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Bullet RPM ......

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    Posted: February/17/2016 at 00:45

So what RPM is a bullet doing?

(Please correct me if my maths is wrong here)

Let us take a bullet fired at 2800 ft/per second. To uncomplicated the matter let us assume an average speed of 2500ft per second.

Let us also assume a barrel twist of 1 in 12 inches.

2500x60/3 = 50000yds per min

So the bullet should turn once per 12 inches or 3 times per yard

Therefore the bullet will turn 50000/3 = 16666 times 50 000 x 3

Therefore an RPM of 16666. 150 000

The amazing fact is that a bullet at 2500 ft per sec takes  0.12 sec to travel 100yds

Therefore the bullet only turns 33.3 300 times before reaching your 100 yd target.



Edited by 8shots - February/18/2016 at 03:21
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Post Options Post Options   Thanks (0) Thanks(0)   Quote sucker76 Quote  Post ReplyReply Direct Link To This Post Posted: February/17/2016 at 02:44
MV x (12/twist rate in inches) x 60 = Bullet RPM

2500 fps x (12/12) x 60 = 150000rpm

It's late (or early?) So I can't single out the issue with your calculation.  I think its when you divided 50000 by 3.  I think you should have multiplied.  I pulled the above formula from accurateshooter.com.  No matter what the rotational speed is mind-boggling. 
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Rancid Coolaid Quote  Post ReplyReply Direct Link To This Post Posted: February/17/2016 at 08:08
Yards per minute is not a common unit of measure for us in the states, 8shot, you lost me there.
Beyond that, yes, avoid contact with fast-moving bullets!
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Steelbenz Quote  Post ReplyReply Direct Link To This Post Posted: February/17/2016 at 10:51
Wouter, if the bullet is traveling at 2500 fps from a 1 in 12 twist barrel it rotates once every foot, ergo 300 revolutions in 100 yards.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Steelbenz Quote  Post ReplyReply Direct Link To This Post Posted: February/17/2016 at 10:53
It's really irrelevant the speed just the twist rate. Theoretically, it would rotate 3000 times in a 1K shot.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote budperm Quote  Post ReplyReply Direct Link To This Post Posted: February/17/2016 at 12:12
My head is now spinning faster than any bullet... Smile
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Steelbenz Quote  Post ReplyReply Direct Link To This Post Posted: February/17/2016 at 15:00
1 in 12 twist give you 1 revolution per foot! 3000 feet=3000 rotations.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Rancid Coolaid Quote  Post ReplyReply Direct Link To This Post Posted: February/17/2016 at 20:44
Except that the bullet starts decelerating the instant it leaves the barrel. But, still, fast!
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Post Options Post Options   Thanks (0) Thanks(0)   Quote 338LAPUASLAP Quote  Post ReplyReply Direct Link To This Post Posted: February/17/2016 at 21:39
MV X 720 / TWIST RATE= EXACT RPM

AT DISTANCE RPM IS FACTOR OF VELOCITY DYNAMICS AND PRESSURE.

Result or Density of Air Humidity Heat holding capacity or Elevation Pressure, Barometric, Temperature, Drag, wind speed wind direction (Charge) and Ballistics Coeff G1 G7 and gyroscopic stability.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote 338LAPUASLAP Quote  Post ReplyReply Direct Link To This Post Posted: February/17/2016 at 21:49
Off the top of my head I think the formula for a 1:8 twist 223 at 3000fps is like 1.5 revs per every 12 inches so in 100 yrds 33 revs sounds slow...
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Post Options Post Options   Thanks (0) Thanks(0)   Quote 8shots Quote  Post ReplyReply Direct Link To This Post Posted: February/18/2016 at 03:12

Originally posted by Steelbenz Steelbenz wrote:

Wouter, if the bullet is traveling at 2500 fps from a 1 in 12 twist barrel it rotates once every foot, ergo 300 revolutions in 100 yards.

The time it takes to reach the target can also be used to determine the number of times it turns.distance?

So on the correct RPM of 150000 and .12 sec = 300

My 33 times was based on the wrong RPM, which I corrected above but not the turns out to 100yds.



Edited by 8shots - February/18/2016 at 03:20
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Post Options Post Options   Thanks (0) Thanks(0)   Quote 8shots Quote  Post ReplyReply Direct Link To This Post Posted: February/18/2016 at 03:13

Originally posted by Steelbenz Steelbenz wrote:

It's really irrelevant the speed just the twist rate. Theoretically, it would rotate 3000 times in a 1K shot.

I think speed matters as RPM is a time factor.

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Post Options Post Options   Thanks (0) Thanks(0)   Quote 8shots Quote  Post ReplyReply Direct Link To This Post Posted: February/18/2016 at 03:24

Originally posted by Rancid Coolaid Rancid Coolaid wrote:

Yards per minute is not a common unit of measure for us in the states, 8shot, you lost me there.
Beyond that, yes, avoid contact with fast-moving bullets!

Neither here, we talk m/s.

