Twist Rates and Velocity?

[MudRunner2005]

No, say a bullet is shot from a 16.5" barrel at 2700 fps. 10 " twist
Or a 26" barrel at 2700 fps 10" twist. Its still going to be doing 1 revolution every 10"

Yes, and longer barrel equals more stabilization b/c of more time touching the rifling. If that's not the case, then why are pistols less accurate than rifles?

 

[MudRunner2005]

IMO, twist effects velocity by resisting travel down the bore.
It takes more energy to push a bullet in a 10 twist than it would in a 20 twist.
In "general" that creates higher pressure due to more bore resistance which MAY increase velocity.
On the other side, if you are at max pressure then you may have to reduce the load and go the same or slower with a faster twist.

Also IMO, a bullet should be shot at the slowest twist for ALL conditions you will face.

First, bullet RPM slows down at a much slower rate than forward velocity so it effectively becomes more stable the longer it fly's;

Second, no bullets are perfect.
While in the bore a bullet rotates around the centerline of the bore.
When the bullet emerges from the muzzle it rotates around the bullets center of gravity.
This "jump" to the CG is dependent on the difference between the CG and the centerline of the bullet formed in the bore AND velocity.

There is an article on the Lilja website on this:
Lilja Precision Rifle Barrels - Articles: A Look at Bullet Imbalance and Twist

Also there was something on the web that I can't find right now that experimented with machining an imperfection in a group of bullets to create an imbalance.
Then they indexed and shot the bullets in 90 degree increments and the bullets all grouped to the same 90 degree group as predicted:

edge.

Thank you very much for this example. It has helped solidify what I said earlier.

 

[paphil]

I don't think barrel length causes instability, short pistol barrels are harder to aim and hold steady, but if a pistol has a scope and is held in a vice, it will shoot amazingly well. shorter barrels flex less and also have less time for the exploding powder to push the bullet. That is why we use faster burning powder for pistols than rifles and also why longer barrels give more velocity with the same load. The spin rate is applied in the first few inches of the barrel and is probably responsible for at least some of the throat erosion noted in hot loads. The bullet goes from a dead standstill to about 80 percent of muzzle velocity and 100 percent of spin rate in the first three inches of travel. Some of you pistol guys can correct me on that but most of the torque is applied in the first few inches. Wear and tear on the bullet and the barrel happen at this time. If the bullet doesn't begin to spin immediatly, the jacket would be stripped from the bullet by the rifiling. Faster twist barrels will wear faster than slow twist barrels in the throat area. Variable twist barrels are an interesting concept but I don't have any experience with them.

 

[Cold Trigger Finger]

Nope. You better do some research on that. Length of barrel has zero to do with RPM's. It's the velocity.

A bullet coming out of a 22" 10 twist barrel at 3000 fps will spin just as fast as a bullet coming out of a 30" 10 twist barrel at 3000 fps.

I guess I should have read more before I replied. :rolleyes

The only time a longer barrel spins a bullet faster is when it has a Gain Twist

 

[Cold Trigger Finger]

I guess I'm just cheap, as I get sick of "Standard " twist barrels.
To get a useful barrel in the 223 , 22/250 , 220 Swift, and most others, a factory rifle needs to be rebarreled .

A 9" twist 223 with a 24" barrel will push a naked 70 gr TSX 3000 fps. An awesome bullet at a very useful velocity. But that combo produces oval holes in the target and from 3/4"-12" groups/ patterns at 100 yards.
If the bullets are pushed fast enough to stabilize at close range when the bullet slows sufficiently it will become unstable again. Most other calibers are similar or worse tho the 6.5 s should be standardized with a 7.5"

Art Alphin / A-Square did a lot of work with twist rates in the larger calibers and had great results. My last 416 Rem had a 10" twist and that twist, tho 4" faster than standard was in no way a liability. It did cause slightly higher pressure. I had to use slightly slower powder. But still got book velocity.

 

[MudRunner2005]

I don't think barrel length causes instability, short pistol barrels are harder to aim and hold steady, but if a pistol has a scope and is held in a vice, it will shoot amazingly well. shorter barrels flex less and also have less time for the exploding powder to push the bullet. That is why we use faster burning powder for pistols than rifles and also why longer barrels give more velocity with the same load. The spin rate is applied in the first few inches of the barrel and is probably responsible for at least some of the throat erosion noted in hot loads. The bullet goes from a dead standstill to about 80 percent of muzzle velocity and 100 percent of spin rate in the first three inches of travel. Some of you pistol guys can correct me on that but most of the torque is applied in the first few inches. Wear and tear on the bullet and the barrel happen at this time. If the bullet doesn't begin to spin immediatly, the jacket would be stripped from the bullet by the rifiling. Faster twist barrels will wear faster than slow twist barrels in the throat area. Variable twist barrels are an interesting concept but I don't have any experience with them.

Wait wait wait wait wait........But Montana said this is false when I mentioned that earlier in the thread.

Based on what you said, this is exactly why Roy Weatherby used free-bore in his calibers and rifles instead of having them touch right up on the lands....Correct?

For the record, I am in 100% agreement with everything you just wrote. Keep up the great work. I see you know your stuff. It is good to have folks like you on this site. That is what makes this place so great.

 

[MontanaRifleman]

Wait wait wait wait wait........But Montana said this is false when I mentioned that earlier in the thread.

No I didn't.

 

[paphil]

Montana said that with the same muzzle velocity, regardless of barrel length ,the spin would be the same. I totally agree with that. With a 1 in 9 twist, the bullet rotates once each 9 inches of travel . If a 1/9 travels 81 inches in one minute , it would have revolved 9 times thus would have 9rpm. Same bullet travels same distance in 30 seconds, now is rotating at 18 rpm. When it travels at 3000 feet/ second ,it rotates at 240,000 rpm. The faster it goes, the faster it spins, but still rotates only once for each 9 inches traveled. If you go to a 1 in14 twist, the 3000 fps bullet only rotates at 154,286 rpm. That is why some light varmet bullets don't do well in high speed fast twist situations. 300 thousand rpms tends to be hard on the pistons!

 

[Bart B]

Here's another "twist" in this very interesting thread. . . .

Which rifling twist presents the greatest angle of the lands to the bullet?

30 caliber, 1:11 twist

22 caliber, 1:8 twist

And thanks, Edge, for finding then posting those "unbalanced bullet" pictures. Those tests are exactly what Dr. Frederic Mann did in the early 1900 testing unbalanced lead bullets in his "Mann rest" barrel. It was published in his book "The Bullet's Flight from Powder to Target" that came out in 1907. Pretty savvy shooting stuff from a well known surgeon at the time.

 

[edge]

The Mann rest is pretty interesting, I had to look it up

I also found the article that those pictures came from for anyone interested in reading:

Bullet Imbalance by Harald Vaughn

Bullet Imbalance

edge.