Twist Rate??

[Mal H]

Jim - the reason we are using RPM's is because that measurement of rotations is, by far, the most common measurement used in every day life. The huge numbers represented can be compared to other rotating things we all know about. We could just as easily use Revs. Per Second.

Of course we know a bullet isn't in flight for a full minute, but the exceptionally high rate of rotation will tear apart a bullet that isn't properly made or isn't properly chosen for the twist rate no matter how long it is in flight past a reasonably short distance from the muzzle (most all bullets can go several feet without destroying themselves at any rotation rate).

An analogy is the engine of a top fuel dragster. The crankshaft only rotates around 500 to 600 times for the entire length of the approx. 1/5th mile run. But, that small number is hard to believe as it screams down the track trying to tear itself apart.

 

[Jim Watson]

To me rpm of a bullet is of no value.
It appears nowhere in barrel specification or ballistic chart.

I have overspun and destroyed bullets. I have underspun and keyholed bullets. Weirder yet, I have bent bullets without breaking them up, with a few lucky shots hitting a target with a C shaped hole.

 

[Swampman]

I don't.
But don't get carried away with a fascination with big numbers.
Just as well measure your next vacation in inches.

How else would you suggest defining bullet rotation?

Your "300 revolutions to a hundred yard target" is useless for anything meaningful, like calculating the rotational stress on a bullet.

A 1/12" twist is going to spin the bullet about 300 times in 100 yards whether it's going 1000 fps or 5000.

Equating bullet RPM with measuring a vacation in inches is nonsensical and even unintelligent.

 

[JohnKSa]

To me rpm of a bullet is of no value.
...
I have overspun and destroyed bullets.

Some bullet manufacturers will provide, upon request, a maximum spin rate (in rpm) specification. With that spec (and a chronograph) one can avoid ever blowing up a bullet by overspinning it.

 

[230RN]

But a bullet is in flight for a lot less than a minute.
12" twist = bullet is turning once every foot of travel, all the way to the target.
300 revolutions to a hundred yard target.
Spin does not decay nearly as much as forward velocity.

Y'all be careful, now, you hear?

Me:
"Don't confuse RATE of spin with TOTAL NUMBER of spins."

I don't.
But don't get carried away with a fascination with big numbers.
Just as well measure your next vacation in inches.

Mmmmkay. Sorry, I got the impression that you were confused on that. And you wouldn't be the only one. I've discussed twist and RPM many times on several boards, and that's a common point of confusion, so I was sort of writing to the world at large.

As I mentioned, the first time I did that RPM calculation, maybe thirty years ago, I was shocked at how big the RPM numbers were and I thought my calculator was broken. (I had just read about .220 Swift bullets disintegrating in flight, so I was curious about it.)

With modern guns it's not hard to get 250,000 RPM.



Terry, 230RN

 

[wiscoaster]

I've usually heard it referred to as "gain twist" rifling.

Yes, your term is the correct, my memory failed me (yet again) and substituted something else. Thanks for the correction.

 

[Jim Watson]

Some bullet manufacturers will provide, upon request, a maximum spin rate (in rpm) specification. With that spec (and a chronograph) one can avoid ever blowing up a bullet by overspinning it.

I would have never thought to request that value.
I know that Sierra Blitz bullets have always had a recommended maximum velocity; don't shoot them in your Swift, etc. That goes back to the pre-M855 era when almost all .22 centerfire varmint rifles had 14" twists. Now they have a warning against fast twist. To show how close you can crowd the line, there was one gunzine writer who said his premium aftermarket barrel would shoot them faster than recommended, but a relatively rough factory barrel would break them up.

My attempt to make a .223 into a Long Range rifle first involved the 90 grain Sierra MK marked right on the box "6.5" twist" in red. So I had a 28" 6.5 twist Kreiger put on an AR. Accuracy was as good as any bolt action I owned but the Sierra maximum load would not stay supersonic to 1000 yards. I confess to overloading it until it would. That was when I started losing bullets on the way to the target. I had one match where all hits were 10s or Xs but there were several misses. I finally caught a couple of those bent bullets on paper at 100 yards and knew the problem. So I bought JLK and Berger bullets which would stand the stress. I loaded the remaining Sierras to what they would stand for midrange shooting where they did very well.

Gain twist is another subject I will leave to another thread.

