Discussions around the fire.. Bullet "rpm"

N

n5lyc

New member
At our local range this morning, we were sitting around the fire discussing bullets and shooting, we started talking about twist rates, and a thought came to me, about the RPM a bullet is spinning when it exits the barrel.

We were all shooting 45-70's today.

I pulled out my phone and did some quick math, i asked the guys what they thought the bullet 'rpm' was as it left the barrel, most guessed 10,000-12,000 RPM or so.

When I told them at 2100fps out if a 1/20 rate barrel it was turning about 75,600 rpm, they were surprised. (It spins about 180 times in 100 yards)
Then we started talking about other cartridges.

The short form is: MV X 720/Twist Rate = RPM

The .22 mini mag at 1235 FPS turns over 55,500 "rpm" at a 1/16 twist rate.(it rotates about 225 times in 100 yards)

The long form: a .223 bullet exiting a 1:12 twist barrel is making a full rotation every 12 inches or foot. If the bullet is moving at 3000 fps (Feet Per Second) to calculate RPM (Rotations Per Minute) you multiply 3000 x 60 which is 180,000 RPM. That same bullet exiting a 1:8 twist barrel is spinning half again faster so it's doing 270,000 RPM (3000 x 1.5 x 60).

A 1/7 twist gives about 300,000 rpm but when shot into a target at 200 yards, it manages only about 1028 rotations.

You can see how the different twist rates produce significantly different bullets RPMs.

Just some ramblings from a morning at the range..
 
Last edited:
Yeah, kind of mind-boggling. You can see it in very high-speed video footages.

I've read accounts of bullets disintegrating in flight from over-rotation; such as very light 5.56 rounds from a fast twist barrel. Hype? dunno...
 
Sierra Bullets released one lot of 30 caliber 168 HPMK's to the US Army and US Navy Rifle Teams back in the early 1970's Shot with max loads in 1:12 twist M14 and M1 rifles, a lot of them flew apart about 20 to 30 yards downrange. In the US Army International Team's 300 meter .308 Win free rifles in reduced loads, they stayed together and was one of the most accurate lots of 168's they ever got. So all the ones the Army had were kept by their Int'l team and Sierra replaced the bad lots for the other service teams at no cost.

Sierra's first 28 caliber 168-gr HPMK bullets had thin enough jackets that when shot in 7mm Rem Mag's, they often flew apart from jacket failure. Boots Obermeyer's 5R twist rifling didn't weaken their jackets and were popular barrels for shooting those bullets until Sierra finally got better jacket material and standard rifling no longer weakened them.
 
well I'll tell you what, if you can find a way to get all of those bullets to stay airborne for a full minute without slowing down(which would also decrease RPM as well)then I will be impressed with bullet RPM.

I guess it's something I never really think about and is pretty irrelevant to my normal shooting calculations.
 
As I said in the first paragraph, as it exits the muzzle.

And I never said it stayed airborne for a minute.

I used rpm as a reference point.

Wether your record player turns at 33 1/3 rpm or 45 rpm.
Wether your muscle car idles at 600 rpm, and red lines at 8000.

It is a reference that EVERYONE can relate to..

Ian
 
no need to get all defensive. I was just saying that RPMS are completely irrelevant to ballistics. yes you need a fast enough twist rate to stabilize a bullet but it's a lot easier to just say I need a 1:8 twist than I need my 223 bullet to be spinning at least 180,000 RPMs at the muzzle.
 
It is a reference that EVERYONE can relate to..
Click to expand...

I relate it to my 24,000 rpm shop router...as in more than ten times as fast...:eek:

Pretty mind boggling, even if only lasts for a second and a half. Just think about the fact that it "spins up" to that rotational speed before it even leaves the barrel.
 
Not getting defensive, I just thought it would be interesting to put a 'pencil' so to speak on one of the aspects of ballistics.

The rate of twist is relevant to exterior ballistics, it is a steady and increasing with range, drift of the flight path called gyroscopic precession, for our projectiles this drift to the right is quite small, on the order of about 15 inches for a .308 caliber bullet at 1000 yards. It is also fairly predictable, and can be seen as a predictable bias to our long range point of impact.

I am sure, some more knowledgable on this board can a lot more info to the discussion..
 
One note, while we're on the subject:

The reason why high velocity varmint cartridges typically have slower twist rates than their lower velocity counterparts, is to keep bullet RPM in the same "butter zone".
High velocity makes up for the slower twist rate, without spinning the bullet fast enough to magnify instabilities or make it self-destruct.
 
"I've read accounts of bullets disintegrating in flight from over-rotation; such as very light 5.56 rounds from a fast twist barrel. Hype? dunno..."

Not hype at all, I've had it happen. Looks particularly interesting though a spotting scope.
 
It's actually an important consideration in accuracy. We don't generally talk about RPMs directly but we do talk about the two things that dictate RPM, twist rate and velocity. We talk about it when we say a bullet is "stabilized" even though we don't use the term. Some extreme accuracy folks do calculate the numbers directly.

Too much RPM can be just as bad as too little. Bullets with inherent defects are thrown off course by imbalances.

And, yes, the first time I did those calculations I thought I screwed up the numbers. 200,000 RPM?! That can't be right... Well, yes it can. :D
 
According to the Speer manual a bullet with MV of 3500fps is spinning at 330,000rpm. This is enough centripetal force to strip the jackets off of some thin jacketed varmint bullets.

I have to agree that RPMs are not as significant to most shooters as FPS, but it's not FPS that causes a bullet to go up in a grey puff, 15' from your muzzel.

While this has never happened to me, it might be kind of spooky, having your bullets disappear.
 
Expressed in RPM, bullet spin is just another big number, kind of like government spending.

How about RPS, a second is a long time for a bullet in flight.
 
Not too fast...

My muzzle loader with a .490 patched round lead ball at ~ 1100 FPS only turns about 12,000 rpm. (1/66 twist.)

It only spins 54 1/2 times in 100 yards..

Slooooowwww moootttiiiooonnnnn
 
It is like that other RPM, rounds per minute. We say a machinegun fires 750 rounds per minute, but that is what is called the "cyclic rate", not the rate at which the gun can be fired as a practical matter.

I didn't do the math myself (too darned dumb) but I have been told that the spin of the bullet does not slow anywhere near as fast as its forward velocity, since the resistance of the bullet surface (and land marks) is not as great as the resistance of the air to its forward progress.

Jim
 
James K said:
the resistance of the bullet surface (and land marks) is not as great as the resistance of the air to its forward progress.
Click to expand...

That's probably right: A back o' the envelope calculation tells me the 200,000 rpm of a hypothetical .223 bullet corresponds to only 195 ft/sec, so it's rotational wind resistance would be far less than it's resistance to moving forward.
 
You must log in or register to reply here.
Back
Top