Sometimes I have question pop into my head that I just can't find information on. Here's my question. If a rifle barrel has say, 1 twist in 10 inches, and a bullet leaves the barrel at a speed of 2500 fps, how fast is the bullet spinning while in flight to the target? Since while traveling down the bore it only completes one revolution in 10 inches, does the same ratio of 1:10 apply while in flight to the target? The reason I ask is someone posted that the petals of an X bullet act like a buzz saw upon penetration of an animals vitals. But, if it is only spinning at a rate of 1 inch every 10 inches, that wouldn't be possible.

Another interesting thing to ponder,
I have heard that a .223 fired from an M16 is doing 130.000 rpm.

quote:

Originally posted by mikeh416Rigby:
how fast is the bullet spinning while in flight to the target?

The reason I ask is someone posted that the petals of an X bullet act like a buzz saw upon penetration of an animals vitals. But, if it is only spinning at a rate of 1 inch every 10 inches, that wouldn't be possible.

Mike,

This is a very complex question.

First you need to look at your bullet as having two velocities.... The forward velocity we always talk about and the rotational velocity. While in flight, the forward velocity slows faster than the rotational velocity. So the bullet starts making one revolution in a lesser distance than the 10" length it did at the exit from your barrel.

Upon impact things really change! The forward velocity is rapidly slowed and the rotational velocity does as well, but the inital revolutions shortly after impact are very close together as to length!

So to answer your question: Yes a "buzz saw" effect is probably a good analogy. Maybe the fastest drill you ever saw with the drill point melting back is a better one because all the helix isn't gone, just shortened considerably for a brief moment.....I don't see the sharp edges of the "X" bullet being any real advantage though. Conventional bullets experience this same phenomenon at impact and blunt force trauma may even be tougher on our target than any buzz saw effect...........

BigRx

quote:

Originally posted by The Specialist:
Another interesting thing to ponder,
I have heard that a .223 fired from an M16 is doing 130.000 rpm.

Specialist,

If it the later 1 in 7" twist M-16 then the RPM at the muzzle for a 3000fps load is 308,571!

BigRx

I think this has more to do with PR "spin" than the bullets effect on game animals.

This is a very simple problem to solve, but seems to come up often. If a bullet leaves a bore with a twist rate of 1 twist per 12 inches, then the bullet makes one full rotation for every foot of travel as it approaches the target. If the target is exactly 12 inches deep and IF you eliminate all rotational friction during penetration, the bullet will only make one full rotation during its path through the target - thus destroying any notion of buzz saw like performance.

In regards to RPM, rotations over time, bullets can achieve extremely high rotational velocities. However, considering rotation over distance, bullets only achieve 250 to 400 rotations within 100 yards.

quote:

Originally posted by Zero Drift:
This is a very simple problem to solve, but seems to come up often. If a bullet leaves a bore with a twist rate of 1 twist per 12 inches, then the bullet makes one full rotation for every foot of travel as it approaches the target. If the target is exactly 12 inches deep and IF you eliminate all rotational friction during penetration, the bullet will only make one full rotation during its path through the target - thus destroying any notion of buzz saw like performance.

In regards to RPM, rotations over time, bullets can achieve extremely high rotational velocities. However, considering rotation over distance, bullets only achieve 250 to 400 rotations within 100 yards.

Nice linear logic Zero Drift.......

Except that it doesn't work that way when two variables slow down at different rates.

BigRx

Granted, forward velocity bleeds off far more rapidly than rotational velocities, however, the bullet is only rotating the same number of rotations as imparted by the twist of the bore.

If it is 1:12, the bullet only makes one rotation per foot of travel no matter if it is traveling at 1 foot per second or 3000 feet per second.

quote:

Originally posted by Zero Drift:
Granted, forward velocity bleeds off far more rapidly than rotational velocities, however, the bullet is only rotating the same number of rotations as imparted by the twist of the bore.

If it is 1:12, the bullet only makes one rotation per foot of travel no matter if it is traveling at 1 foot per second or 3000 feet per second.

