Print Page | Close Window

Bullet RPM ......

Printed From: OpticsTalk by SWFA, Inc.
Category: Firearms, Bows, and Ammunition
Forum Name: Shooting
Forum Description: Techniques, tips, stories, general discussion, etc.
URL: http://www.opticstalk.com/forum_posts.asp?TID=42731
Printed Date: August/13/2020 at 06:54
Software Version: Web Wiz Forums 12.01 - http://www.webwizforums.com


Topic: Bullet RPM ......
Posted By: 8shots
Subject: Bullet RPM ......
Date 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.




Replies:
Posted By: sucker76
Date 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. 


-------------
Stamp collecting since 2015


Posted By: Rancid Coolaid
Date 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!


-------------
Freedom is something you take.
Respect is something you earn.
Equality is something you whine about not being given.


Posted By: Steelbenz
Date 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.

-------------
"Don't argue with a fool! From a distance you can't really tell who's who!"


Posted By: Steelbenz
Date 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.

-------------
"Don't argue with a fool! From a distance you can't really tell who's who!"


Posted By: budperm
Date Posted: February/17/2016 at 12:12
My head is now spinning faster than any bullet... Smile

-------------
"Peace is that brief glorious moment in history when everybody stands around reloading".
--Thomas Jefferson





Posted By: Steelbenz
Date Posted: February/17/2016 at 15:00
1 in 12 twist give you 1 revolution per foot! 3000 feet=3000 rotations.

-------------
"Don't argue with a fool! From a distance you can't really tell who's who!"


Posted By: Rancid Coolaid
Date Posted: February/17/2016 at 20:44
Except that the bullet starts decelerating the instant it leaves the barrel. But, still, fast!

-------------
Freedom is something you take.
Respect is something you earn.
Equality is something you whine about not being given.


Posted By: 338LAPUASLAP
Date 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.


Posted By: 338LAPUASLAP
Date 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...


Posted By: 8shots
Date 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.



Posted By: 8shots
Date 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.



Posted By: 8shots
Date 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.



Posted By: Steelbenz
Date 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.

-------------
"Don't argue with a fool! From a distance you can't really tell who's who!"


Posted By: 8shots
Date 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???



Posted By: Steelbenz
Date 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.

-------------
"Don't argue with a fool! From a distance you can't really tell who's who!"


Posted By: Steelbenz
Date 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.

-------------
"Don't argue with a fool! From a distance you can't really tell who's who!"


Posted By: Steelbenz
Date 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.

-------------
"Don't argue with a fool! From a distance you can't really tell who's who!"


Posted By: dsr
Date 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.

Respectfully,
dsr
       


Posted By: Rancid Coolaid
Date 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?


-------------
Freedom is something you take.
Respect is something you earn.
Equality is something you whine about not being given.


Posted By: Steelbenz
Date 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.

-------------
"Don't argue with a fool! From a distance you can't really tell who's who!"


Posted By: supertool73
Date 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 - http://www.appliedballisticsllc.com/epswerve.html

-------------
Lifetime warranty and excellent customer service don't mean a thing when your gun fails during a zombie attack.

"A Liberal is a person who will give away everything they don't own."


Posted By: 338LAPUASLAP
Date 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.




Posted By: dsr
Date 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.


Posted By: 338LAPUASLAP
Date Posted: February/18/2016 at 14:24

Interesting


http://appliedballisticsllc.com/about-us/ - http://appliedballisticsllc.com/about-us/


http://appliedballisticsllc.com/ballistics-resources/abseminar2016/ - http://appliedballisticsllc.com/ballistics-resources/abseminar2016/


who knew...



Posted By: Steelbenz
Date Posted: February/18/2016 at 14:38
LITZ used the term. Still doesn't matter, the longer bullet needs to spin faster for the very reason I've stated. Litz would also agree. As the bullet slows force applied to the bullet makes it want to fall over just like a top, only on a different plane.

-------------
"Don't argue with a fool! From a distance you can't really tell who's who!"


Posted By: dsr
Date Posted: February/18/2016 at 15:53
Steelbenz,

To see what I am describing read the chapter on bullet stability. Specifically the section on dynamic stability vs gyroscopic stability. (ie - the horseshoe graph).

I do agree with your statement on why a longer bullet needs a faster twist rate but I believe the chapter on bullet stability will clarify your understanding of the dynamics better.

Respectfully,
dsr


Posted By: 338LAPUASLAP
Date Posted: February/18/2016 at 17:22

Vp has much to do with this as well.



Posted By: dsr
Date Posted: February/18/2016 at 17:37
Originally posted by dsr dsr wrote:

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)


Steelbenz statement that rotation is degrading as fast as velocity is false according to Litz. It remains close to its muzzle value.

Looking at the problem from a different view ( remembering that rotation is basically constant for the bullets flight) and using the example earlier in this thread. A .308 with a muzzle velocity of 3000 ft/s in a 1 in 12 twist rate give 3000 rotations per second.

And let's say for sake of argument ( round numbers) that it has a 1000 yard velocity of 1100 ft/s and since we agree that the rotation is still 3000 rotations per second. Reverse solving for twist rate gives 1 in 4.4 twist rate.

