Bullet rotational speed


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bandur60
September 9, 2013, 12:09 PM
While I was in one of my meditative postures (??!!) I got to wondering .... does the rotational speed of a bullet decrease as velocity decreases? I realize there is probably only a fraction of a second to a few seconds until impact and subsequent zero RPM, but what do you think? For ease of figuring I use 3000 FPS and 1:12 twist and arrive at 180,000 RPM exiting the muzzle. Not that that really matters.

Not firearm related, but a previous question in my mind: if there are enough wind turbine generators in a line extending downwind, do they eventually "use up" the wind the further downwind they go? I know, I just have too much time on my hands .... only looking for lighthearted answers.

Should have been clearer in my wording; more like, when a bullet fired at MV of 3000fps slows to 2000fps (for example), does the rotation decrease correspondingly? Again, not seeking an agonizingly detailed answer, just one of those "Hmmm, I wonder..." moments.

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MtnCreek
September 9, 2013, 12:12 PM
does the rotational speed of a bullet decrease as velocity decreases?
Yes.

if there are enough wind turbine generators in a line extending downwind, do they eventually "use up" the wind the further downwind they go?

???

rcmodel
September 9, 2013, 12:21 PM
I don't believe bullets give up rotational speed nearly as fast as they give up velocity.

If they did, they would run out of RPM at long range, become unstable, and tumble well before they actually do.


Wind turbines use up the wind?
Yes.
I know here in Kansas the wind blows hard all the time.
If you go stand beside the tallest natural feature for miles, a barb wire fence?
The wind will blow your hat off.

However, if you go stand at the very end of the 1/2 mile fence line, it won't. :D

rc

zxcvbob
September 9, 2013, 12:31 PM
does the rotational speed of a bullet decrease as velocity decreases?

No. Where would the angular momentum go? The rotational speed starts out less with lower muzzle velocities, but you knew that already.

However fast the bullet is spinning when it leaves the barrel, that's how fast it will be spinning when it hits the target, minus friction losses that I *think* will be insignificant.

230RN
September 9, 2013, 01:19 PM
There's confusion here as to whether the decrease in velocity means "out of the muzzle" versus some time downrange when the bullet is slower.

Out of the muzzle, since you've calculated that 180,000 RPM already, you can see that at reduced loadings, which lower the muzzle velocity, the RPM will go down also, since muzzle velocity is the only change in the equation. Vice-versa for increases in muzzle velocity.

However, downrange, the RPM does not decrease as much as the actual velocity. You can figure the force on the nose of an "average" high velocity bullet at around 2500-3000 feet per second MV is anywhere from 20 to 30 pounds due to air resistance, and is still about 2 pounds at around 1000 feet per second downrange.

But the resistance to the mere spin of the bullet is much less in comparison. I don't have any numbers here, but let's face it, it's not like the bullet has big fins on it or anything to add to the air resistance of spinning. It's such a minor factor that I've never seen any data on that aspect of it. (Not saying it doesn't exist somewhere, but I've never seen any.)

So the RPM stays pretty constant as the downrange velocity decreases.

Terry, 230RN

REF:
Hatcher's Notebook, "Exterior ballistics" chapter, esp p. 569.

MtnCreek
September 9, 2013, 01:38 PM
minus friction losses that I *think* will be insignificant.

That's what I've read. I've read several posts here regarding over-stabilizing a bullet and it's affect on accuracy. I didn't understand it, so did some more reading to see if there was truth to it. One source (I think it was Lilja) stated somewhere that rotational speed does decrease, but it was not a factor on stabilization (if it's enough at/near muzzle, it's enough for the entire flight of the bullet). Going trans-sonic affects flight path / stability before loss of rpm could.

Good reading below. If you've read books on external ballistic, probably nothing new to you, but for me it was some good reading.

http://www.riflebarrels.com/articles/bullets_ballistics.htm

bandur60
September 9, 2013, 04:16 PM
Just the kind of discussional (is that a word?) replies I was hoping for! And thanks MtnCreek, some good reads there. RC didn't disappoint either, does he ever?

osteodoc08
September 9, 2013, 04:38 PM
Agreed with rotation. If you really want to start thinking while on the think tank, imagine rotational velocity physics. It's dependent on the radius of the bullet and rotational speed.

Not sure on the wind thing, but I imagine the resistance would eventually be to great to overcome if you aligned that many turbine fans.

Also, if light produced light while traveling the speed of light, would the light produced be going the speed of light or twice the speed of light? (There is a correct answer)

mxl
September 9, 2013, 05:03 PM
This is right up there with how can robins eat worms and crap white stuff on my car?

Dave P
September 9, 2013, 05:09 PM
And be aware that if you spin them too fast, the bullet will explode long before it reaches your target. Jacket can't hold it together.

