Seriously, re: RPM's on a bullet. I have heard and/or read that although a bullet's forward moment/speed slows over distance from muzzle, that the bullet's ROTATIONAL velocity slows at a much slower pace.

This in my head is hard conceptually, as let's use a 12" twist for ease of discussion.

At 4000 fps, at muzzle it is turning 1x per 12" travel, at a rate of 4k fps.

Now when bullet slows to HALF, in my mind, the RPM's would also cut in half if you consider again, the 1 turn per foot twist rate.

SO, IF what I hear was correct, and I can somewhat conceive it, in order for FORWARD velocity to have been slowed by half, and RPM's to have a faster RPM than 1/2 the initial velocity, the ONLY thing I can conceptualize here is that the bullet begins turning at a faster 'twist rate' i.e. over time increasing the 'rate of twist' if there is not a 1:1 relationship of slowed FORWARD velocity and slowed ROTATIONAL velocity.

Is this correct?

I ask, as I have heard comment that even when velocity on say a hot 22 cal varmint bullet slows to a Long Rifle speed, it can be very destructive.....but surely some of that is bullet costruction as well right?

Any knowledgable 'physicist' want to take a stab in 'layman terms' for me please? Thanks.

Also, if you have the answer for the above, what is your take on how Twist rate affects killing power on a given bullet?

Let's say using a 6.5 mm bullet out of a 7.5" twist vs. a 9" twist. I understand that science says Rotational velocity on a bullet is a VERY small component of kinetic energy. SO, if correct, and I assume so, is it the bullet's CENTRIFUGAL force, that if increased, causes more perhaps hydrostatic shock, and or greater wound channel, and even perhaps helping keep nose forward maximizing straight line penetration? I assume the extra centrifugal force causes more bullet upset and is the main correlating factor in killing power variation.

Thanks for the insight anyone can give.

This is the same phenomenon that makes a baseball curve when a ball is pitched with a horizontal rotation. As the ball gets closer to home plate it's forward velocity slows some but its rotational velocity does not. It results in the curve being much more pronounced near home plate.
In other words air resistance directly opposes the ball's or bullet's forward motion but has very little effect on it's rotation.
I suppose if you fire a bullet straight up, when it reaches maximum altitude and stops, it will still be spinning like a top.

6.5,

Ireload sums it up. The air resistance which slows velocity impacts the forward progress of the bullet more than its rotational speed; therefore, the speed of rotation does not slow at the same rate as the speed of the bullet. Kudude

Off the top of my head I think you are confusing speed of travel and distance travelled. It may take longer to travel a foot as the bullet slows but the rotational rate over the same distance will change much more slowly. Course, that's a wild ass guess too.

Well not being a physics expert or a ballistic expert, I can't see the confusion...

velocity is one thing in the equation..

bullets can be shot out of a barrel at 3000 fps, one having a one in 12 twist and one having a one in 9 twist... both in the same length of barrel, and both with the same Muzzle velocity...

Both are going to loose velocity because of air resistance and forward motion, plus the effects of gravity....they will both loose it at the same speed...and distance...

Twist rate/ rpms have nothing to do with the bullets forward momentum, only their stabilization in flight....

the bullet from the 1 in 9 twist will be having substantially more rpms when it left the muzzle than the one in 12 twist barrel....

considering the high difference in rpms of the faster twist than the slower, neither are going to really have anything porportional to the other bullet's rpm....

Bullets in both instances are going to run out of velocity long before the effect of more or less rpms is going to add anything to the trajectory table...

this may be different in outer space where there is no wind or air resistance, and no gravitational pull to help slow down the velocity.....

they don't have enough velocity to make a diffrerence on the number of rpms the bullets are turning to make any difference in the trajectories...

layman's explanation only...seafire is no rocket scientist.. although I have been told I have the mental capacity to be... then I have also been told that I have the mental capacity to be locked up in the state nut house....oh well...

Thanks, that helps. I guess where my confusion lies most is:

1) the initial RPM is based on twist rate and forward velocity, that is how many turns per given distance, and how that MV affects speed.

as if I understand what is being said:

1) for example: A bullet fired with a 12" twist at a MV of say 2,000 fps has a 120k RPM.

