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Re: [Phys-l] Final velocity of bullets - spin rate

Being unfamiliar with guns I was surprised when Oren Quist wrote:

suppose the bullet spins two revolutions as it moves
through a 75 cm long barrel. (these numbers are somewhat arbitrary,
but reasonably close to actual -- please replace with better numbers
if you have them).

I would have guessed a higher spin rate. However using Bruce Esser's
high power rifle data:

muzzle velocity 2932 fps (or 89,370 cm per sec)
measured muzzle spin at 158,000 rpm (or 2630 rev. per sec)

For a 75 cm long rifle barrel, the number of spins will be,
approximately, (75 cm/89,370 cm per sec)*2630 rev per sec = 2.2 rev

So I learned something about bullet spin rates and certainly agree with
Oren Quist's comment.

Don Polvani
Northrop Grumman Corp.
Undersea Systems
Annapolis, MD

-----Original Message-----
From: phys-l-bounces@carnot.physics.buffalo.edu
[mailto:phys-l-bounces@carnot.physics.buffalo.edu] On Behalf Of B. Esser
Sent: Wednesday, January 24, 2007 8:26 AM
To: Forum for Physics Educators
Subject: Re: [Phys-l] Final velocity of bullets

On Tue, 23 Jan 2007, Quist, Oren wrote:

Of course a rifle bullet is spinning as well as translating. For the
sake of argument, suppose the bullet spins two revolutions as it moves

through a 75 cm long barrel. (these numbers are somewhat arbitrary,
but reasonably close to actual -- please replace with better numbers
if you have them).

Is the bullet still spinning when it reaches its maximum height?

It has been a few years since I participated in high power rifle
matches.
Spin rates for these target rifles varied between 100,000 and 300,000
rpm as measured at the muzzle. While there were various calculations for
minimum spin rates to establish stability, most competitors used a more
empirical method.
In my experience I never saw a tumbling bullet even at 1,000 yard
targets.
The holes in the target were circular. An occasional bounced bullet
would leave an interesting non circular hole. Of course launch angle was
less than 5 degrees and time of flight was less than a second.
Some real bullet data using my chronograph:
30-06 (bullet diameter of .308 inches), bullet weight 150 grains, muzzle
velocity 2932 fps, velocity at 200 yards was 2,318 fps. This was the
average of 25 shots. There was no wind, temp at 76 farenheit. While I
did not measure the actual spin rate for these bullets (Hornady) other
measurements with identical barrels (twist rate/length) measured muzzle
spin at 158,000 rpm.


--
Bruce Esser
Physics Teacher
Marian High School
http://marian.creighton.edu/~besser

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