RPM Test; a tale of three twists, Chapter 2

[Larry Gibson]

Ralf

Let me throw some things at you. Take your 311291 bullet . Make velocity 2500 fps. That gives us 180,000 rpm. First look too rpm...that means revolutions per minute, yes? According to Lyman time of flight to 100 yard target is .131124 seconds. Bullet is not in flight for one minute. Let us say that it took 1 full second instead of what Lyman say. Then my question to you is did bullet turn 180,000 rpm or did it turn 180,000 rpm divided by the one second which come to 3000 turns?

That is correct; you are dividing the 180,000 by 60 (60 seconds in one minute) to get the RPM of one second, i.e. 3,000.

Let me ask different way. If you could stop bullet right after come out muzzle but not stop rotation, all this for observation purposes, would bullet be actually spinning 180,000 rpm? I think not. Another observation. Guns do not torque very much when fired but they do recoil. If you lay rifle on bench and fire with string would not you think if bullet leaves muzzle at 180,000 rpm that not only would rifle recoil back but spin some too from rotational torque?

If you stop the rotation you are correct, the bullet is no longer doing 180,000 RPM. It is doing zero RPM because you stopped it. I'm not seeing a point to this?

There is in fact torque. Shoot a light weiht '06 with a 180 gr bullet without holding the rifle and you will see the rifle recoil to the rear and counter clockwise. Also ask any big bore shooter if there is no torque during recoil. Yes there is torque. However it is proportional to the weight of the bullet, the spin, the velocity and the weight of the rifle. As I mentioned earlier about higher pressure in faster twists. There is higher pressure as the bullet enters the rifling. That also is where the rifle is torques. Some question whether there is further resistance or torque be cause once the bullet is in motion (rotating) in tends to stay in motion. I'm not really sure either was but as I've said; I've not found any evidence on bullets of increased pressure in the rifle marks on fired bullets.

Someone say bullet leaving muzzle like soft copper wire spun in drill. This is such crazy comparison to bullet, how do I explain. Bullet is not like spinning flywheel. If bullet is really spinning like that would you not think that this rotational force would show up in certain targets like gel test or flesh? Take a flat plate aluminum. Put it on drill stand table but no clamps. Chuck in 1/2 inch drill and set drill rpm for say 2000 rpm. Then abruptly bring drill down fast into aluminum. What happen? Aluminun gets spun very violently. Yet bullets don't show this in target medium.

I think what I am getting at is not rpm that destroys accuracy or bullet for that fact but I think pressure.

That's what most every one thinks. Hoever (once again) Pressure effects the bullet in the barrel. RPM effects the bullet in flight. If there was no centrifugal force from RPM then all bullets, even unbalanced bullets, would fly straight. Of course youstill have the effects of wind on the bullet but it is the centrifugal force of the RPM that causes the inaccuracy.

One more thing think Larry. I have seen test where noses of jacketed bullet were damaged to see what happens. You would think the bend noses would unbalance bullet badly but yet they shot say ok. I don't like that voids unbalance bullet so bad and is amplified by rpm because I don't think bullet is really spinning that fast.

You go think and let us know what you come up with to my many questions.

I don't have to go think. You go back and reread this thread and the first thread, Chapte 1. You will find I purposely unbalanced some .308 bullets and fired "before" and "after" ten shot grups. Study them closely, the answer is obvious.

Damaging the nose does unbalance a bullet but not "badly" Shootin "ok" as compared to what? If they shoot into 1 moa undamaged and into 3 moa damaged is that "ok"? It may be for deer hunting but it is not to a measurement of accuracy.

Larry Gibson

[felix]

That's true, Corky. Maybe it was 21 and 3/4. Talking strickly vibes here with a free floated barrel in any caliber they tested. This observation assumes projectiles at 3300 fps and the force required to make that speed. It seems that 300 fps increments provided the "same" vibe trace. We always want the projectile to leave the barrel when the barrel exit is not moving, so the length of the barrel must be exactly at a half-node location. We know in practice that will never happen because that location is so small in the trace, and the acceleration of the vibe is maximum at that point, even though the velocity is zero. The opposite at a nodal location, so we never want to chop a barrel off at a vibe node. ... felix

Remember, this is about free float without a Boss or any kind of weight at barrel end.