2) if a bullet is fired same twist....12"...at a mv of 4,000 fps, ONCE that bullet slows over distance to say 2,000 fps forward speed, the RPM's are still more like 240k, NOT as in example one when fired at 2,000fps which results in 120k RPM.

THEREFORE if RPM's of a bullet fired are highly dependent of MV and twist rate, but not as dependent on velocity downrange, then I can only conceptualize that for a given say foot of travel that the bullet makes downrange say in example 2, that the bullet begins turning at a faster rate per 12", than just one turn every 12". Make sense?

Otherwise, how else can it have a higher SPIN rate than 1 revolution every foot traveled-that being traveled at a decreasing forward speed as the bullet is slowing down, than as the example of a bullet at 2,000 fps MV?

Do you follow my reasoning in my question?

I understand the indents by grooves/lands on the bullet has NO/little affect once bullet is in flight.

I suppose I need to THINK in terms of 'things change' one the bullet leaves the control of the rifling in the bore.....

I guess I should not think about things so much, Seafire, we might share something in common! Thanks guys.

While not an expert, I'm always happy to throw in my 2 cents worth...

I've not read that a .22-250's bullet decelerated to the speed of a .22 LR is very destructive in comparison. If the bullet is spinning faster, it may indeed fragment more easily on impact, but at .22 LR speeds, I doubt either is going to be terribly destructive.

For an 8" twist at 3500 fps, you have 5250 rps (315,000 rpm). At the bullet's surface, the radius is about 0.11", and you have a tangential speed of 300 fps. Compared to a 900 fps impact speed, that extra 300 fps isn't going to make a really big difference, especially since it is only found at the surface of the bullet (the center of the bullet is not affected).

Your have a good grasp of how much spin is imparted to a bullet. I see your concept but you are linking and confusing two disconnected forces, the velocity and rotation, one being the independant forward motion, the other is the independant rotational motion.

Consider a smooth bore; it may have signifcant velocity but no spin. Consider a kids top; it has hight spin but no velocity. Both mostions impart distinct actions due to their motion but they are totally unrelated. That's also the situation with a rifle bullet, once the spin and velocity are imparted each motion then works independantly, after clearing the muzzle a free bullets actions will coast alone until impact.

Spin has no place in computing the energy expanded in terminal ballistics. That's because the centrifical energy of spin is working in a different plan, at right angles to the direction of travel. When a bullet hits something that disrupts its jacket the spin may tear it appart and throw out fragments to the side - in a cone pattern in the target since the particle are still moving forward as they fly out. Bullets are made differently, some will disrupt and some won't but the spin effect itself remains at a right angle to the path of travel. That force's effect cannot be computed in any reliable way.

Forward bullet motion is greatly resisted by air. There is a very real but very small air resistance to the rate of spin. Since the time of exposure (time of flight) is so brief, the spin will remain vertually constant over that few milliseconds of flight time.

The bullet accelerates to muzzle velocity in a RIFLED barrel, which rifling imparts spin.

Subsequently air resistance slows the bullet's forward progress but the air is NOT rifled and has little effect on the spin.

quote:

Originally posted by asdf:
While not an expert, I'm always happy to throw in my 2 cents worth...

I've not read that a .22-250's bullet decelerated to the speed of a .22 LR is very destructive in comparison. If the bullet is spinning faster, it may indeed fragment more easily on impact, but at .22 LR speeds, I doubt either is going to be terribly destructive.

For an 8" twist at 3500 fps, you have 5250 rps (315,000 rpm). At the bullet's surface, the radius is about 0.11", and you have a tangential speed of 300 fps. Compared to a 900 fps impact speed, that extra 300 fps isn't going to make a really big difference, especially since it is only found at the surface of the bullet (the center of the bullet is not affected).

Agreed, thanks for the analysis/insight.

quote:

Originally posted by Jim C. <><:
Your have a good grasp of how much spin is imparted to a bullet. I see your concept but you are linking and confusing two disconnected forces, the velocity and rotation, one being the independant forward motion, the other is the independant rotational motion.

Consider a smooth bore; it may have signifcant velocity but no spin. Consider a kids top; it has hight spin but no velocity. Both mostions impart distinct actions due to their motion but they are totally unrelated. That's also the situation with a rifle bullet, once the spin and velocity are imparted each motion then works independantly, after clearing the muzzle a free bullets actions will coast alone until impact.

