr Noslers and Blitzkings when I find deals.

Normally I go with the theory of going with a faster twist to give myself more options in bullet weights later should I change my mind. But I wonder if I go with a slower rate of twist will I get a decent gain in velocity?

I am strongly considering a CBI barrel in a 28" or 30" Bull, but I see they are listed as a 8" twist only. I am not dead set on a CBI, but I have had great luck with my 6BR in CBI so might as well stick with what works, but if a 1-14 will get me decent gains, then I will start looking elsewhere. I have had multiple barrels for a single caliber in the past but by coincidence they all had the same twists so I have no 1st hand experience comparing drastic changes in twists on the same caliber.

So I guess my question is: Since a 1-14" should easily stabilize 58gr bullets, how much am I going to gain in velocity if I go with a 1-14" over the 1-8"? (assume all variables are equal other than twist)

My current 243AI is a 26" Savage SS Fluted 243 barrel that was rechambered and has the 1-9.25" twist and it shoots the 58's great, but I am just wondering if I can get more from a slower twist or just stay with a faster twist and quit worrying about it ???

[keeki]

with a 1-8 I would think you would have problems getting those 58's to stay together

[82boy]

To start you wil get more velosity from a fast twist over a slow twist rate barrel. (Think about it, the faster twist alows the bullet to build more pressure behind it.) Personaly I would say that a 243AI is SUPER WAY overbore for a 58 gn bullet.

[Kelly Jackson]

Boots - I just rebarreled my coyote rifle with a 26" 243AI 12 twist.
For referance I was shooting a 24" 9.25 twist standard 243 and 55gr NBT's bumping 4000 MV.

The 12 twist got to 4192MV with ease. My load right now is running 4142MV.
My scope hight is 1.8 and when sighted in .7 high at 100 my group at 300 centered .82 low.
Very happy with how this turned out.

I type all this to give you a referance knowing there are many other factors to consider.
Least of all chrono Vel accuracy.

Just rechambered my 9.25 to AI also, mainly to use to fireform brass for my AI's. Have not ran it throught the chrono yet. It is now 23.4" long. Was banging steel at 500 with it late yesterday afternoon fireforming some new brass. The 9.25 is hard to beat. you can run 105's at steel and down to 55's for a lazer beam.

Kelly

[bootsmcguire]

To start you wil get more velosity from a fast twist over a slow twist rate barrel. (Think about it, the faster twist alows the bullet to build more pressure behind it.)


I guess I didn't think about the faster twist keeping the bullet in longer and building more pressure. My thinking was taking me to "Slower twist = less resistance = outta the hole faster".



Personaly I would say that a 243AI is SUPER WAY overbore for a 58 gn bullet.


I suppose it is, but it sure is fun and my current 243AI barrel is very very flat and accurate. Easily hits 4000fps with the 58's. I have been thinking about going to a heavier bullet (maybe a 75gr V-Max), but the 58's are cheap and shoot well so I dunno.



with a 1-8 I would think you would have problems getting those 58's to stay together


I was wondering about that too. My 1-9.25" I have now launches them at 4000fps and have yet to have one come apart but for all i know it could be a hundred RPMs from going "pop" in flight.

Kelly, It sounds like you got a tad faster by going with a slower twist, but then again each barrel is unique. I am considering all the factors, I just was going back and forth on "Twist vs. Velocity" and figured I would see what y'all were thinking. I was just thinking that if I am going to order another custom barrel and by going with a different twist I could get a significant gain in Velocity to flatten it out even more, then why not?
Thanks for posting your findings and I will agree the 1-9.25" is a nice twist, I was just hammering ideas in my head ;)

Looks like if the 1-8" won't jump up the RPM's to awful bad, then it may be the way to go.

[keeki]

given that you use the same amount of powder in the 12 as you do the 8, yes the 8 will build more pressure. But, you will be be able to use more powder in the 12twist before reaching max pressure.

[Jamie]

To start you wil get more velosity from a fast twist over a slow twist rate barrel. (Think about it, the faster twist alows the bullet to build more pressure behind it.)


I guess I didn't think about the faster twist keeping the bullet in longer and building more pressure. My thinking was taking me to "Slower twist = less resistance = outta the hole faster".