Using JBM Ballistics stability calculator with the following inputs at the muzzle.
.308 caliber, 1.5 inches length, 220 grains, 0.0 plastic tip, 3000 ft/s, twist rate 12, temp 59, pressure 29.92. Gives a stability factor of 1.265 at the muzzle.

And as agreed to for this argument the rotation of the bullet at 1000 yards still has 3000 turns/second with a velocity of 1100 ft/s with an effective twist rate of 1 in 4.4. Re-running the JBM calculations with the new velocity and twist rate gives a stability factor of 6.775.

So we see that the bullet has gained stability.

Regards,
dsr


Posted By: dsr
Date Posted: February/18/2016 at 17:43
Originally posted by 338LAPUASLAP 338LAPUASLAP wrote:

Vp has much to do with this as well.



I am sorry 338LAPUASLAP I don't know what "Vp" is.

Regards,
dsr


Posted By: 338LAPUASLAP
Date Posted: February/18/2016 at 18:13
Velocity Pressure Wave and Drag or Shock wave from bullets tip and surface area along with the twist rate loss factor and the left and right running bow waves and tail waves resulting in a turbulent tail wave and turbulent wake wave that apply force/pressure as the bullet is pulled downward by gravity and the density factor of the air it travels through resulting in this gradually slowing revs p/m resultant from force rotational drag factor.  (Anyone remember the old free bore boost effect on rpm of bullet?)


Posted By: 338LAPUASLAP
Date Posted: February/18/2016 at 18:13



Posted By: 338LAPUASLAP
Date Posted: February/18/2016 at 18:16



Posted By: dsr
Date Posted: February/18/2016 at 18:26
Pressures across the surface of a body can be integrated to produce a resultant vector and a moment on the body.

I can can do the pressure calculations on the surface of a right cone relativly easly in supersonic flow but I don't know any relatively easy closed form solutions to a compression corner, expansion fan, expansion fan, compression corner for a body of rotation.

Regards,
dsr


Posted By: dsr
Date Posted: February/18/2016 at 18:31
In the top most shadow graph one can see the shock waves as thin black oblique lines at the front and rear of the bullet. The expansion fans are the less distinct oblique lines between the shock waves.

Regards,
dsr


Posted By: 338LAPUASLAP
Date Posted: February/18/2016 at 18:32
Wah Wah did did you you say say?

We all have much to learn.

I understand some of what you posted above in differing degrees in a variety of other fields and scenarios but what is exactly are you referring or making a inference to in regards to the above posted question or the bullet rpm post?

l


Posted By: 338LAPUASLAP
Date Posted: February/18/2016 at 18:44
I looked back at what I think you were trying to draw conclusion or reference to.  

So that you know you use a factor for the collisions.  Each Lab has their own calculations or Sub factors.


Posted By: Peddler
Date Posted: February/18/2016 at 19:55
I don't care much what it does on the way to the " Boiler Room " just as long as it kills when it gets there!!!

Mine usually do!

-------------
When you are dead, you don't know you are dead.It is difficult only for others.

It is the same when you are stupid.


Posted By: Kickboxer
Date Posted: February/18/2016 at 20:01
The more you know about the flight characteristics of the projectile, the better prepared you are for putting it "on target" at whatever range you choose.  If you are a "control freak", you want to know these things...

-------------
Opinion,untempered by fact,is ignorance.

There are some who do not fear death... for they are more afraid of not really living


Posted By: budperm
Date Posted: February/19/2016 at 05:52

Bet you've lost sleep over  it......



On second thought, you NEVER seem to sleep so probably NOT... Smile



-------------
"Peace is that brief glorious moment in history when everybody stands around reloading".
--Thomas Jefferson





Posted By: Kickboxer
Date Posted: February/19/2016 at 06:17
I think Bud just called me a freak...

-------------
Opinion,untempered by fact,is ignorance.

There are some who do not fear death... for they are more afraid of not really living


Posted By: BeltFed
Date Posted: February/19/2016 at 06:52
Originally posted by Kickboxer Kickboxer wrote:

I think Bud just called me a freak...


Unless the Energizer Bunny is a freak, then I don't think so.

I wonder what the ballistic computer in the M1A1 tank includes in all of it's calculations when making a running shot. But since that round is fin stableized, and the fins impart the spin on the projectile instead of the barrel, then it's probably different in many ways.


-------------
Life's concerns should be about the 120lb pack your trying to get to the top of the mountain, and not the rock in your boot.


Posted By: Stevey Ducks
Date Posted: February/20/2016 at 19:20

Rifling in rifle barrels imparts gyroscopic stability to bullets and that prevents them from turning sideways.



Posted By: Peddler
Date Posted: February/20/2016 at 19:24
I will go with "Bang Dead " WTH ?????

RPM still dead!!!

-------------
When you are dead, you don't know you are dead.It is difficult only for others.

It is the same when you are stupid.


Posted By: Cebe45
Date Posted: October/11/2018 at 12:41
Why?Smile

-------------
https://www.atncorp.com/smart-hd-weapon-sight - riflescope



Print Page | Close Window

Forum Software by Web Wiz Forums® version 12.01 - http://www.webwizforums.com
Copyright ©2001-2018 Web Wiz Ltd. - https://www.webwiz.net