I think I see where I was getting screwed up. Using my original model of a 1 in 10 twist at 2500 FPS, then in 1 minute (if the velocity remained constant, which of course it can't), then the bullet would travel 150,000 feet, or 1.8 million inches. Therefore, with a 1 in 10 twist, over 1 minute, the bullet would have completed 180,000 revolutions. But the bullets flight to the target is only a split second, not a minute. And also, if the animals body is 20 inches wide, and the bullet speed remained constant, then the bullet would only complete 2 full turns while traversing the body.

Sometimes I think too much.

I agree with BigRx here...his post is based on sound logic. Physics dictates that with one variable--velocity--slowing at a greater rate than the other variable--bullet twist--the revolutions will actually become greater over the same distance.

RPM = 720 X Muzzle Velocity/Twist Rate

Bullet leaves the barrel at 3,000 fps from a 1:10 twist = 216,000 RPM = 3600 RPS = 370.8 revolutions per 100 yards. Flight time at 3,000 fps is 0.103 seconds - this takes into account the deceleration of the bullet and the relative constant rotational speed of the bullet...

That's why if you shoot at really low velocities and high barrel twists the buzz saw effect takes place, and a Barnes X can grind a whole buff into hamburger, as its linear stability goes to pot at low velocities, and the bullet keeps rotating around inside the buff until it completely stops. Then all you need is a bunch plastic bags.

Bill - You ain't right in the head...

quote:

Originally posted by Zero Drift:
RPM = 720 X Muzzle Velocity/Twist Rate

Bullet leaves the barrel at 3,000 fps from a 1:10 twist = 216,000 RPM = 3600 RPS = 370.8 revolutions per 100 yards. Flight time at 3,000 fps is 0.103 seconds - this takes into account the deceleration of the bullet and the relative constant rotational speed of the bullet...

Zero,

You were doing fine until you got to revolutions in 100 yards. Why is it 370.8 instead of 360 which is how many 10" sections are in a hundred yards?

But let's talk about 100 yards. What is the 100 yard velocity? Let's pick one....2500fps

216,000 RPM ..... Let's say we lose 2% of this RPM over 100 yards. That's 211,680 RPM.

Do you agree our bullet is going 2500fps and rotating 211,680 RPM at 100 yards per this?

Let's go back to the muzzle. Our bullet exits making one full turn in its helix every 10" per our twist..... A given.

Both velocities are time related. (Forward and rotational.) But one is slowing at a faster rate....

Back to one hundred yards. If you calculate the forward velocity (2500fps) against rotational velocity (211,680 RPM) you will find the length of one rotation of our helix is now 8.5" instead of 10"!

BigRx

^^Once again Big's logic is right on the mark. You can tell he has strong engineering background. His argument is based on hard and fast rules of physics. I completely agree with him.

BR - You seem confused that I am somehow disagreeing with you. I have never stated that the progression between the degradation between velocity and rotational velocity is liner. I have made it a point to say exactly the opposite.

I simply used a liner example to quash the drill bit or buzz saw myth. If you wish to dissect the discussion, please have at it and enjoy yourself...

ZD,

Okay, smarty pants. Awhile back Doctari is calculating, in some article for Rifle mag. I think, the rotational energy of a bullet, and came up with an energy on the same order of magnitude as the linear energy. What a croc.

You wanna try?

quote:

Originally posted by Zero Drift:
I simply used a liner example to quash the drill bit or buzz saw myth. If you wish to dissect the discussion, please have at it and enjoy yourself...

Zero,

Please re-think this. Think about that (your) bullet spinning twice the speed of a dentist's drill. Think about the centrifugal force built up in that spin you have driven up to six figures by the extremely rapid traverse down your spiraled tube....

Then think about that bullet making contact not unlike hitting a brickwall somewhat at the speed it is going. Where does all that RPM and centrifugal force go to? Do you really believe it is gone in just two revolutions?

BigRx

BR - Since you seem content to carry on, I have a question for you - How many bullets have you recovered from game? I have a bunch of them. Many of them Nosler Partitions which like to fragment in front of the partition. In almost every case, the fragments are carried straight back and not twisted to the side like they were drilled into the animal.

Further, if you have ever seen slow motion pictures of bullets hitting ballistic gelatin, you do not see bullets spinning rapidly inside the gelatin. If you shoot a bullet into a pool for recovery, you do not see them spinning on the bottom of the pool for minutes on end.