Spin has no place in computing the energy expanded in terminal ballistics. That's because the centrifical energy of spin is working in a different plan, at right angles to the direction of travel. When a bullet hits something that disrupts its jacket the spin may tear it appart and throw out fragments to the side - in a cone pattern in the target since the particle are still moving forward as they fly out. Bullets are made differently, some will disrupt and some won't but the spin effect itself remains at a right angle to the path of travel. That force's effect cannot be computed in any reliable way.

Forward bullet motion is greatly resisted by air. There is a very real but very small air resistance to the rate of spin. Since the time of exposure (time of flight) is so brief, the spin will remain vertually constant over that few milliseconds of flight time.

Jim, THANKS for the time you spent, I am with you in your explanation. No problem grasping this at all. I think as to my ? on killing effect/wound channel, long ago in a book on handloading author Don Zutz I believe wrote about a friend's '96 Swede using 7.5" twist and 129 grain bullets IIRC, and it's wound channel/destruction perhaps noticeably greater than a 270 130 grain at somewhat more MV. That effect you spoke of must be very real, and although not easily or meaningfully calculated in statistically numbers, definitely seen in bullet tests etc.

Also, a reference in a Handloaders digest, seems author's name may have been 'Amber' cannot recall, late 60's I believe shown 6.5x55 bullet test in newspaper media, penetration, damage, and expansion/wt. retention data. Had 94 mauser IIRC w/faster 7.5 twist and a HVA mauser 'Husky' Std. lenght bbl, with perhaps a 9 twist. Shorter 17.7 bbl 94 w/faster twist looked to have more bullet upset (thought less MV), and damage in media, and less penetration to some degree IIRC.

That said, seems a faster twist might help in field, on game, but it is dependent on how a bullet is constructed and likely of course how much RPM you start it at.

Thanks for your explanation.

And Bill, thanks to you, that really solidifies in my mind what is going on, as I have been in a fog about that concept, but can see it now.

Appreciate all the time you all spent in your posts. Very helpful.

6.5BR, you weren't totally off base. When a bullet fragments on impact, the tangential speed will increase the speed of some of the fragments. These fragments do increase the wounding potential, but as discussed above, it will not be a night and day difference, especially at these low impact velocities. Also, the extra rpm on the penetrating slug can improve its stability in the tissue, a little.

Thanks asdf.

Nice exercise. roger

The rotational energy is a very small part of the total kinetic energy of a bullet. It has very little effect on the killing power. I forget the exact percentage but it's in Rinkers book "Understanding Firearms Ballistics", I don't have it at hand to look it up.
Think about equal and opposite reactions. Does the rifle twist out of your hands as it shoots or kick rearwards?......................DJ

Yes Bartsche, must keep your mind sharp!

DJ, I understood VERY little ke comes from rotational energy, but the difference in bullet upset seems to affect wound channel to some degree, it would have to be tested in ballistic gel media to see, I would reckon a substantially faster twist all else equal would give a larger cavity closer to entrance hole/impact, to what degree don't know, may be more pronounced in frangible varmint bullets, I do know my 6BR 8 twist and 70 TNT's at 3400 is heck on water filled milk jugs, at 200 yds, and even out at 400, have to see out of a say 12 or 14 twist if there was any noticable difference.

As to recoil, yes, rearward and to a smaller degree some twisting perhaps not noticable in rifles but yes with some handguns, not sure how much correlated with twist.

Have heard interestingly at VERY long range bullets veer to right or left depending on DIRECTION of twist, which seems odd, as when you think about it, it is putting same forces 360 around the bullet, seems it would fly true straight line.

Bartsche, hanging with me! Ha. Thanks gang, perhaps it was a different 'read' than normally seen.

The best way I can think of to show that the forward velocity and rotational velocity are independent of each other is a simple coinflip. Your thumb will impart the same basic motions as a bullet out of a rifled barrel. And, when the coin comes down, it will be spinning at pretty much the same rate as when in left your hand. Slower than the bullet of course. Alot more air resistance on a flat coin, but I think it makes a fair example.

quote:

Originally posted by 6.5BR:
Yes Bartsche, must keep your mind sharp!

TOO LATE for that.

