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Bullet RPM ...... |
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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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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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"Don't argue with a fool! From a distance you can't really tell who's who!"
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supertool73
Optics God Superstool Joined: January/03/2008 Status: Offline Points: 11814 |
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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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338LAPUASLAP
Optics Master Scope Swapper Joined: October/17/2009 Status: Offline Points: 2596 |
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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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dsr
Optics Apprentice Joined: May/31/2007 Location: United States Status: Offline Points: 244 |
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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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338LAPUASLAP
Optics Master Scope Swapper Joined: October/17/2009 Status: Offline Points: 2596 |
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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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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.
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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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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 |
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338LAPUASLAP
Optics Master Scope Swapper Joined: October/17/2009 Status: Offline Points: 2596 |
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Vp has much to do with this as well. |
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dsr
Optics Apprentice Joined: May/31/2007 Location: United States Status: Offline Points: 244 |
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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 |
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dsr
Optics Apprentice Joined: May/31/2007 Location: United States Status: Offline Points: 244 |
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I am sorry 338LAPUASLAP I don't know what "Vp" is. Regards, dsr |
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338LAPUASLAP
Optics Master Scope Swapper Joined: October/17/2009 Status: Offline Points: 2596 |
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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?)
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338LAPUASLAP
Optics Master Scope Swapper Joined: October/17/2009 Status: Offline Points: 2596 |
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338LAPUASLAP
Optics Master Scope Swapper Joined: October/17/2009 Status: Offline Points: 2596 |
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dsr
Optics Apprentice Joined: May/31/2007 Location: United States Status: Offline Points: 244 |
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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 |
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dsr
Optics Apprentice Joined: May/31/2007 Location: United States Status: Offline Points: 244 |
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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 |
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338LAPUASLAP
Optics Master Scope Swapper Joined: October/17/2009 Status: Offline Points: 2596 |
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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
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338LAPUASLAP
Optics Master Scope Swapper Joined: October/17/2009 Status: Offline Points: 2596 |
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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.
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Peddler
Optics God Joined: July/04/2012 Location: Oswego,NY Status: Offline Points: 13526 |
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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! |
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Kickboxer
MODERATOR Moderator Joined: February/13/2008 Status: Offline Points: 23679 |
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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...
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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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Bet you've lost sleep over it...... On second thought, you NEVER seem to sleep so probably NOT... |
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