Another question for the experts, or anyone who knows

[gamestalker]

How does the barrel rate of twist effect a bullet's performance?

For instance, I have a 7mm RM and it has a 1- 9 1/2 rate of twist. A common load would be high end velocity bullets in the 130 gr. to 145 gr. range. My past chrono history with those bullets has indicated I'm running between 3250+ fps with a 145 gr., and 3400 fps with a 130 gr. bullet. I also like to load high velocity 100 gr. varmit bullets that are screaming @ 3700 fps.. Am I within the limits of the barrel ROT? I also have a 1-10 barrel for the 7mm RM on a custom built M98 action that I shoot from time to time. The thing is, I haven't noticed any real difference between the 2 except the 1-10 is consistenly a little bit faster barrel.

 

[rcmodel]

Basically, you cannot over-spin, or over stabilize a bullet, as long it is tough enough to stay together on the way to the target.

I don't think you will find any light 7mm varmint bullets that aren't tough enough.

rc

 

[Walkalong]

For the very finest of accuracy, you do not want to over spin bullets, but, as rcmodel noted, as long as the bullet can hold together, it will work.

 

[rcmodel]

Just reading a test report on the STAG 3G with 1/8 barrel.

It shot these 5-shot 100 yard averages:
40 V-Max = .83"
55 FMJ = 3.01"
68 HPBT = 2.07"
69 HPBT = 1.69"
75 TAP = 1.66"
77 HPBT = 1.12"

Not the first time I have seen the same results with very light for caliber & badly "over-stabilized" bullets.

But the bottom line is, you don't know what your rifle will do until you try it.

rc

 

[flashhole]

Faster twist barrels stabilize bullets with a long bearing surface (these are usually heavy bullets). Slower twist barrels work better with bullets with moderate to short bearing surface (these are usually light weight bullets). You can cause damage to really light weight bullets when shooting them with heavy charges in fast twist barrels. I have had light weight bullets come apart and never reach the target.

Can you over-stabilize a bullet? Not unless it is at its physical limit of holding together then you can induce excessive yaw and that will have a negative effect on accuracy.

It's not the barrel ROT that is important to successful performance, it's the integrity of the bullet that will keep it together. That being said, your barrel will tell you which bullet it likes. For example, my 223 Rem with a 1:9 twist barrel will shoot light weight bullets (45 grains or less) very accurately and it will shoot bullets 63 grains and heavier very accurately but it won't shoot the middle weight class of bullets worth a darn. Bearing surface has a great influence on shootability (is that word?) in your gun. You simply have to experiment with a few to determine which ones will work and which ones won't. It is not an exact science.

 

[dbarnhart]

Barel twist and bullet weight are interrelated. There is a nice chart on Brownells website:

http://www.brownells.com/.aspx/lid=11182/GunTechdetail/Calibers-and-Twists

 

[gamestalker]

That deffinitely answered my question well guys. Over the years I had light weight HP bullets I load for varmit hunting, that were producing frag holes on the paper as far out as 200 yds.. And yes, they were moving along at maximum velocity. Of course I knew most of what I was experiencing was the result of exceeding the velocity limits for that bullet and that barrel, but wasn't certain if rate of twist was a factor or not.
Thanks again!
Merry Christmas

 

[amlevin]

Bullet stability requires the right spin rate. Too Slow and it wobbles, too fast and it can come apart. Another often overlooked result of overspinning a bullet is that it may not reach the target "nose first" but somewhat sideways. Overspinning can cause the bullet to not stay tangent to the arc it flies but want to remain more in the same attitude as it left the barrel. Just like a slow bullet can keyhole, so can an over-spun bullet. Perhaps not as big an issue as too slow a spin rate but still an issue.

Here's a calculator for determining if a bullet is stable or not with different twist rates:

Miller Stability formula calculator

 

[ArtP]

Though I can't prove it, I've read from several sources that too fast a spin will amplify imperfections in the bullet, even if it holds together.

I will say in my most accurate rifle, a 243, if I go under a certain weight, I lose accuracy regardless of bullet brand.

 

[Walkalong]

too fast a spin will amplify imperfections in the bullet

Just like an out of balance wheel.

 

[beatledog7]

But given the silly high rpm of a bullet in flight, any surface imperfection or COG-induced wobble would be well in play even for a bullet spun at just the right speed.

MV = 2800 FPS
9-Twist RPM = 2800 x (12/9) x 60 = 224,000 RPM

MV = 2800 FPS
8-Twist RPM = 2800 x (12/8) x 60 = 252,000 RPM

If a bullet is already wonky enough to be sent off flight path at 224K RPM, how would you know if the 12.5% faster spin made it worse?

 

[ArtP]

Another way to approach that is, in your twist examples, it's about a 12% difference. Competition BR shooters do care about a a tenth of an inch, and they will not over twist by 12% just because it seems insignificant.

Sometimes out of balance wheels will shimmy at 45mph, then go smooth until 75mph is reached where they shake again. Sweet spots I guess. I can't explain why at one speed a car shakes, then is smooth at another speed.

I am not saying it matters to the OP or to me. But I do like to know as much as I can about subjects I'm interested in.

If the term "Don Miller twist rate" is Googled, far more information about twist will be revealed than the vast majority of us know. I had always wondered why not error on the side of too much twist when in doubt -- I couldn't hurt, or could it?. Or just have an overly fast twist for everything I shoot -- 1:8 ought to work for everything. I found the answers to those questions when I spent a half hour reading about Don Miller and his explanation of twist.

 

[beatledog7]

Art, good point.

I'll probably never be as good as any of my rifles are, so it'll likely never make much difference to me either way. It's just interesting to think about.

I do pay attention to the twist rate / bullet length conventions. My .223 is 1:9, so I don't bother with really big bullets. Anything the weighs over 70gr is probably too long for my rifle to stabilize, so says the wisdom. More relevant is the fact that I didn't buy it to shoot buffalo in the first place, and >70gr bullets will take all the critters it's meant for.

 

[Walkalong]

I'll probably never be as good as any of my rifles are

That's me with my pistols. I love to shoot them, but I will never bring out their full potential.

 

[NeuseRvrRat]

Sometimes out of balance wheels will shimmy at 45mph, then go smooth until 75mph is reached where they shake again. Sweet spots I guess. I can't explain why at one speed a car shakes, then is smooth at another speed.

i can. it's due to the natural frequency of the system, a wheel in this case. the system will vibrate the most at its fundamental frequency and the harmonics of that frequency (multiples of that frequency). so if the fundamental frequency of the system happens to fall at 20 Hz, it will also experience high vibration at 40 Hz, 60 Hz, 80 Hz, and so on.

this phenomenon is the reason the Tacoma Narrows Bridge shook so much. at times, the frequency of the vibration imparted by the wind over the bridge would match the bridge's natural frequency. the system's displacement will just keep increasing until something breaks.

the same basic principles apply to something as simple as a mass on a spring with a force applied or something as complex as a spinning bullet undergoing the effects of air resistance.