[sundog]

"If you could stop bullet right after come out muzzle but not stop rotation, all this for observation purposes, would bullet be actually spinning 180,000 rpm?"

Yes. Put it like this. If the boolit remained in flight for one minute, other than bleeding off some rpm due to external influence (kinda like a top slowing down), it would be expected that it would turn 180,000 times in one minute.

Don't confuse rpm with number of turns in a given distance. The number of turns in a given distance is fixed, a constant, by the rifling. In one hundred yards (300 hundred feet) a boolit from a 12 twist barrel (one turn in one foot) will turn 300 times, regardless of velocity.

[45 2.1]

45 2.1

The example does not need ammending. You are still leaving the time ellement out. Perhaps you should realise you are wrong and understand the world is not flat. Or put the time element back in as that is what the "Per Minute" means. It is not RPD (Revolutions Per Distance) it is RPM (Revolutions Per Minute).

Larry Gibson

See if you can get this: Revolutions per distance traveled do not change even when velocity is varied. Time has no relevance in that regard. If you chose to make up things, your results will be made up also.

[felix]

The RATE of projectile ACCELERATION changes direction at circa 16 inches. This is the third derivitive of the distance traveled over time. Could be the reason they decided a pistol was defined to be anything under 16 inches. Speculation on my part. Somebody must have lobbied somebody for that specific length. ... felix

[felix]

True, Bob. Distance and time must be considered as the only elemental parameters in your discussion, Larry, about what you are talking about. Velocity is nothing but a function of both. ... felix

[Tiger]

This from internet: An object traveling in a circle behaves as if it is experiencing an outward force. This force, known as the centrifugal force, depends on the mass of the object, the speed of rotation, and the distance from the center. The more massive the object, the greater the force; the greater the speed of the object, the greater the force; and the greater the distance from the center, the greater the force.

With that said I think smaller diameter bullets have less force on their outer surface then larger calibers. Then how does this explain the forums famous 6.5 Swede bullets not being able to be shot at high velocity, according to the above centrifugal force law?

RPM can be viewd, I think, both as a speed and as a counter. I explain. As in speed this would relate to a flywheel...how fast it is spinning. Now put a paint marker on the outer edge of the flywheel and use it to count how many revolutions the flywheel makes in one minute. See what I mean?

In nut shell: I do not believe bullet is spinnning the rpms figured out by the rpm formula. I believe it only turns number of rpm's (as in the counting method I explained above) given it by the rifling twist in the barrel.

Another internet taking: An object traveling in a circle behaves as if it is experiencing an outward force. This force, known as the centrifugal force, depends on the mass of the object, the speed of rotation, and the distance from the center. The more massive the object, the greater the force; the greater the speed of the object, the greater the force; and the greater the distance from the center, the greater the force.

With that said we are looking at tensile strength. Larry if you thing the bullet alloy tensile strength "just begins" to let the bullet deform after the rpm ceiling you have imposed, then why does the bullet disintegrate in a ridiculous rpm figure like over 200,000 rpm? Mine you, you claim that the rpm is starting to do something to the bullet after your rpm ceiling limit.

You need to look else where then rpm causing accuracy problem, this is of course if the rifling rate is correct for the bullet length and weight.

Ralf

[sundog]

Tiger, if your flywheel is turning 12 rpm it will turn 12 times in one minute or 1 time every 5 seconds. During that 5 seconds (and even if it's the ONLY turn it makes), it is still turning 12 rpm, even though it only turned one time.

[Tiger]

Sundog,

I know that. I meant as in the flywheel it is spinning at some rpm. The other I meant like a friend is going to turn shaft connected to a set of gears and he ask you watch output shaft how many time or complete revolutions it turns as he turns the input shaft. See?