Of interest though somewhere back in that foggy brain I recall a revolver whose barrle started to unscrew because the barrel twist was in the wrong direction. The bullets twisting down the barrel did the job. Has anyone else incountered that? roger

One last thought: I might be able to better define the increased speed of the fragments due to rotation. If the tangential speed is 300 fps and the impact speed is 900 fps, a fragment from the edge of the bullet can fly off diagonally to the direction of impact. The speed along this diagonal is computed as the diagonal of a right triangle with sides of 900 and 300. The speed of the fragment is thus only 950 fps, which considering it is only a light fragment means very little increase in wounding potential.

Been gone a while, so am late to this party...

One plain English way to look at it:

Hypothetical bullet exits muzzle at 3000 fps. So, takes it .001 second to travel 3 feet (1 yard).

Rotational spin at muzzle with 1-in-12 twist is 1 turn for every foot of travel, or 3,000 revs per second at 3,000 fps velocity.

At "X" yards, velocity is down to 1,500 fps. Now it takes the bullet .002 second to travel 3 feet, but bullet is still spinning at almost the whole 3,000 revs per second.

So, it is now spinning almost the full 3,000 times per second while it is taking twice as long to cover the same distance.

I.e., the spin rate for every foot of travel has almost doubled because it takes the bullet twice as long to travel that foot of distance but the spin rate has not slowed appreciably.



That, of course, brings up another question; i.e., "Why do we quote spin in rpm's at all, when it is the spin rate per foot or yard (or meter) which is vital to obtain stabilization?"

I think that Canuck nailed it. The two factors are independant but related.
1. RPS number of revolutions in period of time
2. RPF number of revolutions per distance traveld.
3. A projectile could be spun up to a specified rpm or rps and then ejected.
4. Over a long range the projectile will be effected by A. the corrilis effect, and B. Pre-something I learned in flight training and mis-remember right now. It is tehreason that a prop has more thrust in part of its path than others, thus imparting yaw.
5. I feel that rotation has little or no effect on wound channel size. Think if a milling machine or a drill press. The only increase in the diamiter of the "Path" is when the bit wanders off path.
In fact a drill press is a good example to look at as to the RPM/RPF question. It maintains the same RPM no mater how fast it is fed (RPF)
Just my humble opinion.
Judge Sharpe

Inertia ; Is the property of an object to remain in constant motion unless acted upon by external forces .

Said velocity of : For an 8" twist at 3500 fps, you have 5250 rps (315,000 rpm) assuming this is correct . Bullet drop will occur long before rotational spin stops .

The rest of it falls under a Math a magician's calculation .

Shoot Straight Know Your Target . ...

Where things get interesting is at very long ranges where the projectile is fired at a significant upwards angle.

Theoretically the projectile would be "Stabilized" and ignoring curvature of the earth would return to earth with the same nose up attitude at which it was launched.

But areodynamic forces are acting on the bullet as it is in flight and the bullet that tries to "Belly in" will tend to weathercock and arrive on target point on.

In the process of changing it's initial angle to it's final angle there is a certain ammount of spin decay due to precession.

Though the effect is relatively small with rifle projectiles
at "normal ranges" (under 1000yd) the effect is quite pronounced with indirect fire projectiles (artillery projectiles)

Spin DIRECTION can also affect filght path because like a curve ball the rotating bullet will rise slightly due to a crosswind that moves in opposition to the top of the rotating cylinder we refer to as a bullet in flight
Granted the effect is TINY but has been demonstrated.

AD

It is revolutions per minute, not per foot. They will tend to drift to the side at long range, just like a curve ball as someone said.

/

[QUOTE]Originally posted by jstevens:
It is revolutions per minute, not per foot. QUOTE]


WHAT is?


Bullet rotation can be expressed either way, depending on which aspect of the phenomenon you are discussing.

In actual fact, it would appear that if the bullet has NO linear velocity, it is irrelevant how many revs it might spin in a minute.

So, the number of revolutions it covers while traveling a given distance is what counts for stability is it not?

If so, it is quite possible to describe that as either how many revs it performs during a unit of time needed to fly that distance, OR the number of revs per each unit of travelled distance (revs per foot), No?

As a practical matter, all the rpm tells us is useless info if we don't also know the distance travelled during that rotation time...which is what the "velocity" is for in some formulae. (as in "3,000 feet per second",which is not a measure of time but of distance travelled DURING a given period of time.)