 

[CapnMac]

It can't.
A projectile with a length to diameter ratio greater than 5:1 can't be spin stabilized.
Notice that tank AP is fin stabilized.

Actually, it's just difficult, not impossible. The concentricity is a huge issue at such ratios. Which is why it's not much seen. However a number of rockets well beyond 6:1 are spin stabilized.

The reason anti-tank rounds are fin stabilized has to do with the physics of the penetrator rod. Which work best when in 1:14 to 1:18 rations. When the benetrator hits the target the KE/PE equation creates a state change where the tip of the penetrator becomes a plasma "driven" by the remaining mass behind it. If the penetrator is rotating, the angular momentum disrupts the potential for penetration. Which is wanted to reach "across" things like spaced and multi-material faced armor composites.

 

[bandur60]

A little "drift here, but back to the gain twist ..... Am I right in thinking that bullet rotation/ spin/ rpm stops gaining as the bullet leaves the barrel? In other words, just because spin rate is increasing while traveling through the barrel, once it leaves there is nothing acting on it to continue increasing spin rate? Just a hypothetical inane wondering, nothing in my life will change depending on any answers.

Kind of like the question, if you put enough wind turbines in a row can you use up all the wind ....

 

[Riomouse911]

I would think that the forces of friction caused by the atmosphere on a spinning bullet would eventually slow and stop it.

However, since a bullet flies only so far but very fast, there may be an inconsequential amount of rotational slowing during the bullets’ time of flight.

(Remember, if you drop a bullet from the hand or fire a similar bullet from the same height and at a level plane (across a flat surface) from a rifle, both bullets drop to earth at the same rate due to gravitational pull... the fired one just hits the earth a lot further away from you than the dropped one.)

Now Im not an aeronautic dynamics physicist mind you, but I am a bit of a study of science in general.

Stay safe.

 

[Mal H]

In other words, just because spin rate is increasing while traveling through the barrel, once it leaves there is nothing acting on it to continue increasing spin rate?

You understand it correctly. Once a bullet leaves the barrel, whether the barrel has a single twist ratio or has a gain twist, the rate of spin does not continue to increase.

 

[lysanderxiii]

It can't.
A projectile with a length to diameter ratio greater than 5:1 can't be spin stabilized.
Notice that tank AP is fin stabilized.

Nope.


Length to diameter ratio = 5.44

5.25

Some go higher than 6 . . .

 

[Jim Watson]

Yeah, yeah, I was out of date. Science marches on.

 

[lysanderxiii]

I would think that the forces of friction caused by the atmosphere on a spinning bullet would eventually slow and stop it.

However, since a bullet flies only so far but very fast, there may be an inconsequential amount of rotational slowing during the bullets’ time of flight.

(Remember, if you drop a bullet from the hand or fire a similar bullet from the same height and at a level plane (across a flat surface) from a rifle, both bullets drop to earth at the same rate due to gravitational pull... the fired one just hits the earth a lot further away from you than the dropped one.)

Now Im not an aeronautic dynamics physicist mind you, but I am a bit of a study of science in general.

Stay safe.

That is where the 300 turns to the target information comes in handy. How fast is something spinning 150,000 rpm going to slow down in 300 turns? A top with the drag of the nose on the table takes a minute or so to wind down from around 300 rpm, so with just the air to retard the spin of a projectile, it will take a few minutes. The time of flight for a 16 inch projectile to maximum range is only around 90 seconds.

 

[Riomouse911]

I’m sure there’s a formula somewhere that can show the drag caused by the atmosphere on a spinning object and the rate at which the drag would slow the spin down. That being said, you’re right about the extremely brief time-of-flight making such a question more of an exercise in theory rather than a practical one...

Good stuff

Stay safe.

 

[Jim Watson]

I once saw somebody trying to compute spin decay and not having much luck.
A gedankenexperiment. Shoot to find the minimum twist/velocity to stabilize a given bullet at 100 yards.
Then shoot it at greater ranges and see where it falls off. Be sure to stay supersonic so you don't involve that turbulence.

 

[P5 Guy]

I’m sure there’s a formula somewhere that can show the drag caused by the atmosphere on a spinning object and the rate at which the drag would slow the spin down. That being said, you’re right about the extremely brief time-of-flight making such a question more of an exercise in theory rather than a practical one...

Good stuff

Stay safe.

The last couple of chapters in Hatcher's Note Book has the equations.