Comments on: Calculating Bullet RPM — Spin Rates and Stability

By: Steve Blair

Steve Blair — Sat, 03 Oct 2009 18:27:18 +0000

Regarding post #13:

The M193 ball used in Vietnam had a 55 grain bullet. The 62 grain M855 was adopted as NATO STANAG 4172 in 1980.

By: Bob MacLaren

Bob MacLaren — Thu, 24 Sep 2009 11:29:25 +0000

What I’d like to know is whether the bullet gets heated by passing through the air at Mach 2 like a small Apollo capsule. I’m prepared to believe that the bullet gets heated to around half the
temp of molten lead by barrel friction (from the rifling) plus a small effect at the base from hot propellant. But is there an appreciable extra heating (and later presumably a cooling)from flying through the air?

By: R Rod

R Rod — Mon, 21 Sep 2009 19:35:23 +0000

Anyone who had seen a very slow speed photography of a bullet hitting the weaved surface of an armor vest could readily notice the twisting effect imparted by the spinning bullet on the material it hits. Yes, there is a “buzz saw” effect, at least until all the rotational inertial energy of the bullet is used up!

By: sw

sw — Wed, 10 Sep 2008 22:53:26 +0000

Regarding the “buzz saw” effect, I’ll explain this little known fact. When a bullet is spinning at, let’s say 200,000 rpm on impact with the animal, the foreword motion of that bullet immediately slows down from the impact. But until ALL foreword motion completely stops, the bullet is still spinning (although also rapidly slowing down as well). So there IS a momentary “buzz saw” effect. Using a non-frangible deer hunting bullet, the folded back sharp petals act like an end mill; cutting on it’s front and sides at the same time. Do you remember the “shot in the stomach, but the bullet wound up in the guy’s shoulder” stories that came back from Vietnam? Guess how the little 68 fmj bullet got there? It spun it’s way up there. With retained RPM.

By: sw

sw — Wed, 23 Jul 2008 02:36:02 +0000

If you want to see what a RPM induced radial burst pattern looks like, take a piece of 24″ X 36″ construction paper (or your wife’s favorite Brad Pitt poster), roll it up into a 12″ diameter tube (stapled or taped together), lay the tube on the ground, stand up a piece of 1/2″ plywood in front of the tube opening, then get back about 10 feet and fire a high velocity frangible .22, .20, or .17 bullet thru the middle of the plywood. You will see the radial burst pattern show up as VERY tiny little holes around the circumference of the tube. You will also notice the pattern isn’t very far beyond the plywood. How far behind varies with the foreword velocity and RPM of the bullet on impact. When the bullet bursts, it bursts APART at a higher velicity than it’s foreword velocity. That’s the reason why you cannot kill 2 ground squirrels with a single frangible bullet, even if one squirrel is standing right behind the other.

By: steve westbrook

steve westbrook — Thu, 19 Jun 2008 13:04:35 +0000

To get a good idea of what happens when a “frangible” projectile bursts (either from excessive rpm, or on impact), read a chapter or two on “flywheels” in any good mechanical engineering book. When the centrifical forces exceed the tensile strength of the material, there is a sudden, EXPLOSIVE release of the stored energy in that material. In a Hot Rod Magazine published back in the 60’s, there was a picture of the interior of a dragstrip operated ‘55 Chevrolet. Or what was left of it anyway, as when the flywheel burst, the floorboard was blown up into the dashboard, the dashboard was blown clear to the roof, the windshield was blown completely out, and the front seat was totally demolished. By a miracle, the driver survived.
This is what happens when a high rpm “frangible” bullet strikes the animal. It not only expends it’s stored energy foreword into the animal; it also VIOLENTLY releases it’s stored rotational forces in a radial burst as well. The faster it spins, the more violently it comes apart.

By: steve westbrook

steve westbrook — Thu, 19 Jun 2008 08:08:48 +0000

consider this: The .015″ thick jacket of a “frangible” bullet gets immediately reduced to an effective .010″ thick jacket the moment the rifling gets “impressed” (engraved insinuates the removal of metal) into it’s thin surface; weakening the soft copper jacket’s tensile strength by about 33%. Then, the lead core gets heated to a “flash” temperature approaching it’s melting point, from the hot gasses pushing it and the copper jacket’s friction with the barrel; further reducing it’s (their) tensile strength. So both metals, already seriously weakened, get wound up to a rotational speed of 150,000-250,000+ rpm and CAN violently burst apart in a radial (not foreword) burst pattern the moment they leave the confinement of the barrel. I extensively studied the radial burst patterns of high velocity .22’s many years ago, and the radial burst patterns they produce explain why you cannot “skip” a high velocity .22 bullet into a ground squirrel even if the bullet hits the ground only 6″ ahead of it. The moment the bullet’s thin copper jacket gets scuffed (such as a “skip” contact with the ground), the bullet violently explodes in a radial pattern; none of it hitting the squirrel.

By: Avi Nahsohn

Avi Nahsohn — Thu, 12 Jun 2008 02:01:51 +0000

am pleased with your definitions thue, it needs to be clear that bullet max rpms dose require a bullet to trevel quite long distance in order to spin that many times so this is in fact a formula which is used to determain best twist rate??!! some folks i know thinks the bullet spins 237.600 rpms at the muzzle as stated in speer manuel #14 page 21. that is what i’ v been trying to get a clear answer from the ‘big boys” but still no luck. in fact no bullet can stay in flight that long it will reach next state if so again could you make it clear so all people could remember the time fraction it takes a bullet to leave the barrel so in no way it could spin that many times right at the muzzle right?? i do thank you for a proper respond.
respectfully Avi.

By: Tom McElwayne

Tom McElwayne — Fri, 06 Jun 2008 12:15:49 +0000

JOSH— sorry to have to disagree with you but, that “buzz saw” effect exist. I’ve shot well over 10,000 varmints and let you tell fast twist of the identical caliber and bullet do make a notiable difference! (it’s the direction the energy travels when imparted into the mass at point of termination)

By: Josh

Josh — Wed, 04 Jun 2008 20:04:38 +0000

Please be a little more careful to differentiate conventional wisdom (the things that ‘everybody knows’) from fact. The lore about the tremendous effect of bullet rotation on terminal performance (the ‘buzz saw’ effect) is often repeated, but just can’t happen. For example, a 70 grain, 6mm bullet at 3000 fps has about 1400 ft-lbs of energy, and less than 1% of it is due to rotation. When a bullet comes apart in flight it is largely due to temperature from frictional heating due to passage through the bore. A rifle bullet spinning at typical rpm would never fail from centrifugal force at room temperature. It would be easier to destroy the bullet by firing it a very high velocity from a smoothbore and letting friction do the work. This is just plain physics.