I even suspect that the Greenhill Formula doesn't include ANY reference to time per se, only to how FAR the bullet can travel while making one rotation, to achieve & maintain stabile flight.

/

quote:

Originally posted by ALF:
Alberta Canuck:

Sadly Greenhill is a poor substitute for the Stability theorem ( actually Molitz's instability equation) Greenhill is a simplified derivation of the stability theorem but valid only within narrow boundries.

In practice what we are dealing with are the effects of angular and linear momentum of a spinning body, it explains amongst other why we can have precession and nutation.

It is simply mass spun around an axis at a set distance from the point of rotation and at a specific velocity ( as vektors as they have direction as well) this mass then also has linear velocity.

As such then time is important as both linear and angular velocity is time dependent. The angular velocity given in radians per second.

What is very important is distance travelled as this influences the angle of attack of the spinning bullet.

If the Gyro theorem criteria are fulfilled ( S value > 1) and the projectile is spin stabilized the initial angle of attack at muzzle will dampen out so that it becomes very small, between 50 and 100 m as long as the bullet remains supersonic, once the bullet's linear velocity drops off to transssonic phase the bullet may become unstable because at around mach 1 the overturning force coefficient spikes causing the angle of attack to become bigger.

I am aware that Grreenhill's is an old and vastly over-simplified approach first applied mainly to artillery. It is, however, the one most riflemen still have some feel for and some understanding of...and is very useful in a rifleman's daily sporting life; that's why I cited it.

I also understand that time, in and of itself is not THE major variable here, which is why I prefer to think of the problem as one of defining how often a bullet must rotate on its long axis over a given distance to maintain practical stability (and accuracy) over hunting ranges.

It is true that decay and a zillion other things affect a bullet's flight, along with how many angels can dance on the head of a pin simultaneously.

Perhaps I should not be so inclined to give short shrift to the incidence of people always quoting the bullet's rpm as THE important thing, but without distance travelled as a primary consideration, I still see the whole exercize as exactly that, exercising.

Anyway, I believe the original question has been answered in layman's terms. The added, incidental, comment I made on rpm vs. rpf was merely intended to get people thinking about it, not to set us all off on a whole new thread.

quote:

Allan DeGroot

O.K. The bullet 'flies' right? It is tilted into its flight path by aerodynamic forces and that tilting is resisted by gyroscopic forces right? That would cause the bullet to ‘twist’ to one side of it’s flight path, which will cause it to 'fly' off course - just a little. Also, the bullet does not ever travel fully nose first down range, so the air flow over it will be affected by its spin, with just a little bit more 'lift' on one side than the other. This is the same effect as 'wind drift', right? The bullet is actually carried a little further by 'lift' caused by that slight angle of attack. But do the two 'systems' add together or reduce each other’s effects?

quote:

I suppose if you fire a bullet straight up, when it reaches maximum altitude and stops, it will still be spinning like a top.

Nice & simple explanation.

Warrior

If you take a piece of string, wind it around a top, and set the top spinning on it's point on a flat surface, the top will continue to spin even though the top has no forward motion at all.

When you fire a bullet through a rifled bore, the bullet has two separate and distinct vectors-a forward vector that causes it to fly through the air, imparted by the powder gases pushing it forward, and an angular vector imparted by the spiral grooves acting to cause the bullet to rotate. These are separate and independent forces; neither depends on the other. They are both imparted separately, and will stop separately.

The forces acting to slow the bullet down are dependent on the friction caused by its' forward motion through the air, acting on the large surface that is exposed to the atmosphere it is flying through. The rotational vector, consisting of the bullet's angular motion (rotation) is also acted upon by the atmosphere, but to a much lesser extent.

Consequently, the rotational energy is expended much more slowly than forward motion.

In such cases, the rifling is in fact playing the same roll as the string you used to spin the top, even though the force it exerts to spin the bullet is acting at a lesser angle relative to the axis of the bullet (depending on rate of twist) than the string does in the case of a top which is not being given forward motion as well as angular motion.

WOW, this exercise has been very enlightening, as I had one physics course a couple of years ago as my undergrad was business. The knowledge on this forum is really amazing and I did digest most of what has been said and it is much clearer now.

I thank you all for your time and thought spent.

Bartsche, you still hanging? Just ribbing!