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# revolutions per minute debate

#1
04-21-2005, 09:38 AM
 Platinum Member Join Date: Sep 2004 Location: on the rifle range in Utah Posts: 2,704
revolutions per minute debate

Ok. I have several friends whom I have been talking with that have disagreed about this topic yesterday. I will ask the main questions here in a way as to not "steer" the answer so I can't be accused of wording the question in a way that leans the answer in my way of thinking. I will not say now what I believe to be the answer.
Ok, here it is:

If a bullet leaves a barrel that has a 1-8" twist, is it still spinning 1 time in every 8" all the way through it trajectory say out to 1000 yards? In other words, if we viewed the bullet at the 786th yard and followed it in slow motion throughout that yard, would it still be revolving 4.5 times in that yard? (lets assume the bullet has no flaws in the outer surface of the jacket, and there are no voids inside the bullet)

If you were to shoot a 55 grain bullet out of an 8 twist barrel at 4000 fps, and then shoot a 80 grain bullet out of the same barrel at 3000 fps, they have different rpm's, but do they both still spin 1 time every 8 inches?
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#2
04-21-2005, 09:52 AM
 Posts: n/a
Re: revolutions per minute debate

A bullet's rpm slowes in flight but not as quick as its forward motion. The forces slowing the forward motion are much greater than the forces slowing the spin.
8 twist is 8 twist no mater what fps. rpm increases as fps increase.
#3
04-21-2005, 09:57 AM
 Gold Member Join Date: Dec 2004 Location: Black Hills,South Dakota Posts: 669
Re: revolutions per minute debate

Yes,but as Hary says,the rotation will slow somewhat but nothing compared to the velocity loss.When compared to time and distance traveled,the bullet is spinning hundreds of thousands rpm.
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#4
04-21-2005, 10:05 AM
 Platinum Member Join Date: Jun 2007 Posts: 1,515
Re: revolutions per minute debate

Yes GG the "rate of twist" as we use it in shooting is fixed by the rifling. 1:8 is 1:8 and that is it. The RPM however is governed by the velocity of the bullet leaving a barrel. the faster the bullet the faster the rpm OR if the muzzle velocity is constant RPMs wil increase with a faster twist rate.

In other words, If your gun has a 1:36 twist (assuming the revolutions of the bullet dont slow down during the few second flight) the bullet wil make 100 revolutions in 100 yards regardless of how fast the bullet is.

There is no "snap" factor to rifling. The bullet does NOT spin faster than the rifling just after it leaves the muzzle. I know of many people that think the bullet speeds up after leaving the barrel for a few yards as well and this is not the case either.
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#5
04-21-2005, 11:12 AM
 Platinum Member Join Date: Jul 2003 Location: Central PA Posts: 1,307
Re: revolutions per minute debate

Yea, what they said, it only loses RPM's because the velocity is dropping off.

why does this matter anyway?

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#6
04-21-2005, 11:13 AM
 Platinum Member Join Date: Jan 2004 Location: Blackfoot, Idaho Posts: 8,574
Re: revolutions per minute debate

GG, Go here: http://www.nennstiel-ruprecht.de/bullfly/fomo.htm

and go down to "Spin Dampening Moment".

BTW: CTR = Choose The Ruger (#1 that is.) Or maybe its Remington. Or maybe its Come-on Truckers Roll.
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#7
04-21-2005, 11:52 AM
 Silver Member Join Date: Jul 2004 Location: Upland, CA Posts: 423
Re: revolutions per minute debate

I agree. Have heard, many times, that the RPM degrades much slower than velocity, and it makes sense, to me. However, other than the necessary (paper) computations, I really wonder if it can be ACTUALLY MEASURED, ....say, 500 yards downrange? Whatever, I accept the theory.

Good hunting. LB

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