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texasmac
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Bullet drift due only to twist related torque?
Dec 26th, 2009 at 11:25am
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Hey Guys,

On another forum an experienced shooter made the following comment.  
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“If one were to go to the 100 or 200 yard line and get a DWZ aka DEAD WIND ZERO such that the rear tang is on MZ aka MECHANICAL ZERO (the rear tang sitting in the middle of the grid lines) and then go back to the 1000 yard line and shoot under the same no wind conditions....with a perfect hold and break...the shot (using a .45 caliber with say a 550 gr. bullet and having a right hand rate of twist) would probably impact somewhere betwixt 35" to 40" right of the targets center. This is because of rotational drift aka torque applied to the bullet by the right hand twist. Therefore to offset the bullets 'drift'....it's necessary to shim the right hand side of the sight base of the rear tang such that when a shot is fired it will offset the drift to a point striking the targets center.”
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Being a relative long time shooter, sometimes out to the 500 meter range, I certainly can understand horizontal and vertical bullet drift as a result of cross winds and bullet spin, but I’ve never heard, until now, of horizontal drift caused by torque alone, which is applied to the bullet as a result of twist.  Is there any validity to the guy’s comments?

Wayne
  
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westerner
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Re: Bullet drift due only to twist related torque?
Reply #1 - Dec 26th, 2009 at 11:39am
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Mornin Tex. 

Ive read about it but havent experienced it while shooting at a long distance. Maybe it happened and I didnt notice. 

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              Joe.
  
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podufa
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Re: Bullet drift due only to twist related torque?
Reply #2 - Dec 26th, 2009 at 11:47am
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Isn't this what the Buffington sight took care of?
  
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harry_eales
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Re: Bullet drift due only to twist related torque?
Reply #3 - Dec 26th, 2009 at 11:55am
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Not only the Buffington sight on the Springfield 45/70, but later in WW1 on the long barrelled and shoulder stocked Artillary Lugers as well.

Gyroscopic precession movement of bullets in flight does exist and it is always in the direction of the bullets rotation. Of course it can only be recorded in dead calm conditions and shooting from machine rests to get as much of the human factor taken out of the equation.

Harry
  
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Schutzenbob
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Re: Bullet drift due only to twist related torque?
Reply #4 - Dec 26th, 2009 at 12:33pm
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If the light is behind me during calm conditions I can sometimes see the bullet hit the target, and with a right hand twist barrel I always see the bullet fall from the upper left toward the center of the target. 

Bob
  
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boats
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Re: Bullet drift due only to twist related torque?
Reply #5 - Dec 26th, 2009 at 4:57pm
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Texasmac hope it's not snowing on you down there.

They do drift due to spin, but not much at Schuetzen distances.  My 7mm 08 Silhouette rifle correction is 1 moa at 500 meters and that's probably over correcting.  385 Turkeys I don't change the mechanical windage at all.   I think off a chart at 500 for the 7mm 168 gr Match King it's just over 1/2 moa. I use 1 moa as a rule of thumb rather that make small corrections. It probably re-sets my mind to pay attention to the rams more than anything else.

This is a wild guess but would expect the drift to be more with the faster rotation of a jacked bullet compared to Lead Single shot bullet velocities. Look at a Trapdoors Buffington, see how the slide is skewed to account for spin drift at long range. Takes a lot of yards before the skew is noticable.

As a pratical matter I say it's a waste of time trying to shim a tang sight to compensate. Better to have a table of mechanical zeros for match distances and dial them in than trying to make the sight track with the bullets rotation.

Little off topic but there is a bias with arpeture sights on NRA Silouette targets. Rams facing one way or the other need to be corrected for. The arpeture finds center of mass and it's slanted toward the head.  Not noticable on the other 3 critters but real on Rams.  Might be what that guy is confusing spin drift with.

Boats
« Last Edit: Dec 26th, 2009 at 5:10pm by boats »  
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John in PA
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Re: Bullet drift due only to twist related torque?
Reply #6 - Dec 26th, 2009 at 8:26pm
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Drift occurs because the bullet is spinning and riding a cushion of air out to the target.  If it's spinning to the right  and pressure on the bottom is greater than on the top (as it is when falling through the air) it crawls to the right.  Left hand twist is the opposite.  It will do this even if the air is perfectly still.  Wind drift is in addition to this.
  
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MartiniBelgian
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Re: Bullet drift due only to twist related torque?
Reply #7 - Dec 27th, 2009 at 3:12am
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I think it's called the Magnus effect - but that has nothing to do with torque, it is just spin drift.
  
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stebbinsgr
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Re: Bullet drift due only to twist related torque?
Reply #8 - Dec 28th, 2009 at 6:35pm
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Being a retired artillery officer and a fair student of ballistics what you are referring to is drift caused by bullet rotation.  Yes the torque imparted on the bullet will cause drift, but, in comparison artillery shells which are fired several thousand meters (up to 22,500 meters or roughly 14 miles for a 155mm howitzer at 2/3 of max range) and in those cases the drift can be several meters.  Other factors of course are wind, curvature of the earth (sorry it is not flat - for all you flat earth types) and the earth's rotational effect.  Other factors being wind, powder temperature and ballistic pressure.

But, in the case of a bullet firing at 1,000 yards you talking in Minutes of Angle being measured at a factor of ten.  If you have a rifle that will group 3 rounds in a one inch circle at 100 yards, you will be lucky if that rifle will group three rounds in a 10 inch circle at 1000 yards.

So if you have a rifle that can only hold a 3 inch group ( or 3 MOA at 100 yards) that group could open up to 30 inches at 1000 yards.

Is drift caused by bullet rotation also going to come into play, yes.  But also remember your target group will expand at the greater range.
  
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e15cap
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Re: Bullet drift due only to twist related torque?
Reply #9 - Dec 28th, 2009 at 7:55pm
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Boats, Could you expand on your statement about Rams please. I miss more than my share and am always looking to learn.
                                                   Best, Roger
  
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boats
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Re: Bullet drift due only to twist related torque?
Reply #10 - Dec 28th, 2009 at 8:16pm
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Ok but will start seperate post, windage adjustment on Rams with BPCR

Boats

  
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