Ralf

[felix]

This might help. Take a 100 grainer at 22 caliber versus one at a 45 caliber, both shot at 16 twist with the same barrel length. How much powder energy (powder speed optimized for bore) will it take each to make each projectile independently go 1600 fps? This will decipher how much energy it took to rotate the outermost particle on each projectile to reach the same RPM. ... felix

[HORNET]

If you spin the boolit up to 180000 rpm out of a 12" twist at 2500 fps, then its turning at 180000 rpm regardless of what the forward velocity may change to subsequently. If it slows to 1250 fps, its still turning 1800000 rpm but only traveling half as far during the same amount of time. This would now give it 1 turn in 6"... think about it.
Sundog, the boolit will continue accelerating in the barrel as long as the force exerted on the base by the pressure (F= pressure x area) exceeds the friction (F=Mass x Acceleration). The required pressure can be remarkably low as in the cat-sneeze loads.

[felix]

Any one particle on the boolit accelerates the same down the axis of the barrel. But, the particle in the center of the boolit has zero lateral distance to move, and therefore cannot have any lateral acceleration or velocity. ... felix

[sundog]

The boolit accelerates as pressure is decreasing?

Then how do you explain stuck boolits? In order to get stuck, it had to slow down.

And what about those crazy secondary spikes on pressure trace graphs?

[felix]

Impossible, Corky. Stuck boolit is equal to zero dynamic pressure. Static pressure is released as you uncork the beer bottle (or open the action, assuming the action caused a perfect seal). Get the barrel long enough, then we will get a stuck boolit. ... felix

[sundog]

That's what I'm saying. Friction overcomes pressure, boolit stops. There had to be pressure to get it going, eh?

[leftiye]

Sundog, I really hate to help larry out here, he is so good at bending or ignoring what was said that he really needs no help. But where the wires got crossed here with the pressures decreasing thing (after the peak of the pressure curve) is that though the pressures becomes lower after the peak pressures occur, they are still plenty high enough to further accelerate the boolit. A lot of the magnum cartridges have exit ressures in a longer (26")barrel of about 7000 psi (from memory -30 years old data), enough to push a boolit through a bore (maybe not engrave it though) and to continue accelerating it. Yep, acceleration, and increasing revolution rate occur clear to the muzzle.

Larry, as Bass said no one even thought for a moment of suggestion that you cut the barrels off of your guns. Nor is the question of whether or not such shortened barrels would be legal an issue here. The proposition was a theoretical one suggesting that while the RPMs would decrease to ideal levels, the exit pressures might totally destroy accuracy, or at least offset any gain from lower rpms.

Bass also has good points in saying that the rpm effect should show up with jacketed bullets too [ It actually should effect all bullets, as with my previous post about why come the paper patched boolits can be shot so fast and still not lose accuracy - This has to be that plain lead boolits recieve much more deformation than is visible, and any sort of protection from the bore will reduce this] and also that the lower RPMs encountered in larger, slower velocity, slower twist calibers should make them better candidates as accuracy guns than small bore calibers.

[sundog]

Leftiye... no, pressure is still plenty high enough to force the boolit out of the barrel in a normal situation. I don't see how decreasing pressure can equate to acceleration.

Apparently the frog thinks deceleration is possible, too. About 2/3 the way down under the pressure curve graph.

http://www.frfrogspad.com/intballi.htm

[felix]

Yep, Corky, that would be close to 16 inches. Keep in mind the frog is talking about the RATE of the pressure change reversing directions, and not the pressure itself. It is still being generated, but now at a decreasing rate. ... felix

[joeb33050]

[QUOTE=sundog;321310] I don't see how decreasing pressure can equate to acceleration.

QUOTE]

Say the bullet's going 37 fps and pressure falls from one zillion psi to .999999 zillion psi. The bullet accelerates to 38 fps in some (small) bit of time.
joe b.

[sundog]

Yeah, so if the peak is too early you have exactly what I'm talking about.

Remember the stuck boolit? If the peak were later, the boolit might clear the muzzle.