Spin drift question

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Don McDowell
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Re: Spin drift question

Post by Don McDowell »

Glen Ring wrote: Sun Feb 14, 2021 5:56 pm So does anyone use a Horus reticle to compensate for spin drift? It looks like a very busy scope and range finding with it seems a mathematical challenge but some of the Precision guys like those busy scopes.
Bptr shooters or at least some do this process
Put a 100 ya target up at the bottom of. 6 ft tall piece ovf cardboard
Using a level make a verify level line from the
Con the target up to the top of the cardboard then a strip of 1 inch painters tape up the line you just drew on the left side of it
Now put in the 800 yard setting and aim at the bullseye with zero wind age on the sights
Onserve the bullet strike and adjust the front sight until the bullet strike barely clips the left edge of the tape
That will put you very near a no wind zero at 800
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desert deuce
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Re: Spin drift question

Post by desert deuce »

Yep !
Sometimes you get the chicken, and sometimes you get the feathers!
bruce m
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Re: Spin drift question

Post by bruce m »

good point don.
and fclass and palma shooters do the same.
that method can also demonstrate how plum your staff is if you shoot groups from a 100 yd to 1000 yd zero.
or you can use a windage foresight to make adjustment for spindrift.
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Glen Ring
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Re: Spin drift question

Post by Glen Ring »

That's cool Don...thank you. No math involved either and I like that.
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Don McDowell
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Re: Spin drift question

Post by Don McDowell »

You’re welcome
Well it’ll get you close but it will take some of the guess work from reading windflags and mirage and make your spotters life a bit easier 😀
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Glen Ring
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Re: Spin drift question

Post by Glen Ring »

Jeanne and I want to shoot some long range stuff with our BPCR rifles this summer. We may not have the ideal set up, but we'll have fun.
When I set up our Highpower silhouette rifle I develop an accurate load at 100 meters, then crunch the numbers in a ballistic program, and set the 100 yard adjustment before we go to the range. It's surprising how accurate those programs are and it saves me a lot of time. I guess i can almost get there with my 45-70 the same way...or at least give me an idea where to start.
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Distant Thunder
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Re: Spin drift question

Post by Distant Thunder »

Don,

What you're saying then is you get a little more that 8 inches of spin drift at 800 yards or 1 MOA?

I think the problem I'm having with this spin drift is that it is not constant. The amount of drift increases as the rotational speed of the bullet decreases and therefore increases with distance.

So there really isn't an angle that you set your tang sight at that will compensate for the spin drift at all distances. That makes the only real way to deal with it being to know your no wind zero at each distance that you shoot, record that and have that in your note book along with your sight settings. That might work, but I ain't never going to get a no wind zero at 1000 yards!

Being that spin drift is caused by the air pressure on the nose of the bullet, which is sitting above and to the right of the line of flight (RH twist), I would think that changes in air density would have an affect on the amount of spin drift at any given time. Is that correct?

The variables can get quite deep.

I would really like to understand this subject better and I'd like to know how best to deal with it, correct for it, but as I feared in discussions of this sort the answer seems rather elusive. I think I'm going to do some math :roll: and come up with a shim thickness that I can put under the right side of my sight base that will move me over to the left about 2-3 MOA and see what that does at 1000 yards to correct for the spin drift. I will of course have to reestablish my 200 yard no wind zero.

I don't see how doing this can hurt anything and I'm hoping it will make my windage adjustments at long make more sense. I know that it always seems that I have more left wind or less right cranked in than the conditions would call for.

I'll let you all know how it works out, but that's going to be a bit of a wait,
Jim Kluskens
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Don McDowell
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Re: Spin drift question

Post by Don McDowell »

Yes a 45-70 will get you to 1000 might not be the best choice but it’s certainly not a bad one either
You might want to consider a midrange match to start the target rifle game sort of sneak up on the long range stuff

I suppose if some one were inclined it would be possible to crunch the numbers and figure the spin drift for ram line or 600 yd midrange and use the same principle to get a no wind zero on a gun for those matches
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Don McDowell
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Re: Spin drift question

Post by Don McDowell »

Jim all I can tell you is the process was explained to me and it appears to work well
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bruce m
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Re: Spin drift question

Post by bruce m »

jim,
here is a fictitious set of spindrift figures.
yards and moa.
100 0.3
200 0.6
300 0.8
400 1.1
500 1.5
600 1.8
700 2.1
800 2.5
900 2.8
1000 3.2
these are taken from jbm ballistics, using a wind speed of 0 mph and at a pressure of 29.92 ins of mercury.
changing that pressure to 29" mercury the only differences are
300 0.9
400 1.2
900 2.9
you would have to know pressure variations you might be in to do appropriate calculations.
it looks like small pressure variations don't make a meaningful difference compared to the accuracy of the windcall and the difficulty of adjusting a vernier that finely.
if you can adjust something for it that would be better than nothing.
as don says, if you are close on this adjustment, your wndcalls are meaningful, and learning to gauge the wind will become more meaningful.
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desert deuce
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Re: Spin drift question

Post by desert deuce »

The Definition of, "Close enough for Government Work."

