spin drift

[Eaglet]

Also, those that own LoadBase 2.0 can obtain the SF by either running the desk top version or the PDA version.
For the PDA version here are some screen shots:

Run the program, click on Analyzer...

Now the question is, which number do we plug into Meichele's equation, the "calc SF" number or the "actual SF" number?

 

[Michael Eichele]

I came up with 1.396 SF for the 7mm and 1.680 for the 300.

Giving you 5.83" of SD from the 7mm at 0', 29.92 and 59 degrees using the factory published BC.

The 300 calculated out to 6.52" of SD in the same conditions with the factory BC.

If I remember right, you were seeing about 8" of SD? 8-6=2

2" of difference could be scope missalignent, shooter error or mathematical descrepancies or all of the above. Also you were likley shooting much colder air than 59 degrees. Regardless, I have found this system to be scary close in any practical scenario up to 1K. I have seen it very close at 2K as well. However, close at that range could be 8" instead of 2" Still closer than not factoring it in at that range.

Also using Eaglets SF of 1.84 you are just over 7" at 1K. This makes the margin of error even closer assuming his program is better than mine.

 

[mikebob]

This thread is pretty darn good I must say. I was just wondering why couldnt a guy set up a target at 1000+ yards, wait a very long time here in OK for a windless day and zero windage at that range for dead center impact. Then shoot at 100 yards, the distance you are off would have to be spin drift wouldn't it. Then zero back at water ever range you zero at and add that distance as wind drift (break down into mph) like shawn. Would that work I now it wouldn't be perfect but it would close. I think it would be more off at closer range that at 1000 but then it would be easier to account for at closer range also. Heck I am tired just babbling somebody slap me if im getting off base into stupidness.

mike

 

[Michael Eichele]

This thread is pretty darn good I must say. I was just wondering why couldnt a guy set up a target at 1000+ yards, wait a very long time here in OK for a windless day and zero windage at that range for dead center impact. Then shoot at 100 yards, the distance you are off would have to be spin drift wouldn't it. Then zero back at water ever range you zero at and add that distance as wind drift (break down into mph) like shawn. Would that work I now it wouldn't be perfect but it would close. I think it would be more off at closer range that at 1000 but then it would be easier to account for at closer range also. Heck I am tired just babbling somebody slap me if im getting off base into stupidness.

mike

IMHO Mike, you have described about as good a way to figure it out! Assuming ones scope is dead level ect....You can predict fairly well these days how much it will be, but nothing beats getting out and trying it. That is exactly how I used to do it. I would just do a real world test and memorize those figures. The problem is that as conditions change, so does SD. Not enough for practical applications but for an anal perfectionist like myslef, I would just loose sleep over it.

 

[Eaglet]

Sorry, I did not see phorwath's last post.

Using LoadBase 2.0 I got the following results:

SF for 7mm = 1.52
SF for 300 = 1.79

 

[mikebob]

thanks for the reply Michael

I have always added about .5 moa at 500 and 1moa at 1000 while shooting targets but seem to forget about it hunting. I am going to do it the next windless day ( somewhere around winter 2012 probably) and see how it works, has to be better than what i am doing. If i ever get this exbal figured out i might mess with the spin drift function in it.

Mike

 

[Pdvdh]

I came up with 1.396 SF for the 7mm and 1.680 for the 300.

Giving you 5.83" of SD from the 7mm at 0', 29.92 and 59 degrees using the factory published BC.

The 300 calculated out to 6.52" of SD in the same conditions with the factory BC.

If I remember right, you were seeing about 8" of SD? 8-6=2

2" of difference could be scope missalignment, shooter error or mathematical discrepancies or all of the above. Also you were likely shooting much colder air than 59 degrees. Regardless, I have found this system to be scary close in any practical scenario up to 1K. I have seen it very close at 2K as well. However, close at that range could be 8" instead of 2" Still closer than not factoring it in at that range.

Also using Eaglets SF of 1.84 you are just over 7" at 1K. This makes the margin of error even closer assuming his program is better than mine.

I didnt realize I could calculate SFs with Loadbase 2.0 until I read Eaglet's post. So I calculated SFs of 1.44 (7mm) and 1.69 (308) for my conditions of fire.

With these SFs I calculated 4.9" spindrift for the 168 gr VLD (7mm Rem Mag) and 6.5" spindrift for the 210 gr VLD (300 Win Mag).

My actual spindrift was about 9" with the 168 gr VLD and 7" with the 210 VLD.

So I'm pretty darn close to your formula's prediction with the 210 VLD, but I experienced about 80% more spindrift with the 168 VLD.

I would note that Loadbase 2.0 calculated 4.48" Coriolis drift at 1000 yds for my 168gr VLD, and 4.74" Coriolis drift for my 210 VLD. I subtracted these from my total drift in order to calculate spindrift from my measured POIs.

You're correct that there are a number of factors that could explain the discrepancy with my 7mm Rem Mag, and I'm not going to lose any sleep over it. I've determined my drift by field measurement and that's what I'm running with. Thanks Michael, and Eaglet too, for pointing out that SF can be calculated with LoadBase 2.0.

 

[Pdvdh]

This thread is pretty darn good I must say. I was just wondering why couldnt a guy set up a target at 1000+ yards, wait a very long time here in OK for a windless day and zero windage at that range for dead center impact. Then shoot at 100 yards, the distance you are off would have to be spin drift wouldn't it. Then zero back at water ever range you zero at and add that distance as wind drift (break down into mph) like shawn. Would that work I now it wouldn't be perfect but it would close. I think it would be more off at closer range that at 1000 but then it would be easier to account for at closer range also. Heck I am tired just babbling somebody slap me if im getting off base into stupidness.

mike

That's precisely what I did also. I first zeroed dead on at 990 yds on windless days. Then I came back and shot at 300 yds and measure the leftward impact at 300. From there I could calculate the total left to right drift from 300 to 990 yds. Then I subtracted Coriolis drift from my total drift to determine spindrift. You could also shoot at 100 yds or 200 yds, and those might be better yardages. I used 300 yds because that's where I shoot to check my rifles zero prior to heading out hunting and I wanted to know the left dope required at that yardage to be dead on at 1000 yds.

 

[jwp475]

Sorry, I did not see phorwath's last post.

Using LoadBase 2.0 I got the following results:

SF for 7mm = 1.52
SF for 300 = 1.79

Is that 1.52 & 1.79 inches at 1K?

 

[Jeff In TX]

This thread is pretty darn good I must say. I was just wondering why couldnt a guy set up a target at 1000+ yards, wait a very long time here in OK for a windless day and zero windage at that range for dead center impact. mike

Mike,

I'll make you an offer, then next time we get a totally windless day in here in Texas or OK let's get together and give this SD a whirl? Well, we'll probably never meet under those conditions, but if you ever venture down to TAC PRO here in Texas and want to shoot at a grand under tornado conditions (those are pretty much our calm days) let me know.

 

[mikebob]

Mike,

I'll make you an offer, then next time we get a totally windless day in here in Texas or OK let's get together and give this SD a whirl? Well, we'll probably never meet under those conditions, but if you ever venture down to TAC PRO here in Texas and want to shoot at a grand under tornado conditions (those are pretty much our calm days) let me know.

Thanks for the invite,
i am in texas often on business I just might drop in if I am close to that area.

If your ever this way give me a shout I have some brand new targets I made without a bullet hole one in them yet.

 

[Eaglet]

Is that 1.52 & 1.79 inches at 1K?

That's the bullet SF = Stability Factor to plug into Michael's formula to calculate Spin Drift.

SD = (2SF)(Time of Flight)²