Now just suppose a bullet

[Charles]

leaves a 1-15 twist barrel at 1,500 fps.

1) Does the bullet maintain a constant 1-15 spin for it's entire flight?

or

2) Does the bullet, free from the constraints of the barrel, spin faster after it leaves the muzzle?

or

3) As the bullet loses velocity, due to the increased distance from the muzzle, does the rate of spin also slow down?

Ah, the things our minds turn too, when sleep seems hard to find.

[dbateman]

i belive the rotation remains pretty much constent thrue the bullets flight

[Ram Hammer]

I'll go with door number three, Montie

The atmosphere is imparting friction on the bullet acting both on velocity and rotational speed. We have observed the frictional effects on bullet rotation by the change in point of impact under varying wind conditions. Here is the scenario. You are shooting in a Black Powder Silhouette match sighted in on the Ram at 500 meters with no cross wind. Your rifle has right hand twist 1:18 rifling. When a right to left cross wind comes up, the point of impact changes to the left and up! The reason is that the rifling engraved on the bullets surface is acting as a paddle that catches the air. The cross wind makes the air on one side of the bullet denser thus giving the paddle (engraved rifling) more to grab onto. A cross wind from left to right would lower the point of impact for the same reason. No cross wind simply puts uniform friction on the bullet slowing down the rotational velocity but not changing the point of impact.

If the frictional effects of air on the rotation of the bullet can be observed, then the bullet rotation must slow down through its flight path.

[Andrew]

I'll go with door number three, Montie

The atmosphere is imparting friction on the bullet acting both on velocity and rotational speed. We have observed the frictional effects on bullet rotation by the change in point of impact under varying wind conditions. Here is the scenario. You are shooting in a Black Powder Silhouette match sighted in on the Ram at 500 meters with no cross wind. Your rifle has right hand twist 1:18 rifling. When a right to left cross wind comes up, the point of impact changes to the left and up! The reason is that the rifling engraved on the bullets surface is acting as a paddle that catches the air. The cross wind makes the air on one side of the bullet denser thus giving the paddle (engraved rifling) more to grab onto. A cross wind from left to right would lower the point of impact for the same reason. No cross wind simply puts uniform friction on the bullet slowing down the rotational velocity but not changing the point of impact.

What he said. I didn't know about how that effected POI though, thanks for sharing the info.

[Ysabel Kid]

I also think I read somewhere that rotational spin decreases in flight. I think it is extremely minor for most rounds over the course of their path, given speed and distance, but if you were to launch something like a .45-70 in a high arc, trying to get maximum distance, it would really have an impact (no pun intended!).

[Pisgah]

Like a spinning top, the rotation does begin to slow the instant the bullet leaves the barrel. But the time of flight is very short, so the spin slows very little during that time. For either velocity or RPMs to increase after the bullet leaves the barrel is a physical impossibility .

[KirkD]

From my observations of exploding aircraft engines on the ground, the turbine hubs keep spinning long after they have stopped their forward movement, provided they are on concrete (don't ever go near one until it falls over). Similarly, the bullet will spin a slowly decreasing rpm that is not locked its forward velocity. rpm is not to be confused with twist, which is one turn every so many inches.

[Buffboy]

Like the others have said, the rpm of the bullet(72,000rpm in your example, if I did the math right) starts slowing as soon as the bullet leaves the barrel. The bullet won't loose as much rotational velocity in a normal flight path as it looses in forward velocity because the the drag is higher in that direction but there is drag slowing both.

[Malamute]

I recall a writer (Cooper?) mentioning seeing/hearing bullets that had been fired upwards, falling back to the ground and "hissing wickedly" in the sand from the bullet's remaining rotational momentum.

[Pete44ru]

#3

[adirondakjack]

The RPMs of the bullet will slow as time passes, but since the time of flight is very short, and since the bullet loses FORWARD velocity at a much higher rate than rotational velocity (more drag on the front than the "skin" of the bullet), Visualize a bowling ball, which hooks as forward speed drops off and rotational spin, retained better than forward speed, eventually gains traction and "grabs" the lane. So in truth the rate of spin expressed as a ratio of forward flight actually INCREASES because forward speed falls off so fast.