Your thinking is correct. As Kelly stated, the slower twist will give a higher MV over the faster twist. This is basically due to the reduced friction of the slower twist. When a bullet hits the rifling it doesn't want to spin, it wants to keep going just like it is. So the more it is required to spin the more energy that is required as well as the additional friction. I am not going to tell you you will get 200 fps faster every time, but if 58 grain bullets are what you want to shoot then a 1:8 twist will slow you down compared to a 1:14.

[thomae]

To start you wil get more velosity from a fast twist over a slow twist rate barrel. (Think about it, the faster twist alows the bullet to build more pressure behind it.)


I guess I didn't think about the faster twist keeping the bullet in longer and building more pressure. My thinking was taking me to "Slower twist = less resistance = outta the hole faster".


Your thinking is correct. As Kelly stated, the slower twist will give a higher MV over the faster twist. This is basically due to the reduced friction of the slower twist. When a bullet hits the rifling it doesn't want to spin, it wants to keep going just like it is. So the more it is required to spin the more energy that is required as well as the additional friction. I am not going to tell you you will get 200 fps faster every time, but if 58 grain bullets are what you want to shoot then a 1:8 twist will slow you down compared to a 1:14.


I am not going to agree or disagree, but rather ask all of you to back up your respective "assumptions" with hard data.

The reason I ask this is because when you lubricate (let us use a moly-coated bullet as an example) a projectile, you also reduce the bullet-to-barrel friction, so the bullet goes down the barrel with less pressure behind it, and the resulting velocity is slower than it would be with the exact same conditions and a non-moly coated bullet.

A slower twist may give a higher muzzle velocity, perhaps. I am not sure. But perhaps, as in the case of a moly-coated bullet, because there is less friction, it might actually result in a slower muzzle velocity.

I have an opinion, which I am careful to keep to myself, but also have an open mind. Mostly, I would like to hear actual experimental results, not guesses.

This is neither a flame nor a rant and is very respectfully submitted.

[keeki]

true. but since it is moly coated your pressures are lower, so you can add more powder to get the pressures back up which is gonna= more velocity

[bootsmcguire]

thomae, you raise a good point, and so does keeki by being able to add powder and thus in theory gain velocity.

I am guessing that any velocity gains gotten by such a slow twist as a 1-14" vs. a 1-8" are going to be minimal meaning less than 100fps either way and not be able to justify the cost of a custom vs. an high quality prefit.

I dunno, keep the thoughts and hard data coming and lets see where this goes. :D

[keeki]

Im with ya on the minimal gains in velocity. I have a 25-06ai 10twist and a 12twist. I can get more velocity out of the 12 twist via more powder before I hit my max load but its only like 125fps. However the 12 does shoot the small stuff better

[Jamie]

Thomae, I don't blame you for asking. It is similar to the moly bullet view but the friction from the 1:8 twist is a good bit more and if you shot an identical round down both the slower twist would really be faster. The slower twist barrel will allow you to use a little more powder because the pressure is slightly lower. Like Keeki, mine came from having a 22-250 in a 1:12 and 1:7 twist and shooting with a buddy who had two 6mm Rems with different twist. The slower twist were always faster. Plain and simple. I don't think 125 fps is minmal though.

[JASmith]

On the negative side, the fast twist results in an almost trivial reduction in muzzle velocity. Look, for example, at the rotational energy for the fastest twist vs the rotational energy in the slowest twist. You will find that the difference is less than a few 10's of ft-lbs. Conservation of energy tells us that the difference in muzzle velocity, while lower for the faster twist, is so small that one would be hard pressed to detect on a chronograph.

On the side of going to slow twist, the higher rotational rates tend to cause small variations in bullet balance to magnify more than would be the case for slower twist rates. This is why Berger, for example, recommends going with the slowest twist that will stabilize the bullet to get the best accuracy potential.

[darkker]

Well, If you want "options" in your VARMINT weight bullets, stay off of the fast twists.
As has been touched on, Centrifugal force is what destroys bullets, NOT velocity.

