Spin Drift and Acceleration

[Limbic]

I shot a doe this past week at a pretty good distance for Mississippi. 428 yards. 168 VLD worked well and the elevation was spot on. 7MOA and I was dead center. The problem was that I was off to the right a few inches. There was absolutely zero wind so other than shooter error I was wondering how much spindrift could I count on. Ballistic AE on the iphone said about 1.25 inches.

What do yall use to calculate drift and acceleration. I was about 4-5 inches too far right and I know not all of that is due to the above but wanted to see how the brain trust manages that.

 

[Korhil78]

Spin drift at that distance is not going to be much at all maybe 1/4"-1/2" if that much. If it wasn't shooter error, then what I would be looking at is if your scope is mounted level. I would also go out to the range and see where it shoots at 400 yds.

 

[Broz]

1 1/4" of SD, the accuracy of the rifle ?? Lets guess 3/4 moa so there is another 3.3" at 428 yards. that is a total of over 4 1/2".

Jeff

 

[Korhil78]

Broz, are you saying there is 1.25" of spin drift at roughly 400 yards?

 

[Broz]

Broz, are you saying there is 1.25" of spin drift at roughly 400 yards?

That was the OP's number. My 300 win shows less at .1 moa which is closer to 1/2 inch. But I don't have all his ballistics so I went with his number from his program. A pretty good rule of thumb is 5" or 1/2 moa at 1000. So an inch at 428 is not way out there.

One other comment. I have never seen it where there was "NO wind" somewhere during the shot. That sure could be but it would be very rare.


Jeff

 

[Korhil78]

Oh ok...I was scratching my head trying to figure out how I was figuring it wrong! haha

 

[Joe King]

Broz, are you saying there is 1.25" of spin drift at roughly 400 yards?

you can get that, depends on your bore dia, twist rate and MV

 

[Joe King]

That was the OP's number. My 300 win shows less at .1 moa which is closer to 1/2 inch. But I don't have all his ballistics so I went with his number from his program. A pretty good rule of thumb is 5" or 1/2 moa at 1000. So an inch at 428 is not way out there.

One other comment. I have never seen it where there was "NO wind" somewhere during the shot. That sure could be but it would be very rare.


Jeff

When ever I see that I think, very little so it was ignored. I think folks forget that a 2mph wind at 400yrds will drift your bullet 2" or more

 

[Broz]

When ever I see that I think, very little so it was ignored. I think folks forget that a 2mph wind at 400yrds will drift your bullet 2" or more

And a 1/2 to 3/4 MPH wind is hard to detect on your face.

Jeff

 

[Limbic]

So very true with the wind. The Mississippi delta is flat. Very flat but occasionally the wind settles. What wind there was that day was coming from the 7oclock. Could have been a few MPH in there. Usually are.

The app I used was the Ballistic AE app for the iphone. It shows 1.53"@425

Gun is a GAP 280AI with 168vld at 2800fps. 1/8 twist. 7"Dixie Precision Rifle Suppressor

I didn't think about the scope base. I have Kelby two piece dove tail bases and rings. I don't recall the gun and scope being out of line when shooting it. Perhaps I'll practice more and get a one piece base

 

[Broz]

Did you dial in the shot?

Are you using a scope level and have you trued your reticle to bullet path?

Jeff

 

[rick523]

As stated he didn't give any specifics on ballistics I ran numbers for my 7mag came up with 5.8 moa up and .1moa of drift = 0 clicks.

 

[Limbic]

Spin and Coriolis Drift

11.5" at 1000 is more than I expected. Not sure what caliber, MV ect. I dialed up 7MOA for my shot and held middle of the shoulder. Not much wind from 7oclock. I'm ok with shooter error if that is what it is. It seems the consensus is that 1-2 clicks a most for drift.

 

[Broz]

So do you have a level to eliminate cant? If not that would be high on the possibility list.

Also, I am not comfortable with the 11.5" at 1000. I would be looking for data input error there. That is to much. I don't even use coriolis till beyond 1200 as it is such a small correction. And I have never seen much more than 1/2 moa for SD at 1000 in many rifle tested.

Jeff