I have seen many gray puffs of "smoke", about 30 feet out front, followed by the shooter yelling "that should have been a 10 - what do you mean there is no hole in the target ??"

Mostly with hot loads out of 223 space guns, as I recall.

pseudonymity
September 9, 2013, 05:10 PM
does the rotational speed of a bullet decrease as velocity decreases?

Yes. Spin a top on a table and it stops eventually, correct? The friction on the point (from the table) and the sides (from the air) gradually slows it down. A spinning bullet has the same forces, friction from the air will eventually slow the rotation.

Spin the top on the table, and then watch as it falls off the table and hits the floor - it is still spinning pretty quickly. The rotation slows down a whole lot less than gravity acts on the bullet - so does the rotation slow down, yes - does it matter much in a practical sense, no.

bergmen
September 9, 2013, 06:53 PM
Along the same lines: The ballistic coefficient I see published in various loading manuals is obviously a representation of the bullet shape as manufactured.

What happens to the B.C. when the bullet gets deformed due to rifling and general swaging as it travels down the barrel? Not enough to worry about?

Dan

MtnCreek
September 10, 2013, 07:33 AM
What happens to the B.C. when the bullet gets deformed due to rifling and general swaging as it travels down the barrel? Not enough to worry about?

I assume any change in shape, even slight, has some effect, but it should be minor and consistent. If excessive runout is a problem, either from the cartridge or the chamber to action, it can have a negative affect on accuracy. Not sure how (or how much) that would change the bullet's BC, but it could change the bullet's center of gravity, creating more wobble in flight. This is based on my reading, not testing, so take w/ ample salt...

Krusty783
September 10, 2013, 08:22 AM
The speed of light [in a vacuum] is a physical constant. No matter how fast 'you' are moving, light always travels at c w/r/t you regardless of your inertial frame of reference. It's quite non-intuitive, but an accepted fact.

Wind turbines extract energy from the wind; physics, thermodynamics, and evidence suggests that the more wind turbines you put up, the more you disturb the wind patterns. Taken to the extreme, you would begin affecting weather patterns because the boundary layer at the ground level will disappear and this will affect the wind patterns at altitude-the velocity deficit will be filled by air from above or from the sides.

Yes, frictional losses will slow the rotation of a bullet, but these are definitely 2nd or 3rd order effects. The transonic drag rise and change in aerodynamic stability as the bullet slows to and through the transonic region will likely have a much greater affect on the trajectory. (the aerodynamic center of pressure moves aft as one passes the sonic velocity. This shift with respect to the center of mass will likely affect the bullet's dynamic stability.)


Sent from my iPhone using Tapatalk.

CarolinaChuck
September 10, 2013, 10:01 AM
There will be some amount of friction to the atmosphere. There is also friction to rotation by the force of gravity. Yes, projectiles can and do become unstable; twist rate is one variable in function, if you change the variables you can get some really bad results.

As an earlier poster mentioned, over rotation can cause the jacket to separate. Under rotation can cause key-holing even at modest distances. Shoot a fouled barrel and see for yourself. Some folks with micro grooved barrels have seen the same issue with shooting lead projectiles in them. The micro grooving may not have enough grip on the soft lead projectile to induce enough spin to keep the projectile stable.

Maximum effective range of the M-16 A1 was 460 meters: we qualified with them at 500 yards. Point being, at some distances over 460 meters the projectile was already showing signs of stability issues. They were know to make a nasty hole in someone at long ranges because they were often tumbling by the time they found home.

Twist rate is a variable of function, and a very important one at that. You can not change the twist rate of a barrel; it is what it is, and therefore becomes a constant in the equation. It should tell you that the rotation of the projectile is critical to a stable and accurate round. You are talking about the holly grail to a hand-loader: finding the right muzzle velocity to match up with bullet weight, twist rate and barrel length is what it is all about.

A load that works well in my rifle, may not work so well in your rifle even though they are the same caliber.

CC

ATLDave
September 10, 2013, 10:11 AM
There is also friction to rotation by the force of gravity.

What?

Jim Watson
September 10, 2013, 10:28 AM
As said, the rotational speed of a bullet is determined by muzzle velocity and rifling twist.
It does not slow down nearly as much as the velocity does due to air drag.
A GOOD barrel and bullet will give stable flight to as long a range as the bullet remains supersonic. Some designs will even tolerate dropping subsonic. So the spin is not decreasing much with range. The Sandy Hook tests showed .45-70s landing point first at the design's absolute maximum falling out of the sky range around two miles.

Yes, rifling engraving does change the shape of the bullet and affect its flight. But that is reflected in the BC if it is derived from shooting tests. If by computation, not.

Yes, it is possible for a large array of windmills to "use up" the wind. Which is why they are spaced widely. Read about sailing ships and "wind shadow."

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