Spin or Corkscrew?

[Ray B]

I have always thought that when a bullet was fired through a rifled barrel that it had two movements that were unrelated to each other. The bullet had forward velocity and when it left the muzzle it was measured as muzzle velocity; and due to the rifling it had spin referred to in rotations per second. So for example a bullet fired to a MV of 2000 fps out of a barrel that had a 1 rotation per 1 foot, the bullet would be spinning at 2000 rps. And for example lets say due to air resistance the forward velocity of the bullet slowed to 1000 fps, my thoughts have been that the spin of the bullet didn't face such air resistance so would have continued to spin with only slight loss in rps.

Now I talk with a fellow that "for years" was a firearms (handguns) instructor for a LE agency. He claims that the bullet isn't actually spinning, but rather is traveling through the air much like a corkscrew- initially at 2000 fps and at 2000 rps, but as the bullet slowed the spin decrease was proportional; so if the bullet was at 1000fps the bullet would be spinning at 1000 rps.

So I ask this highly experienced group- which is it? and is it possible that bullets at low velocity with very low ballistic coefficients (such as 148 gr wadcutters from a 38Spl) would tend to follow the corkscrew example that the instructor was referring?

 

[swd]

If you go quite a ways back in the bulletin Bryan Litz did a really good article on it. More of a tip wobble than a cork screw.

 

[scubohuntr]

There is a bit of "looping" visible if you watch bullet tracks through a spotting scope at long range. I'm not sure it isn't some sort of optical illusion, though, as the scope is generally off to the side a bit. A little windage and a little drop could combine to look like a loop. If you look at the vectors involved, there's really no way a corkscrew trajectory could work. If the center of rotation is offset from the centerline of a bullet (poorly constructed bullet), the dimensional centerline could follow a somewhat spiral path, but the center of rotation is still on trajectory. The factors working on velocity and drag are very different, as evidenced by a bullet coming to rest still spinning (Mythbusters did a segment on this a while back- very entertaining).

 

[ED3]

There is a bit of "looping" visible if you watch bullet tracks through a spotting scope at long range. I'm not sure it isn't some sort of optical illusion, though, as the scope is generally off to the side a bit. A little windage and a little drop could combine to look like a loop. If you look at the vectors involved, there's really no way a corkscrew trajectory could work. If the center of rotation is offset from the centerline of a bullet (poorly constructed bullet), the dimensional centerline could follow a somewhat spiral path, but the center of rotation is still on trajectory. The factors working on velocity and drag are very different, as evidenced by a bullet coming to rest still spinning (Mythbusters did a segment on this a while back- very entertaining).

"There is a bit of "looping" visible if you watch bullet tracks through a spotting scope at long range..."
I've seen the same thing at close range shooting BBs and viewing through the gun's scope...
shooting into clear air, at low velocity. I attribute it to something along the lines of parallax error, if that's the correct term.

 

[Dusty Stevens]

if it corkscrewed and you moved the target back 5yds how big would the group get?