Gentleman, while we haven't created fact, nor really solved the "problem" question we have unearthed some definate opinion on the subject. I agree that rotational forces are minor in comparison to the kinetic energy of the projectile, HOWEVER, how that projectile will react to impact in game is what this is about. If greater/more rapid expansion is generated then bulet construction becomes a greater concern. Velocity is of course an extremely important concern in matters of trajectory and energy potential. It is the release of that energy over what amount of time (depth of penetration) which would appear most salient. I do appreciate the input, but I am still curious as to if there might be a definitive answer! Thanks for the serious responses. Good hunting.

When I do my math it would seem that a bullet fired from a 1 in 8 twist barrel at 2500 feet/sec. would have a rotational (in RPM) velocity of 225,000 RPM. The same projectile from a 1 in 12 twist would have a corresponding rotation of 150,000 RPM. So my humble question is how does this effect the terminal ballistics of an expanding bullet subject to these variables? Would expansion be more rapid with less penetration? Or somehow the converse? Yes, it will be a Sako based .308 Winchester for those who just have to know.

Spin doesn't affect expansion to any large degree. There may be some second order effects, but I wouldn't worry about it.

If you are going to put a custom barrel on it and you plan on hunting with it, you might opt for a slightly faster twist as it might help with stability in tissue. It is my opinion that factory calibers with faster than normal twist seem to have better acceptance by hunters, although this is not consistently true.

RobertD

So, your question is would a bullet expand faster if it rotated 2.5 times through a deer rather than twice?



Joe

I have no doubt that rotational velocity has great effect on bullet performance. It's quite easy to demonstrate with thin jacketed varmint bullets - drive them too fast, and they will simply self destruct in midair from centrifugal force.

I have no doubt that a bullet could tear itself apart at the moment that the jacket begins to open on contact with an animal, resulting in a dramatic surface wound with little or no penetration. If you over-rev a grinding wheel, and touch it to a surface, centrifugal force will cause it to explode. I can't see why a bullet wouldn't do the same thing.

It wouldn't take very high falutin' math to calculate the forces acting on a bullets jacket at various rotational velocities, but I'm not going to tackle it at this time of night...

I'll just keep shooting big heavy bullets at modest velocities, and not strain my brain.

It seems that the RPM of a bullet may make a difference in how it expands. There is a report that when Winchester developed the Fail Safe bullet that they fired them into a medium at reduced velocities to simulate the effect of that bullet at long range. This testing was flawed as the RPM was less from the start.

Long ago Barnes introduced a line of cartridges called the QT's for quick twist. These details may not be exact but there may have been a 6.5 mm on a magnum case with a 180 gr bullet and a 1-5 twist. Barnes claimed greater effect on game due to the QT. There was never any follow up on this.

I wouldn't think it would have much affect, if at all. Once the nose of the bullet hits something, it's going to start slowing down rotationally in a hurry. It basically goes from greater than 100K-200K RPM to a dead stop on a non pass-through impact. So for a 20 inch terminal path, on a 1 in 10 twist barrel, it's going to make 2 full revolutions at the most.

I'm sure the forces on the outer edge of the jacket as it expands out will be greater just due the larger radius, but figuring out if they're strong enough to fling the jacket off is WAY beyond my mathematical capabilities.

The twist rate factors in on the in-air characteristics more than the terminal perfomance. My vote is that impact speed and construction (bullet and target) have vastly more effect on expansion than rotational speed.

irwin

I've long thought that RPM is an overlooked and little understood aspect of a bullet's terminal effect and several of the answer's here have only reinforced my suspicion. It'd be interesting to see the scientific method applied to the question.

Brad,

Bill Davis answered much of this in a American Rifleman article. I wish I could remember all of it but he did state that there is very little energy in the rotational force.

I stand on the side that says it does matter however.

Tumble Weed's post is right on target. A 6.5X.284 I made up on a chrome steel Jap barrel had 140 gr. bullets of all makes coming apart in the air. The real Barnes bullets at that time held together going through the air but going through deer they went off like a hand grenade.On one deer it completely jellied all the orgins in the chest cavity. On another small buck it went through one shoulder and all but blew off the shoulder on the other side. These bullets were not expanding because of velocity they were blowing up inside the animals because of resistance of the centrifical force by body tissue and bone. The same bullets at the same velocity or greater out of a .264 magnum reacted quite differently. Is this relative to what you were asking? roger

Anyone with some free time.....calculate the force the bullet exerts on itself via rotational force and compare that to the force exerted when it strikes flesh/bone/gristle. I suspect the rotational component is trivial.

Another easier test, and one I have read about several times, is to load an otherwise identical cartridge in several rifles of different twist rates and shoot it into expansion medium. Most tests showed some differnece, but seems like it was on the order of 10% or less.

Yes, you can rotate a bullet fast enough to make it come apart, but it comes apart on impact MUCH easier.

Count me in the 'makes little difference in the on game performance' category.

"Another easier test, and one I have read about several times, is to load an otherwise identical cartridge in several rifles of different twist rates and shoot it into expansion medium."

That's why I pointed out that same bullet out of a slower twist .264 Win. Mag.The expansion media was deer.

Point of fact!!!!! If the bullet gets in deep enough, like the Barnes' bullet was doing,as it is coming apart the killing affect is going to be far greater than the degeneration of the bullets velocity alone while going through the animal. Those who have witnessed such an event know what I'm talking about. If you are going from a 12 twist to a 10 twist I doubt there would be any noticeable difference in expansion. I admitt that the scenario I presented was out of the realm of normality but it happened and more than once.

If you are happy calculating and reading about it enjoy your life. roger

That twist has a effect on bullet expansion is evident in the fact a recovered expanded bullet has the petals twisted in the direction of it`s spin. If this has any effect on game I`m not sure but tend to doubt it has much unless it comes apart.

While velocity has the most effect on expansion, the next highest contributor would be the depth of lands in the barrel. Given two equal guns firing the same bullet at the same speed, the one with the deeper rifling (cutting deeper into the jacket) would expand bullets quicker. The twist rates would have to be drastically different to become a major factor, and this would be from increased stress factors for the bullet jacket.