When a bullet strikes an animal, the rotational energy of a bullet is quickly transferred into heat and mechanical force just as the forward momentum is. The bullet does not stop it’s forward progression and then spin down inside the animal.

quote:

When a bullet strikes an animal, the rotational energy of a bullet is quickly transferred into heat and mechanical force just as the forward momentum is.

Actually mechanical work not force but I'm being snippy.

I once shot a buffalo looking straight on to me with a 470 NE Woodleigh soft, from a right hand twist barrel. When the bullet hit the buffalo, it flip over onto its left side, which I am sure was because of the torque generated by the rotational energy of the bullet.

quote:

Originally posted by Zero Drift:


When a bullet strikes an animal, the rotational energy of a bullet is quickly transferred into heat and mechanical force (energy) just as the forward momentum is.

Very true

The bullet does not stop it’s forward progression and then spin down inside the animal.??????????

I will give up on trying to explain a "compressed" helix....

Just know as we finish that your bullet has many more than two revolutions in the animal right at the very first and none after terminated......

BigRx

I once shot a buffalo looking straight on to me with a 470 NE Woodleigh soft, from a right hand twist barrel. When the bullet hit the buffalo, it flip over onto its left side, which I am sure was because of the torque generated by the rotational energy of the bullet.[/QUOTE]

Did the torgue from the bullet flip you over when you shot, or did the gun simply twist out of your hands?

Dean

No, it did not flip me as I was standiing against a tree on my left, or was it may right?

So why do faster twist barrels yield deeper penetration, all else being equal? (example: 1:10 twist compared to 1:14 twist for .458 500 grain pills)

quote:

So why do faster twist barrels yield deeper penetration, all else being equal? (example: 1:10 twist compared to 1:14 twist for .458 500 grain pills)

500,

What data have you seen on this?

How much more did one penetrate over the other?

Are there other caliber comparisons?

George Hoffman did a lot of penetration testing and opined that a 12" twist is preferable to the slower twists for penetration with his .416 Hoffman.

Andy who posts here has done a lot of penetration testing and has seen better penetration with the fast twists.

Anecdotal is all I have seen.

I better get to work compiling anecdotes until some real data results.

Ted,

Art Alphin is the one who figured out that 1:10 works best, but he did not publish his data. His A-Square reloading manual establishes this as the reason that A2 rifles have fast twist rates. The older Weatherby's in 460 with a 1:16 twist were known in the field to be crap penetarators.

quote:

Originally posted by Will:
That's why if you shoot at really low velocities and high barrel twists the buzz saw effect takes place, and a Barnes X can grind a whole buff into hamburger, as its linear stability goes to pot at low velocities, and the bullet keeps rotating around inside the buff until it completely stops. Then all you need is a bunch plastic bags.

Sean

500,

Thanks, that is a piece of information I have not factored into decisions in the past.

This is similar to the question of bullet rotational energy. I do not believe that bullets "drill" their way through animals. I believe that the linear force of the bullet causes the penetration. I don't think that the rotational energy would overcome the resistence of bone and muscle. That is why I don't believe that the "wings" on X bullets drill through animals. While the rotation of a bullet might increase in relation to its forward travel upon impact and the rapid deacceleration of the bullet, I do not believe that bullets behave as high speed drills once they make contact with animals.

quote:

Originally posted by 500grains:
So why do faster twist barrels yield deeper penetration, all else being equal? (example: 1:10 twist compared to 1:14 twist for .458 500 grain pills)

Because the straight line penetration by supercavitation is more stable and less susceptible to disturbance with the faster twist.

George Hoffman and I used to dwell on this spin and bullet rotation a great deal, and its relationship to killing effect, but after you reach a point you realize it makes very little difference in the first place, just something to make good conversation over a toddy, and that ain't all bad, at least for awhile..I wish he was around to discuss it with me some more, thats something I miss....

Once upon a time I was privy to some handgun tests involving expansion, penetration, wounding etc.
In the 9mm tests two different handguns were used. With the faster twist bbl the "wound" cavity was larger with all bullets tested with the faster twist bbl.
Also in some "shooting through brush" tests I have seen, faster twist and tougher bullets have done better.