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Bullet RPM ...... |
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8shots
Optics Jedi Knight Lord Of The Flies Joined: March/14/2007 Location: South Africa Status: Offline Points: 6253 |
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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 Therefore an RPM of The amazing fact is that a bullet at 2500 ft per sec takes 0.12 sec to travel 100yds Therefore the bullet only turns Edited by 8shots - February/18/2016 at 03:21 |
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sucker76
Optics Journeyman Joined: June/13/2013 Location: Texas Status: Offline Points: 392 |
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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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Stamp collecting since 2015
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Rancid Coolaid
MODERATOR Joined: January/19/2005 Location: United States Status: Offline Points: 9318 |
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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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Steelbenz
Optics Jedi Knight ROLL TIDE ROLL Joined: January/03/2006 Location: Heart of Dixie Status: Offline Points: 5153 |
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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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"Don't argue with a fool! From a distance you can't really tell who's who!"
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Steelbenz
Optics Jedi Knight ROLL TIDE ROLL Joined: January/03/2006 Location: Heart of Dixie Status: Offline Points: 5153 |
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It's really irrelevant the speed just the twist rate. Theoretically, it would rotate 3000 times in a 1K shot.
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"Don't argue with a fool! From a distance you can't really tell who's who!"
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budperm
Optics Retard show me your sheep!! Joined: January/01/2009 Location: Pennsylvania Status: Offline Points: 31710 |
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My head is now spinning faster than any bullet...
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"Peace is that brief glorious moment in history when everybody stands around reloading".
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Steelbenz
Optics Jedi Knight ROLL TIDE ROLL Joined: January/03/2006 Location: Heart of Dixie Status: Offline Points: 5153 |
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1 in 12 twist give you 1 revolution per foot! 3000 feet=3000 rotations.
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"Don't argue with a fool! From a distance you can't really tell who's who!"
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Rancid Coolaid
MODERATOR Joined: January/19/2005 Location: United States Status: Offline Points: 9318 |
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Except that the bullet starts decelerating the instant it leaves the barrel. But, still, fast!
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Freedom is something you take.
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338LAPUASLAP
Optics Master Scope Swapper Joined: October/17/2009 Status: Offline Points: 2596 |
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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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338LAPUASLAP
Optics Master Scope Swapper Joined: October/17/2009 Status: Offline Points: 2596 |
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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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8shots
Optics Jedi Knight Lord Of The Flies Joined: March/14/2007 Location: South Africa Status: Offline Points: 6253 |
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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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8shots
Optics Jedi Knight Lord Of The Flies Joined: March/14/2007 Location: South Africa Status: Offline Points: 6253 |
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I think speed matters as RPM is a time factor. |
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8shots
Optics Jedi Knight Lord Of The Flies Joined: March/14/2007 Location: South Africa Status: Offline Points: 6253 |
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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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Steelbenz
Optics Jedi Knight ROLL TIDE ROLL Joined: January/03/2006 Location: Heart of Dixie Status: Offline Points: 5153 |
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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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"Don't argue with a fool! From a distance you can't really tell who's who!"
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8shots
Optics Jedi Knight Lord Of The Flies Joined: March/14/2007 Location: South Africa Status: Offline Points: 6253 |
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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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Steelbenz
Optics Jedi Knight ROLL TIDE ROLL Joined: January/03/2006 Location: Heart of Dixie Status: Offline Points: 5153 |
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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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"Don't argue with a fool! From a distance you can't really tell who's who!"
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Steelbenz
Optics Jedi Knight ROLL TIDE ROLL Joined: January/03/2006 Location: Heart of Dixie Status: Offline Points: 5153 |
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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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"Don't argue with a fool! From a distance you can't really tell who's who!"
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Steelbenz
Optics Jedi Knight ROLL TIDE ROLL Joined: January/03/2006 Location: Heart of Dixie Status: Offline Points: 5153 |
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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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"Don't argue with a fool! From a distance you can't really tell who's who!"
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dsr
Optics Apprentice Joined: May/31/2007 Location: United States Status: Offline Points: 244 |
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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 |
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Rancid Coolaid
MODERATOR Joined: January/19/2005 Location: United States Status: Offline Points: 9318 |
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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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Freedom is something you take.
Respect is something you earn. Equality is something you whine about not being given. |
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