If you want to shoot on the "cheap" with light jacketed bullets, they do not tolerate high RPM's before destruction. Here is the formula for RPM: MV * 720 / twist = RPM
I won't pretend to say that this translates to other diameters, but here is what I found with 22-cal bullets after calling the manuf's.

http://www.shootersforum.com/ballistics-internal-external/78073-f-y-i-varmint-bullet-rpm-limits.html

Remember that if the maker won't tell you, or say "You can't spin them apart"; to hang-up the phone, they have no clue what you are talking about...
Speer's monkey's only answer in FPS, so you have to specifically ask them the twist tested, then do the math with the formula. Nosler claims(specifically asked their head ballistician) that ALL of their bullets(22-cal) survive 4400fps with a 6-twist.... Bullets that can survive 528,000 RPM, will not open with any somewhat lower velocity; it's physics. So given my experience with their BT's, I would say they are around Hornady V-max's strength.

[bootsmcguire]

Looks like for what I am wanting to do a 12 or 14 twist may be the ticket. Looks like a 100,000 rpm difference between the 8 and 14 twists with my target velocity so looks like that will be my direction at this moment.

Keep the opinions and facts coming in since it may be a bit before I order, I am going to have to sell off a few items to pay for the new barrel so changing my mind is still possible :D

[308law]

Well, If you want "options" in your VARMINT weight bullets, stay off of the fast twists.
As has been touched on, Centrifugal force is what destroys bullets, NOT velocity.

If you want to shoot on the "cheap" with light jacketed bullets, they do not tolerate high RPM's before destruction. Here is the formula for RPM: MV * 720 / twist = RPM
I won't pretend to say that this translates to other diameters, but here is what I found with 22-cal bullets after calling the manuf's.

http://www.shootersforum.com/ballistics-internal-external/78073-f-y-i-varmint-bullet-rpm-limits.html

Remember that if the maker won't tell you, or say "You can't spin them apart"; to hang-up the phone, they have no clue what you are talking about...
Speer's monkey's only answer in FPS, so you have to specifically ask them the twist tested, then do the math with the formula. Nosler claims(specifically asked their head ballistician) that ALL of their bullets(22-cal) survive 4400fps with a 6-twist.... Bullets that can survive 528,000 RPM, will not open with any somewhat lower velocity; it's physics. So given my experience with their BT's, I would say they are around Hornady V-max's strength.

I think the Noslers are a little tougher than most other varmint bullets. I have shot them in 223, 22-250, and 243 at prairie dogs, I seem to only get good performance with very high impact velocity's. I also think(just an opinion based on experience, not a proven fact) that bullets with a faster twist will expand more violently with a similar velocity.

[scope eye]

A fast twist barrel like an 8 twist will be slower FPS wise, due to the fact that they energy is being used to rotate the bullet "RPM" VS propelling it forward, the hundred thousand RPM difference between a 8 twist and a 14 twist is a substantial amount of forward mass energy loss, so basically you are using they energy to spin the bullet rather than project it in a forward motion, twist equals friction which causes resistance.

Thanks Dean

[bootsmcguire]

I also think(just an opinion based on experience, not a proven fact) that bullets with a faster twist will expand more violently with a similar velocity.

Now that makes sense since the higher RPMs should be putting stress outward against the jacket making it more likely to upset upon impact.

In my case I know that the 58gr V-Max upsets nicely at 250,000 RPMs (using darkker's formula and plugging in my data for the 6BR), at my desired minimum velocity from a 12 twist should yield between 240k - 260k RPMs so that should work out nicely as far as upset goes.



A fast twist barrel like an 8 twist will be slower FPS wise, due to the fact that they energy is being used to rotate the bullet "RPM" VS propelling it forward, the hundred thousand RPM difference between a 8 twist and a 14 twist is a substantial amount of forward mass energy loss, so basically you are using they energy to spin the bullet rather than project it in a forward motion, twist equals friction which causes resistance.

Thanks Dean
Sounds very logical as well. Wish I had the funds to buy 2 Identical barrels with different twists and prove all this one way or the other.

[airaddict]

Uh oh.....when boots posts in the classifieds it usually good stuff !! ;D

i did enjoy all the opinion. Good stuff in here!

brian

[nsaqam]

It takes VERY little energy to spin a bullet quicker since the diameter is so small and the majority of the mass is in the center of the bullet.

The ONLY downside to a fast twist is that lousy bullets can come apart.
The solution of course is to use good bullets.