In other words, suppose the bullet starts out at 1 turn per 15 inches, while moving forward at 1500 fps. What we have is one turn per 1/1500 seconds, (nevermind the forward speed). Now suppose at some point in it's flight the bullet has slowed to 750fps forward speed. The rotational velocity is probably not slowed too much by then, maybe one turn per 1/1200 seconds. So if rotational speed has only slowed by say 20%, and forward speed has slowed by 50%, then the "rate of twist" in terms of inches of flight per rotation has INCREASED.

Fackler showed us with some very neat photography how a bullet might spin as many as 5 or 10 or even 50 revolutions after forward motion had ceased upon stopping against a hard mass. clearly the rate of spin expressed in inches per turn had gone waay up as forward speed was almost nil.....

This is one of the problems with "over-stable" bullets, as seen in the sandy Hook tests, tended to be facing parallel to the ground even as the arc of flight had them lobbing in at a 30 degree angle.


FWIW, a bullet has reached maximum rpms at the muzzle, after which there is no more force applied to spin it, so it must begin slowing.

[AJMD429]

I recall a writer (Cooper?) mentioning seeing/hearing bullets that had been fired upwards, falling back to the ground and "hissing wickedly" in the sand from the bullet's remaining rotational momentum.

As a kid I shot a pellet gun straight up, just to see how long until the pellets hit the ground, then back-calcuulate the muzzle velocity and height achieved (didn't have a chronograph), and even as a kid, was nervous (I actually wore SAFETY GOGGLES - as a KID!) about being hit.

Imagine doing that with a "real" gun! I'd want to be standing under a steeple made of armor or Kevlar!

[Hillbilly]

for most projectiles traveling somewhat parrallel to the earths surface I'd think gravity would win before the bullet ran out of spin.

I guess a guy with an SMLE or such with volley sights could experiment and see what happens. If you could get it on paper at maximum range wouldnt the bullet tumble if it ran out of spin? You could see evidence of tumbling on the paper target.

Anyone of you modern guys have a software package that would model this? I am curious too.
Hill

[centershot]

1) Yes, the rate of rotation is constant unless acted upon by something other than the bullet's flight through the air.

2) No, the rate of rotation, imparted to the bullet by the rifling, remains constant, HOWEVER, the RPM begins to slow as the bullrt sheds velocity dur to drag with the atmosphere.

3) No, the rate of spin (twrst rate) remains constant, as stated above. RPM begins to slow.

Also, I would like to claeify something that Ram Hammer stated: Shooting in the wind as he described, your bullet will move exactly as he stated. It has nothing to do with thr "paddle effect" he refers to. Rather, it occurs due to the force of lift acting on the bullet. Anytime you have air moving over a curved (cunvex) surface, you are creating lift. With a RH twist and wind from the right, the bullet will be blown left by the wind and will also go up. Same bullet, with wind from the left, the bullet goes right and lift causes it to drop.

centershot

[bj94]

As a kid I shot a pellet gun straight up, just to see how long until the pellets hit the ground, then back-calcuulate the muzzle velocity and height achieved (didn't have a chronograph), and even as a kid, was nervous (I actually wore SAFETY GOGGLES - as a KID!) about being hit.

Imagine doing that with a "real" gun! I'd want to be standing under a steeple made of armor or Kevlar!

You may have been listening to the news media too much. Gen. Julian Hatcher did exactly as you said. He fired .30 cal. bullets straight up and tried to detect them when they came back down. In their experiments it was difficult to find the fired projectiles, so they went out onto a wooden dock in the middle of a big pond and tried again. IIRC they were firing a .30 cal. Browning MG straight up. They did finally manage to land a few projectiles on the wooden dock. They discovered that first of all the projectiles fall at a relatively slow terminal velocity. One projectile fell on the wooden dock and only dented it, one fell in a metal bucket and did not damage the bucket. Secondly they discovered that the projectiles maintained spin all the way up and all the way back down, to the point where the projectiles stayed pointed up the whole time and fell base-first. This is all documented in the book _Hatcher's Notebook_.

So the next time you hear that it is dangerous to fire a gun up in the air, you will know the truth. When there are claims that someone was injured by bullets fired up in the air, there is more to the story. For instance maybe the gun was just aimed a little up in the air and not straight up, or maybe a gangbanger took the oppertunity to take out one of his enemies by getting up on the roof and shooting down.

[Chuck 100 yd]

#3