Spin Drift

[t.t]

I went out the other day shooting targets out to 550yds, no wind and noticed my poi was moving right had to put .5moa @550yds to corect this.

how do you compensate for spin drift

[1967spud]

IIs your scope on square as it can also track out that much over that distance.

[brown dog]

what's your bullet, mv and twist rate?

 

or - for a cuff answer - what's your bullet's time of flight to that distance?

[Chris-NZ]

You're doing well to be certain it's not either an imperfect wind zero and an imperceptible drift causing this. Even a change in position may cause it.

 

If it were more than a full minute, I might be wondering but hell, half a minute ain't nuthin' at 550yds

 

Chris-NZ

[duey]

I wouldn't expect spin drift to be percepable at 550yds

most likely an unseen wind downrange

 

ian

[snc_2010]

My guess:

No wind, high confidence, pulled the shot(s).

[brown dog]

I think you doubters need to look at your toes for a bit before you rush to judge.

 

Half a minute at 550 yards sounds in the parish for an 'averagish' round's (spin)drift.

 

Spud's observation is a valid explanation for what a great shot might spot on a no-wind day. But so is (spin) drift.

 

 

The fact that you doubters can't spot it doesn't mean no one can

 

 

Let's wait for some more detail.

[baldie]

Very few people actually put a scope on straight using proven squaring methods. The idea of a key on the Spuhr mount is genius, and does away with the problem.

If the scope is even a degree out , you will see left/right shots at long range. You have dialled the turret up, and also pulled the left/right out because its not square.

Then there is spindrift too.

If i had a quid for every mismounted scope i look through in the shop....i could retire. Its very common.

Then there's Cant.

[duey]

BD not rushing to judge at all just believe thats there plenty of other variables to consider before jumping to spin drift as the only correction

 

have shot in a few places where no visable wind from firing point but a few small areas between FP and target that funnelled wind to disrupt POI

 

or as Baldie has said could be a scope fractionally out of cant/alignment

 

just my thoughts

 

Ian

[t.t]

what's your bullet, mv and twist rate?

 

or - for a cuff answer - what's your bullet's time of flight to that distance?

 

My bullet 168gr berger VLD, 1:12 twist rh, 2639fps 0.77 sec to 550yds

[t.t]

I was with a friend with his 308 his was doing exactly the same, my scope was put on by ronin off here so thats done correctly.

wind was nil even checked downrange

[Scotch_egg]

Just for reference I use feeler guages under my scope tubes on my picatinny rails. With the addition of a bubble level.

 

This hopefully negate's the can't influence.

 

Ronin know's his beans and will put you right.

[Scotch_egg]

oops I mean't cant not can't.

[Ronin]

Spin drift or un noticed wind influence.......

 

The scope is aligned.

[brown dog]

My bullet 168gr berger VLD, 1:12 twist rh, 2639fps 0.77 sec to 550yds

I was with a friend with his 308 his was doing exactly the same, my scope was put on by ronin off here so thats done correctly.

wind was nil even checked downrange

 

Give yourself a pat on the back.

 

Did this using Litz's point mass solver for your bullet MV etc. (I was interested to see how close the SWAG came based on your TOF - and my answer is that it's tuneable to an almost exact match to Litz, which I found nerdly interesting, anyway here's the Litz version) the gross-error check for non-believers is the received wisdom for palma that drift is approx 1 minute at 1000 :

 

[6mmBR]

I think there is also a condition caused by the rotation of the earth called the coriolous effect. As the earth has rotated slightly during bullet flight, so in effect the target is in a slightly different position on impact. This effect I have been told needs a 1 MOA adjustment left @ 1000 yards to cancel out?

Cheers

Dave

[t.t]

Give yourself a pat on the back.

 

Did this using Litz's point mass solver for your bullet MV etc. (I was interested to see how close the SWAG came based on your TOF - and my answer is that it's tuneable to an almost exact match to Litz, which I found nerdly interesting, anyway here's the Litz version) the gross-error check for non-believers is the received wisdom for palma that drift is approx 1 minute at 1000 :

 

 

Thanks alot bd it was doing my head in i went through all possibilties why this was happening, then came up with it MUST be spin drift. I just ignored the doubters and listen to the folk "in the know" :)

[1967spud]

Thanks alot bd it was doing my head in i went through all possibilties why this was happening, then came up with it MUST be spin drift. I just ignored the doubters and listen to the folk "in the know" :)

Err actually carefull how you diss the doubters they all had valid reasons to what could be the trouble in this case it may have been spin drift but could have easily been any off the other troubles. you asked what was wrong and were given some possibilities, so please dont knock people who are trying to help you even as in this case they were om the wrong track.

[1967spud]

I went out the other day shooting targets out to 550yds, no wind and noticed my poi was moving right had to put .5moa @550yds to corect this.

how do you compensate for spin drift

 

just as an after thoughtto get your drops for this range what software did you use

[t.t]

Err actually carefull how you diss the doubters they all had valid reasons to what could be the trouble in this case it may have been spin drift but could have easily been any off the other troubles. you asked what was wrong and were given some possibilities, so please dont knock people who are trying to help you even as in this case they were om the wrong track.

 

the question was "how do you compensate for spin drift" nothing else and bd answered my question

[brown dog]

I think there is also a condition caused by the rotation of the earth called the coriolous effect. As the earth has rotated slightly during bullet flight, so in effect the target is in a slightly different position on impact. This effect I have been told needs a 1 MOA adjustment left @ 1000 yards to cancel out?

Cheers

Dave

 

Dave, that's people muddling themselves; the 1 MOA left 'standard' at 1000yds is precessional drift for a palma-ish 308. Coriolis is negligible at these distances, and would vary (down as low as nil value) with latitude and the bearing of fire.

 

S'pose it's worth pointing out that '1MOA left drift correction' assumes a clockwise rifled barrel!

[6mmBR]

Cheers Matt. I think in the long run if we forgot about these and just got on with shooting.

Much more fun than worrying about theoretical mathematical effects.

Happy Xmas one and all

Dave

[brown dog]

They're not theoretical, they're real

 

...interest level depends on whether you chase the first round hit;

 

or can ignore the known elements of the error budget because you expect to fire sighters

 

- I agree, nothing works out how a bullet will fly through a given set of conditions better than flying one through them

 

(although I do have your attitude to exterior ballistics in my attitude to all that tosh about reloading and bullet jump and neck turning and other anal reloading 'stuff' )

[t.t]

I'd like to go watch guys actually shooting long distances. I thirst for knowledge regarding long range shooting, im learning every time i go out with my rifle.

any offers for next year guys would be greatfully recieved as long as not at the other end of the country, im in Cumbria

[1967spud]

join the f class assoc then you can shoot with us at long ramges tt