Spin Drift

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GSSP

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This morning, I was doing some testing at 478 yds. The bullet is the Parker 300 gr Black Max using .320 BC (suggested to me by Levi Reed of LR Customs) @ 2530 fps. The elevation was spot on. I didn't hold for wind, more concerned with the vertical and making sure the BC was where it needed to be. Taking into account the wind of 4 mph from my 7, I should have been holding 1.1 MOA to the left yet the groups was more like 1.4 moa to the right of POA. In regular rifle shooting, with higher BC bullets, I rarely hold for the extra spin drift but am wondering most LR MZL shooters "do" input that data into their dope due to the nature of the bullets?

I'm a somewhat accomplished LR shooting, so please, don't hold the info or think i'm a noob.

Thank you,

Alan
 
Spin drift is not linear. It cannot be expressed as an MOA number. You might expect zero at 100 yards. But as you move out, it becomes noticeable. No idea what to expect for muzzle loaders. I know fast spin like 1:7 is worse case and enough time of flight is required to have a measurable effect.

The OP reads pretty savy. He should do a google, find the formulas and plug in the numbers.

I do remember a very slight error in scope mounting can look like spin drift as you move down range.

Edit: Visualize this: an imaginary world with no gravity to drop the bullet to ground. With spin drift the bullet would follow a circle or more like a spiral (cork screw) path. It is nothing like 1 MOA (arc minute to the right). Nothing like wind. More like the rudder on an airplane, maybe with some added up draft depending on CW or CCW spin.

Edit2: I would be a rare situation for wind to be the same velocity and direction over a 500 yard distance. I would suspect that was responsible for discrepancy in POI. This can get way complicated. It is not my sport. I have used wind flags shooting 22LR at 100 yards. It is a mess to deal with.
 
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Coriolis Effect must be taken into account when compensating on the horizontal plane. The error due to Coriolis force is always toward the right on the northern hemisphere, and always toward the left on the southern hemisphere. It is in function of latitude and bullet speed. It is negligible until about 1000y.
 
Spin drift is not linear. It cannot be expressed as an MOA number. You might expect zero at 100 yards. But as you move out, it becomes noticeable. No idea what to expect for muzzle loaders. I know fast spin like 1:7 is worse case and enough time of flight is required to have a measurable effect.

The OP reads pretty savy. He should do a google, find the formulas and plug in the numbers.

I do remember a very slight error in scope mounting can look like spin drift as you move down range.

Edit: Visualize this: an imaginary world with no gravity to drop the bullet to ground. With spin drift the bullet would follow a circle or more like a spiral (cork screw) path. It is nothing like 1 MOA (arc minute to the right). Nothing like wind. More like the rudder on an airplane, maybe with some added up draft depending on CW or CCW spin.

Edit2: I would be a rare situation for wind to be the same velocity and direction over a 500 yard distance. I would suspect that was responsible for discrepancy in POI. This can get way complicated. It is not my sport. I have used wind flags shooting 22LR at 100 yards. It is a mess to deal with.
The original post stated 478 yds and bullet and load .

So i responded 1 moa right is typically the spin drift at that distance and load.
 
You understand 1 moa, is approx 5 inches at 500 yards !

MOA is one minute of angle, not one inch. 1moa= 1" at 100, 2" at 200, 3" at 300, etc.
 
Yes, I am asking for clarification. Because 5 inch is a very large number. Much larger than reported by the OP. And as posted, it has other implications at other ranges that merit elaboration.
 
Yes, I am asking for clarification. Because 5 inch is a very large number. Much larger than reported by the OP. And as posted, it has other implications at other ranges that merit elaboration.
Would it surprise you that I concur with bestill's theory? I also find that shooting the bullet weight first mentioned and with BH209 at 500yds, there's very close to 1moa of drift on a COMPLETELY CALM range.
 
That is 5 inches, correct?
Yes, I can have anywhere between .5 and 1moa of drift in relative calm. (2.5" to 5")
I start correction normally at 300yds, even with single hole accuracy at 100yds.

Example target fired with BH209. The rifle shot a single hole at 100 dead on, on a calm morning. I was shooting to create my range card vertical. Clearly the first two rounds were 1moa right. I made the vertical adjustment and only .5moa left.

IMG_1717.JPG
 
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I remember watching a program on Outdoor Channel or Sportsman's that Gunwerks or somebody like that was mounting their scopes with a small degree of crosshair cant that would account for spin drift as the elevation dial was turned.
 
I remember watching a program on Outdoor Channel or Sportsman's that Gunwerks or somebody like that was mounting their scopes with a small degree of crosshair cant that would account for spin drift as the elevation dial was turned.
That would be tough. Only one bullet and one charge could be utilized. I believe I'd trust the drift indicated by a program, such as JBM.
 
Gentlemen Question? Is your long range shooting being done with a bubble level on your guns? Also are you shooting from a bench or prone on the ground ?
 
Gentlemen Question? Is your long range shooting being done with a bubble level on your guns? Also are you shooting from a bench or prone on the ground ?

Yep. Be it Scope, or Open Sight Long Range

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Not to beat a dead horse but to show why I made one of my prior statement here is an quote that I believe is accurate:

In Bullet Trajectory part 2, we found that spin drift is a deflection generated by the gyroscopic motion of the bullet. The direction of the deflection is toward the direction of the spin, and the amount of deflection is dependent on bullet length, flight time and, not surprisingly, spin rate. Its effect is noticeable, and must be taken into account, starting around 500yds. To make an example, for a 175gr SMK bullet, fired at 2700fps from a .308, with a barrel twist rate of 1:12”, the amount of spin drift would be about 1½ in at 500yds and about 9 in at 1000yds. But why does the bullet’s spin generate a deflection in the trajectory?

Note in the above example at 500 yards the drift is 1.5 inch. That would be 0.3 MOA. But out at 1000 yards it is 9 inch. That would be 0.9 MOA. That is 3 time more MOA. Therefor drift should not be expressed as a linear MOA (angle) number. The bullet arcs or curves more and more as it flies. That is why MOA is not generally part of the spin drift vocabulary. What is the MOA on a boomerang?

I can find more source, this was the first I hit with a search that sounded credible:

https://thearmsguide.com/5346/long-range-shooting-external-ballistics-spin-drift-13-theory-section/
 

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