I converted to yards to get to a 100yd target scenario, a distance that we mostly target shoot at.

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Post Options Post Options   Thanks (0) Thanks(0)   Quote Steelbenz Quote  Post ReplyReply Direct Link To This Post Posted: February/18/2016 at 07:11
8, yes you can find out the revolutions the bullet will spin in a given amount of time but its all based on the twist of the barrel. Barrel twist determines the length of the bullet that barrel can reliably stabilize.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote 8shots Quote  Post ReplyReply Direct Link To This Post Posted: February/18/2016 at 07:27

Originally posted by Steelbenz Steelbenz wrote:

8, yes you can find out the revolutions the bullet will spin in a given amount of time but its all based on the twist of the barrel. Barrel twist determines the length of the bullet that barrel can reliably stabilize.

You are quite correct, that is why I stated the barrel twist as one of the factors.

The RPM will be determined by barrel twist and bullet speed.

A 1:8 twist at the same speed will give a higher RPM then a 1:12. At the same time a 1:8 twist at 600ft per sec will give a slower RPM than a 2500ft per sec. through a 1:8 twist

And me thinks that is where the balance needs to be found for a specific weight bullet: What RPM does it need to stabilize. Can your rifle handle enough pressure to send a 200gr bullet at the right RPM? Etc etc.

Am I wrong or am I wrong???

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Post Options Post Options   Thanks (0) Thanks(0)   Quote Steelbenz Quote  Post ReplyReply Direct Link To This Post Posted: February/18/2016 at 07:34
You're not really wrong, just the reason behind the number needs to be addressed. The reason the twist needs to be faster on a heavier bullet is the length of the bullet, not the weight of it. A solid copper bullet of the same weight will be longer than a traditional lead core copper jacketed bullet. Ergo, it needs a faster twist barrel.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Steelbenz Quote  Post ReplyReply Direct Link To This Post Posted: February/18/2016 at 07:37
Think of 2 tops spinning, both of the same weight. One short and fat, the other tall and skinny. The skinny one needs to spin faster to stay stable due to the amount of area in Z that forces can be applied to it.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Steelbenz Quote  Post ReplyReply Direct Link To This Post Posted: February/18/2016 at 07:50
As RC noted rotation is degrading as fast as velocity as soon as it leaves the barrel so you've really gotta push that 220gr pill hard in a 1 in 12 or 1 in 14 to keep it stable and it will not be stable long.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote dsr Quote  Post ReplyReply Direct Link To This Post Posted: February/18/2016 at 09:15
Originally posted by Steelbenz Steelbenz wrote:


As RC noted rotation is degrading as fast as velocity as soon as it leaves the barrel ...
(My emphasis added)


Rotation does not decrease as fast as velocity. The bullet becomes more stable until it reaches sonic / transonic flight then a large change in center of pressure on the bullet makes it less stable. With long range (very low drag) bullets having larger upsetting moments than shorter high drag bullets.

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Post Options Post Options   Thanks (0) Thanks(0)   Quote Rancid Coolaid Quote  Post ReplyReply Direct Link To This Post Posted: February/18/2016 at 11:04
I've heard the "settling" argument before, but haven't seen the math behind it, do you have that readily available?

Could you explain how the bullet becomes more stable in flight?
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Steelbenz Quote  Post ReplyReply Direct Link To This Post Posted: February/18/2016 at 11:12
I too have heard the " you have to let the bullet go to sleep" statement but like RC, I can't see the math.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote supertool73 Quote  Post ReplyReply Direct Link To This Post Posted: February/18/2016 at 11:26
Bryan Litz pretty much disproved the whole going to sleep theory in his book applied ballistics.  At least as far as the bullet having a lot of "yaw" when it first comes out of the tube and them stabilizing more and more as it goes farther and farther to the point that it actually changes accuracy at farther distances.  Here is the section on it.

http://www.appliedballisticsllc.com/epswerve.html
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Post Options Post Options   Thanks (0) Thanks(0)   Quote 338LAPUASLAP Quote  Post ReplyReply Direct Link To This Post Posted: February/18/2016 at 13:25

I can describe and possible give you the calculation exactly what do you need from me? Be specific.

I have had many interesting discussions in person and email and phone and have developed things(formulas and tools) that were theories to some of the mentioned people and we put them into practice.


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Post Options Post Options   Thanks (0) Thanks(0)   Quote dsr Quote  Post ReplyReply Direct Link To This Post Posted: February/18/2016 at 14:05
Originally posted by Rancid Coolaid Rancid Coolaid wrote:

Could you explain how the bullet becomes more stable in flight?


As the bullet travels down range the aerodynamic force is reduced which can be considered applied at the center of pressure which is not located at the center of gravity. The resulting two forces applied, one not varying (mass) applied at the center of mass, and one decreasing with speed applied at the center of pressure reduces the overturning fores (moment). Making the bullet more stable.

No where in my preceding post did I mention the bullet "going to sleep".

All the above is contained in Litz's book.
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