Prove Bullet Tractability Doesn't Exist

[MistWolf]

I started this thread to keep another from being hijacked. Some folks on several sites worry that too much twist in an AR can cause a bullet to lose tractability and not have it's nose tip down to follow it's downward path in it's trajectory.

Lungbuster asked about a 1:7 twist, surprised to see barrels listed that tight. Other folks on other sites worry that it's too much for lighter bullets, some citing problems with bullet tractability. I brought it up to show that it's of no concern to the AR shooter.

Ballisticians say it exists-
http://www.nennstiel-ruprecht.de/bullfly/tractf.gif
http://www.nennstiel-ruprecht.de/bullfly/tractf.htm
http://www.nennstiel-ruprecht.de/bullfly/fig15.htm

From another site dealing with naval weapons fired at high angles: "If the projectile is spinning too rapidly, then the bullet is said to be "overspun" or overstabilized, and in such cases one does indeed find that the so-called 'summital yaw' -- i.e., the cant at the top of the trajectory (where air resistance is least) is too large. It can be -- in extreme cases -- too large for the projectile to 'turn over' at the summit, and it will then fall base down."
http://www.navweaps.com/index_tech/tech-043.htm

From a naval academy thesis paper: "A round’s tractability describes the rounds ability to let its longitudinal axis follow a bending trajectory. A round may cease to be tractable if it is over-stabilized. A round becoming over-stabilized is more common with high-angle shooting. Over-stabilized occurs when that the round is rotating too fast and becomes unable to follow a bending trajectory (see Figure 2.1) [Ref. 4].
http://www.nennstiel-ruprecht.de/bullfly/fig15.gif
http://www.dtic.mil/cgi-bin/GetTRDoc?Location=U2&doc=GetTRDoc.pdf&AD=ADA435653

John Burns claims this phenomena does not exist-

As I said this is not true.

Simply because it is possible but very unlikely in indirect artillery fire does not mean it can happen in the realm of direct small arms fire.

If it were possible then we could drastically reduce wind drift by simply shooting bullets and twist combination that provided an extremely high stability factor but sadly this does not work.

This part here from the above quote-

If it were possible then we could drastically reduce wind drift by simply shooting bullets and twist combination that provided an extremely high stability factor but sadly this does not work.
-makes no sense at all.

That's the challenge. Show data showing bullet tractability exists or not

(PS- I already know it's nothing for an AR shooter to worry about)

[Ned Christiansen]

FWIW, maybe very little: I read, or maybe saw on the History Channel, that the wreck of the Bismark (I believe) had a hole it its deck the shape of an incoming shell-- that is, not a round hole but a hole showing the side profile of the shell.

[kartoffel]

Bullets are not footballs. They absolutely have tractability or "spin drift".

See: http://en.wikipedia.org/wiki/External_ballistics#Gyroscopic_drift_.28Spin_drift.29

This is nothing new or unexpected. Long range shooters know that right handed rifling makes bullets drift to the right on long shots.

[John_Burns]

The only concern is spinning the bullet so fast the attitude of the bullet remains the same after it reaches the apex of it's trajectory and does not nose over so the nose of the bullet continues to point along it's flight path. It stays nose up as it drops. This is only a problem when shooting very long ranges for precise accuracy.




Dude,

This is exactly what you said and it is not true.

This is not a “concern” with any “bullets” and is not “a problem when shooting very long ranges for precise accuracy.”

You statement is simply wrong and it had no place in any discussion about the merits of the 1/7 twist barrel.

If you don’t understand the wind drift comment you might want to give your Google Fu another workout.

[kartoffel]

Maybe the confusion is this: projectiles don't fly straight like arrows with the pointy end going exactly into the wind...

...but projectiles ALSO don't remain at the exact same attitude as they left the muzzle.

There's a bunch of effects at work: precession, nutation, plus aerodynamic drag, plus factor in that the center of pressure of the projectile changes with respect to Mach and sideslip angle.

So on long shots with appreciable drop, the bullet tip does tend to stay high, yet it's not a perfect gyroscope either. Nennstiel's "tractability factor" is a quantitative way to describe how much the bullet wants to fly "pointy end into the wind" across a long trajectory arc. An arrow shot from a bow, for example, would have a very high tractability factor, whereas a gyroscope would have a very low factor.

Check the wikipedia link I posted above: note that M193 (5.56) has 23 inches of gyroscopic drift at 1000 yards, whereas M118 (.308 Win) has only 11.5 inches of drift at the same range. Some listed VLD bullets have even less, thanks to having a higher "tractability factor".

