RPM of a bullet.. incredible

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Cannonman1

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I just ran the numbers for the rotation of a bullet fired at 1200 fps from my Whitworth muzzleloading rifle (twist of 1 turn in 22 inches) and come up with just short of 40,000 rpm !!! .. That is over 40 times the speed of rotation of a tire on your car traveling at 60 mph... I knew it was fast but WOW !! That bullet goes from a standing rest and in the distance of 33 inches and a flash is at 40,000 rpm! Never appreciated that but now I do.
 

M. De Land

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I just ran the numbers for the rotation of a bullet fired at 1200 fps from my Whitworth muzzleloading rifle (twist of 1 turn in 22 inches) and come up with just short of 40,000 rpm !!! .. That is over 40 times the speed of rotation of a tire on your car traveling at 60 mph... I knew it was fast but WOW !! That bullet goes from a standing rest and in the distance of 33 inches and a flash is at 40,000 rpm! Never appreciated that but now I do.
Yes, a real eye opener, no? The rotation deteriorates much slower than does the forward velocity. Run the numbers on a high power varmint bullet some time for a real eye brow raiser!
 

centershot

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Yes, the rotation rate does seem impressive, but remember, that is revolutions per MINUTE. Your bullet is never flying for a minute. In a round ball barrel, 1:66 rate of twist, the ball is rotating ONCE for every 66 inches it travels. Some guys look at the RPM numbers and get the idea that the ball or bullet is acting like a miniature buzz saw, that's not the case!
 
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His example was a rate of 1:22 and that's true it only makes one revolution in 22 inches, but 40,000 rpm's it the rate it's spinning at for sure. No saw blade is spinning anywhere near that fast.
Years ago I was goofing around with Swedish Mauser rifles, 6.5 x 55 - reloading short (85 grain I think) bullets going pretty fast. Those rifles are designed for real long bullets and have steep rifling - don't remember what it is though. Sometimes the bullet wouldn't make it to a target 50 yards away, they would "blow up" and turn to schrapnel half way there from spinning too fast.
 

Loyalist Dave

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I just ran the numbers for the rotation of a bullet fired at 1200 fps from my Whitworth muzzleloading rifle (twist of 1 turn in 22 inches) and come up with just short of 40,000 rpm !!! .. That is over 40 times the speed of rotation of a tire on your car traveling at 60 mph... I knew it was fast but WOW !! That bullet goes from a standing rest and in the distance of 33 inches and a flash is at 40,000 rpm! Never appreciated that but now I do.
Lets see...,
My 1:48 twist rotates one revolution for every 48" of travel...
at 100 yards, to the target that's 300 feet or 3600 inches.... my bullet rotates a full rotation 75 times....
The flight time is ....., .2 seconds...., that would give me 22500 rpm at the muzzle...So yeah yours with that more than 2x twist rate would be like 2X mine...

Question..., since the bullet slows as it flies down range, does the rate of rpm also slow ??? So my round ball starts... at 22,500 rpm but is it impacting at 22,500 rpm ???

LD
 

freedom475

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This is kind of a silly notion. Politicians love to use it to increase the "danger" of a bullet. Remember the "BlackTalon" fiasco?
The fact is with a 1:70 twist the target would have to be over 20 miles thick! (108,000 ft)To have the RPM rotational forces of flight applied to it.

In truth, the axis of a bullet shot through a deer does not "spin". The rotational forces could only turn it from 12:00 o'clock to 3:00. (90degrees) And by then, the bullet would have exited the deer.
 
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Math and physics are never a silly notion and the ball really is spinning that fast (if the patch gripped the rifling hard enough to prevent any slipping). The rotational momentum is so small that it wouldn't make any difference to a target - it probably stops spinning almost immediately when it hits something.
As to whether the spinning slows down in flight, that is an interesting question. I think it would maintain most of it's spin because there wouldn't be much air resistance in that direction. It would be a really difficult thing to measure!
 

Gunny5821

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There was this icicle on the garage and it come down and broke my glasses.......😂
I think if I was Ralphie's daddy, I would have used the Red Ryder BB Gun on the Bumpus dogs, especially after they stole the turkey.

Speaking of RPMs, I collect 78 RPMs, I especially love my 1940s-1950s Hanks; Hank Williams, Hank Snow, Hank Thompson, and Hank Locklin.
 

M. De Land

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Lets see...,
My 1:48 twist rotates one revolution for every 48" of travel...
at 100 yards, to the target that's 300 feet or 3600 inches.... my bullet rotates a full rotation 75 times....
The flight time is ....., .2 seconds...., that would give me 22500 rpm at the muzzle...So yeah yours with that more than 2x twist rate would be like 2X mine...

Question..., since the bullet slows as it flies down range, does the rate of rpm also slow ??? So my round ball starts... at 22,500 rpm but is it impacting at 22,500 rpm ???

LD
Yes, it deteriorates but not nearly as fast as forward travel. I believe there is a mathematical equation to figure this out but I've never run the numbers.
In mid and long range shooting one can often see this as accuracy drops off at some point down range where gyroscopic stability is lost if rifling pitch is to slow. The .45 cal bullets we use generally are over 500 grains and need a 1-18 twist to stabilize them to 1000 yards.
With bullets and high rates of rotation speed another factor in trajectory calculation is "spin drift" which is a bullet moving in the direction of rotation without any wind influence.
 
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I always found it interesting that if you dropped a bullet at the same time that another left the barrel, they would both hit the ground at the same time. The fired bullet would stay up a bit longer due to wind resistance…or so I have read.
 

M. De Land

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I always found it interesting that if you dropped a bullet at the same time that another left the barrel, they would both hit the ground at the same time. The fired bullet would stay up a bit longer due to wind resistance…or so I have read.
This is true when the bore is level but wind deflection does not make a bullet fly longer.
Another fable folks often believe is that a bullet rises as it leaves the muzzle when shot level. This happens because people see diagrams of trajectory and line of sight examples of how to set up point of aim intersections at various distance. Gravity and air resistance go to work on a bullet as soon as it leaves the muzzle and both cause the imparted energy to deteriorate.
 
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The rate at which the bullet would lose its initial RPM is dependent on the viscosity of media the media in which it is traveling. The rifling leaves ridges on the bullet and these ridges react to the media around them. Air has the least viscosity of anything commonly encountered so would slow RPM the least. Water and blood tissue have more and something like roofing tar has a lot more. For normal use associated with gunfire at normal ranges, the rpm loss is negligible. A bullet from a 1-10 twist barrel at 2700 ft/sec is spinning at near 200,000 RPM.
 

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As to whether the spinning slows down in flight, that is an interesting question. I think it would maintain most of it's spin because there wouldn't be much air resistance in that direction. It would be a really difficult thing to measure!
AH for sure!

BUT since we're on a topic that's likely negligible for just about everybody who is shooting, EVEN out to 1000 yards, why not?

I know for example that stuff is very different when shooting at 1000 yards (I won't say what was being used to shoot) when humidity is different or the altitude has a big difference between one shooting range or another as the air density is thicker when it's humid and at low altitude....so velocity of the projectile in flight changes more..., why not the velocity of the spin changing more when the air is thicker ???

So would my bullet doing 22,500 rpm in New Mexico at 5000 feet still be doing that at sea level with 95% humidity in North Carolina..., or would it be doing only 22,050 rpm when it hit 100 yards? That's a 2% loss in rotation....... that's probably too much loss.... 🤪

This is like a black powder, ballistic version of how many angles can dance on the head of a clothing pin... but we have that saying because... folks discussed and debated that at one time....

LD
 
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