Bullet rotation vs velocity

[RockyMtnMT]

I have questions about bullet physics brought up by the HAT bullet test thread by Goodgrouper. I did not want to ask these ?'s on that thread for fear of being misunderstood in my intentions. My ? is in no way directed towards GG's testing.

What I want to know is, what is the rate of decline for the bullet spin as it travels down range? I have never heard that a bullet has to start off w/ enough rpm's in order to have enough down range to remain stable.

Does rpm slow down at the same rate that velocity slows down? The only info that I know of is that it takes a certain bullet stabilization factor, depending on the length of the bullet. That it takes less stabilization for longer ranges, but that a bullet w/ proper stabilization for 500yrds and beyond will not function well for hunting under 500yrds. The longer the bullet gets the more critical this becomes because of the issue of tumbling on impact. In my simple mind, the more spin the better. I know this doesn't work, but it seems that way. I know some bullets will come apart due to centrifugal force if over ratated.

I am getting a little long winded for a short question. How much rpm is lost per yard down range?

Thanks, Steve

 

[Varminator 911]

What I've heard is that rpms don't drop much with distance. There's not much to slow down the spin other than the roughness of rifling engraved on the bullet. Would seem to be a pretty hard number to measure. But guess you could use a high speed camera.

I'm thinking it's important to get the needed rpm out the muzzle. When the speed has dropped to half muzzle velocity, the rpm is probably much above half. That's my thought anyway.

 

[AJ Peacock]

Here's a good discussion about it. (make sure you read past the first page or 2, to get down to the brass tacks).

http://www.longrangehunting.com/forums/f19/revolutions-per-minute-debate-11293/?highlight=decay

AJ

 

[Varminator 911]

My take on rpm loss with distance

My take from reading the discussion AJ posted is that most believe that spin slows very very little with distance. This is probably especially true for a long streamlined bullet like the HAT. But there were no absolute numbers given probably because they would be very hard to obtain and aren't significant enough to affect anything. My take is that the decay in rpm over 1000 yds is probably less than 1 percent and certainly much less than the loss in velocity.

 

[jwp475]

A bullet fred from a 1 in 10 twist is only going to make one revolution per 10 inches of travel, no more no less..

 

[Varminator 911]

jwp475:

You need to read the discussion AJ posted. Because the forward speed decays much faster than the spin, as the bullet travels away from the muzzle, the bullet will spin increasingly faster than 1 revolution per 10 inches. The rpm stays nearly unchanged while the forward speed rapidly decreases.

Pretty simple really, but not important.

With regards to bullet testing, it is important that the necessary spin be imparted at the muzzle. Spin is a combination of twist and muzzle velocity. You need an adequate mix of both to stabilize the bullet. The bullet remains stable downrange because the rpm remain virtually unchanged as the velocity decreases.

 

[royinidaho]

Here's a good discussion about it. (make sure you read past the first page or 2, to get down to the brass tacks).

http://www.longrangehunting.com/forums/f19/revolutions-per-minute-debate-11293/?highlight=decay

AJ

AJ,

What an underhanded thing to do. Post that link, that is.

I can't believe I read that whole darn thing again

 

[AJ Peacock]

AJ,

What an underhanded thing to do. Post that link, that is.

I can't believe I read that whole darn thing again

Me too! and I laughed just as hard the second time at BuffaloBob's message on page 6. That was classic.

AJ

 

[MontanaRifleman]

The physics that control spinning would be basically the same as the physics that control the velocity of a bullet.

An object in motion tends to stay in motion.

A bullet shot space would never stop or slow down unless it met with amother object. Nor would it ever stop spinning.

It takes force to put an object in motion, and it takes equal force to stop that motion. The explosion of the powder charge is the force that puts the bullet into motion, and the force that causes it to spin is the velocity of the bullet combined with the frictional force of the rifling.

The force that causes the bullet to slow down is the friction it experiences from passing through air which is also the same force that slows the spinning down. Friction from air has a lot more effect on bullet velocity than bullet spin.

If you fired a bullet straight up into the air it would still be spinning when the bullet finally stopped and started falling back to earth and still be spinning when it hit the earth. Try it with a football or baseball or whatever... toss it straight up with a spin and watch while it falls back to earth with almost the same rate of spin it had going up.

That's basically it in a nutshell.

 

[eddybo]

I would vote for that post as my all time favorite on this site. Now having cast my vote I must try and get the beverages off my keyboard.

Me too! and I laughed just as hard the second time at BuffaloBob's message on page 6. That was classic.

AJ

 

[jwp475]

jwp475:

You need to read the discussion AJ posted. Because the forward speed decays much faster than the spin, as the bullet travels away from the muzzle, the bullet will spin increasingly faster than 1 revolution per 10 inches. The rpm stays nearly unchanged while the forward speed rapidly decreases.

Pretty simple really, but not important.

With regards to bullet testing, it is important that the necessary spin be imparted at the muzzle. Spin is a combination of twist and muzzle velocity. You need an adequate mix of both to stabilize the bullet. The bullet remains stable downrange because the rpm remain virtually unchanged as the velocity decreases.

I agree. I probable didn't word my reply well enough, bit that was what I was trying to say, the spin remains relitively consant.

 

[adam]

Time of flight is extremely short as well, even for extreme range. I've heard figures up to 150,000 rpm. even a t.o.f. of several seconds wouldn't be long enough to slow the spin much.

 

[RockyMtnMT]

No feather ruffling is intended here.

I seems that proper spin on a bullet is necessary to accurately determine what a bullet will do at any given velocity. So from what I am gathering, the only way to do this is to fire at full velocity in order to get the proper rpm's, and set the target far enough to get the desired impact velocity?

Again, I do not want to upset anyone. I just want to know, this stuff is fascinating.

Thanks, Steve

 

[Varminator 911]

Steve:

That how I read it. You need a fully stable bullet to be sure your test is legit.