"Measure with a Micrometer, mark with a piece of chalk and cut with an axe."
Sometimes you get the chicken, and sometimes you get the feathers!
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Re: Spin drift question

Post by gunlaker »

desert deuce wrote: Mon Feb 15, 2021 8:17 am The Definition of, "Close enough for Government Work."

"Measure with a Micrometer, mark with a piece of chalk and cut with an axe."
That's about right :-).

In this case I think it's best to shoot and see. As far as I can tell it's around 2-3 moa at 1000 yards for my .45's, but I haven't seen very much at all at 600. Maybe a minute, probably less. I've never had much luck with shims. This is where a windgage front sight is nice. In my scopes I just keep a log of what I think zero is on each day at each distance. The true no wind zero is in there somewhere, and includes any wind drift.

Chris.
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Don McDowell
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Re: Spin drift question

Post by Don McDowell »

Also sort of important as a spotter to shoot regularly enough with the same shooter to know that shooters gun's trajectory. For instance we've never set Eddie's Fruend gun for no wind zero, but it appears that no wind zero at 800 would be about 3 minutes right on the rear. Takes a bit more math in your head to give windage adjustments during sighters, but then it's just a matter of calls off the spotting disc.
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Kenny Wasserburger
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Re: Spin drift question

Post by Kenny Wasserburger »

I have tested zero at 200 yards in a no wind condition, adjusted the front sight to the point that no wind zero is mechanical Zero... repeat Mechanical Zero.

With a no wind Zero and mechanical zero, at 800 yards I need 2.5 MOA left to compensate for spin Drift. Which matches up to Bruce’s JBM calculations at 800 yards. This is also confirmed with my MVA scopes where I adjust the front mount till no wind Zero is.....Mechanical Zero...this takes a shit load out of the guess work.

Don I would submit that Eddies no wind Zero is off by a good 5.5 MOA of mechanical Zero, that doesn’t help much in a reversal.

Since there is no staff to shim on my scopes. I zero my long rang rifles at 200 yards to impact about 2.5 MOA to the left of my aiming point as zero on the rear mount. Using the MVA front mount to attain this. I believe it is of some advantage in Creedmoor, where at Raton or Byers or Phoenix reversals can happen. But what do I know. :roll:

Kenny Wasserburger
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Distant Thunder
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Re: Spin drift question

Post by Distant Thunder »

desert deuce wrote: Mon Feb 15, 2021 8:17 am The Definition of, "Close enough for Government Work."

"Measure with a Micrometer, mark with a piece of chalk and cut with an axe."
Yea, cut with an axe from seven paces!

I've been researching this a good bit and what Chris said about 2-3 minutes at 1000 yards seems to be the general average. It seems impossible to get a solid answer as to how much even from people who claim to understand spin drift. I think the variables involved are too many for most people to compute.

If I want to be more precise I could record sight settings including windage and conditions over the next 10 to 15 years. I could then take the collected data and come up with a windage correction for each yardage that would be pretty close. I'm not going to do that. I could do the blue tape thing, but I'd have to guesstimate the windage for the other yardages and then remember to adjust the windage each time for each yardage and I would still have to some dreaded math :shock: to come up with corrections from zero when dealing with reversals. I need a system with less thinking involved!

I understand that one number MOA wise is not the perfect answer for every yardage, but there has got to be some kind of average between 800 & 1000 yards that will be close enough to be useful.

I'm looking for a simple way to minimize the spin drift error at 600 to 1000 yards. Something that I could do with any rifle and be much closer than I am using the "spin drift denial" method I've been using until now. I'm not admitting that spin drift exists, but I have noticed over the past few years that my windage settings coming off the line just don't jive with what I see in the conditions and in thinking about it they do seem to been favoring the left by a few minutes. So something needs a shim.

At this point I have decided I will tip my tang sight to the left something in the neighborhood of 2.8 MOA at about 900 yards and then see what happens when I start shooting in the spring. That is my best guess.

So I will ask once again, if some one has good numbers for BPCR velocities/bullets that would give me something closer to the amount of spin drift that would be expected at 800, 900 & 1000 yards please speak up and give me some numbers and I will use those.
Jim Kluskens
aka Distant Thunder
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