Rifle Scopes Spin Drift Questions / Solutions?

[preston pritchett]

Guys
I am over my head and need advice. I copied some stuff from some different websites on the Magnus and Coriolis effect. I remember the back site on the trapdoors was made at an angle to compensate for part of this. I know that it is not going to be 100% but. We have started making scope rings and I was wondering if because our scope rings have the bubble in them. If when you mounted it on your rifle and took a feeler gage and made sure the gap on the top cap on the left side was the same on the right side. Could you then take a pc of plywood at 100 yards and put a nail in the top portion and tie a string to this with a weight attached so it would be vertical due to gravity. If my 308 drops 39 minutes at 1000 could I drive another nail at 40.83” (39 x 1.047) then measure over to the left 1.1 moa (1.151) because of the data below. Drive another nail here. Now hang my string on this nail and with my bubble straight with the world, cant my scope to match the cant of the string. Would this delete “most” of the effect mentioned below? I


For example (typical 1000 yard small arms trajectory), if you always shoot in the northern hemisphere where the horizontal drift is always to the right, and you have a right twist barrel as most of us do, then your bullet will drift to the right approximately 9" due to gyroscopic drift, and an additional 2.5" due to Coriolis, resulting in 11.5" right drift, even in zero crosswind. (1moa = 10.47” 1.1 moa =11.51” this is my figures in parenthesis)
http://www.appliedballisticsllc.com/index_files/SpinandCoriolisDrift.htm

 

[crumpmd]

Re: Spin Drift Questions / Solutions?

Preston

Louis C (hide handle later)understands these things very well. I'll send him a pm and ask him to check this out.

Dr.C

PS I am still loving my Scalpel.

 

[TCA4570]

Re: Spin Drift Questions / Solutions?

What is the Corilis effect, if I have to come up 211.6@1000?

 

[Rich Emmons]

Re: Spin Drift Questions / Solutions?

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: preston pritchett</div><div class="ubbcode-body"> \ Would this delete “most” of the effect mentioned below? </div></div>

Me thinks just level it the best you can, there's almost never "0" wind at a 1000. So drift or none you still gotta read the wind within 2 mph. Besides wouldn't the bubble be nearly in the center anyway.

Was shooting at a 1000 last night about 8:30pm and any wind that was there was coming from 2-3 o'clock... it settled to what i considered no wind... held center of clay target.. hit left by a couple of inches. go figure!

Good looking Rings, BTW

 

[Michael]

Re: Spin Drift Questions / Solutions?

Might work in 1 direction? What about shooting east/west; west east, south/north?

 

[LoneWolfUSMC]

Re: Spin Drift Questions / Solutions?

Sometimes we think about things too much.

I prefer to just shoot.

 

[sobrbiker883]

Re: Spin Drift Questions / Solutions?

I guess you could negate spin drift using such a method, but as Michael pointed out for the other "effects" your bearing determines the value the effect will have at ELR...

 

[Nocalphoenix]

Re: Spin Drift Questions / Solutions?

Go to a left twist barrel to offset the difference to total 6.5" instead of 11"

 

[CoryT]

Re: Spin Drift Questions / Solutions?

While you could setup to negate the spin drift, and indeed G David Tubb's reticle does work just like that, the Coriolis amount is variable. The spin drift amount is also bullet and velocity dependant, so it's only 'correct' for one ammunition type.

I find it's just as easy to dial on the required correction, if the precision of the shot dictates that level of concern.

At 1000 yards with a .308, 1 mph of wind is the same as the spin drift. Since neither I, nor anyone else can read to 1 mph over 1K, it's kind of lost in the noise.

With a .338LM, the windage problem is greatly reduced, so getting a true zero windage setting will increase the first round hit probability, as you are more likely to get more of the CEP on the target.

 

[TiroFijo]

Re: Spin Drift Questions / Solutions?

As CoryT said, I think dialing is just as fast, and it is easier to just mount the scope prefectly level.

The Coriolis effect varies with latitude, so even if you are shooting in the northen hemisphere (always moves POI to the rigth) it is not constant. And it is very small at 1000 yds.

The spin drift is variable, easily lost in the noise at under 1000 yds, but much larger than the 1000 yds value for calibers that allow for 1500+ yds shooting, so you will have to dial anyway.

 

[One-Eyed Jack]

Re: Spin Drift Questions / Solutions?

For 308Win and a 175SMK, spin drift is roughly equal to 1 mph of wind, or about 1 MOA. So, it's pretty much in the noise, as someone said.