So to answer MistWolf's question: yes, tractability is real, and so is the phenomenon of bullets falling ass end first when shot up into the air.

Long range rifle shooters already know about this shit and compensate for it. Cannon cockers know about it to, and as the Naval Academy paper confirms, they take it into consideration as well. In both scenarios it's not so much a "concern" as just another factor that's understood and accounted for.

[GunnutAF]

I guess what the OP stated would be true if a given projectile never lost velocity, or the spin rate never decreased over distance! Which could only be achieved in a comple vacum.:rolleyes: I guess when we achieve the ability to have a camera in a bullet along with speed and rotation sensors we can prove or disprove this.:D Then again all those videos of how Smart Arty work on the History channel and Military channel don't show the Arty hittng like his graphic!

[kartoffel]

I guess when we achieve the ability to have a canmera in a bullet along with speed and rotation sensors we can prove or disprove this.:D

We've been observing all this stuff with doppler radar for decades. You can watch exactly how the bullet yaw and precess as it goes downrange.


Then again all those videos of how Smart Arty work on the History channel and Military channel don't show the Arty hittng like his graphic!

Projos for indirect fire are tractable, i.e. the pointy end tends to go into the wind.

Tractability is much less of a concern with direct fire weapons such as small arms. I don't care if my 5.56 rifle bullets come back down ass end first because it's not an indirect fire weapon. Note however that spin drift can be reduced by choosing appropriate projos and twist rates, and this is a part of extreme long range shooting.

[John_Burns]

I have never heard of anyone who really shoots long range selecting bullets or twists with any consideration to spindrift.

Bullets are selected for you if you are .Mil

If you can choose your own, then bullets are selected for accuracy, high BC, and, if hunting, terminal performance and then the twist rate is selected according to the bullet.

Spin Drift is a constant and is easily calculated and compensated.

[kartoffel]

Spin Drift is a constant and is easily calculated and compensated.

Remove "is a constant" and I agree. ;)

[R0N]

Maybe the confusion is this: projectiles don't fly straight like arrows with the pointy end going exactly into the wind...

...but projectiles ALSO don't remain at the exact same attitude as they left the muzzle.

There's a bunch of effects at work: precession, nutation, plus aerodynamic drag, plus factor in that the center of pressure of the projectile changes with respect to Mach and sideslip angle.

So on long shots with appreciable drop, the bullet tip does tend to stay high, yet it's not a perfect gyroscope either. Nennstiel's "tractability factor" is a quantitative way to describe how much the bullet wants to fly "pointy end into the wind" across a long trajectory arc. An arrow shot from a bow, for example, would have a very high tractability factor, whereas a gyroscope would have a very low factor.

Check the wikipedia link I posted above: note that M193 (5.56) has 23 inches of gyroscopic drift at 1000 yards, whereas M118 (.308 Win) has only 11.5 inches of drift at the same range. Some listed VLD bullets have even less, thanks to having a higher "tractability factor".

So to answer MistWolf's question: yes, tractability is real, and so is the phenomenon of bullets falling ass end first when shot up into the air.

Long range rifle shooters already know about this shit and compensate for it. Cannon cockers know about it to, and as the Naval Academy paper confirms, they take it into consideration as well. In both scenarios it's not so much a "concern" as just another factor that's understood and accounted for.

TFTs contain data for drift correction.

The new SAW sight actually has a bent reticle to account for spin drift.

[John_Burns]

Remove "is a constant" and I agree. ;)

How about I add "for a given bullet, velocity, twist, atmospheric condition and range".:)

[kartoffel]

That'll work :D

[4thPointOfContact]

One additional tiny difference between naval artillery and long range rifle gunnery....

The barrel of a naval canon is going to be aimed 20 to 30 degrees in the air, or more.
How many degrees of elevation is a rifle barrel at to hit at 1,000 meters?

[R0N]

The angle of elevation is based on range to target and desire angle of fall. Once you get above 800 mils, you start getting into high angle fire and the projectile goes higher but land nearer to firing point as departure angle increases.

Even with small arms you can do indirect fire, machine gun indirect fire is still taught and done.

[120mm]

Long range tank fire control computers and lenses also account for spin drift.

I was at the Kabul National Museum last month, and they have an antique wall gun there with a very large rear sight with a series of peeps drilled in it. These peeps "drift" to the right as they went higher. It's as if they knew about this back then, too.