spin drift

[Jeff In TX]

Folks, nobody has been able to explain how you separate just the spin drift from all of the shooting variables I've mentioned in my above post. How do you separate out the effects of crosswinds, trigger control, shooter induced error and group margin error?

Spin drift is a fact of life and physics I will agree. Ya'll say you can measure it, but how? Explain how you separate out and measure only the SD from all of the above variables namely wind. I'm sure finding a dead calm day with an absolute zero wind is more of an anomaly than an everyday occurrence.

So my question for those who swear that they always factor in SD for LR shooting; if your shooting at say a 1000 yards and you have a 7mph wind blowing from 9 to 3 o'clock for the first 200 yards, then a 4 mph cross wind blowing from 10 to 4 for the next 300 yards, then a 6mph wind blowing from 2 to 8 for the next 300 yards and then a 4 mph wind from 1 to 7 for the last 200 yards, approximately how much SD do you have and how much windage you add?

You see I've shot long ranges all over the US and these are type of conditions I normally face when shooting at a grand or more. I've never been lucky enough to have a zero calm day to shoot in. Without guessing, please tell us how you separate out only measure your SD from the wind? I'm not trying to be a jerk about this I'm just a realist who works on facts and not assumptions?

Since everyone seems to believe that SD equals roughly 6" at 1000 yards or there abouts +/- with there rounds, why not just sight your rifle in .5" left at 100 yards and be done with it?

 

[Michael Eichele]

Folks, nobody has been able to explain how you separate just the spin drift from all of the shooting variables I've mentioned in my above post. How do you separate out the effects of crosswinds, trigger control, shooter induced error and group margin error?

Spin drift is a fact of life and physics I will agree. Ya'll say you can measure it, but how? Explain how you separate out and measure only the SD from all of the above variables namely wind. I'm sure finding a dead calm day with an absolute zero wind is more of an anomaly than an everyday occurrence.

So my question for those who swear that they always factor in SD for LR shooting; if your shooting at say a 1000 yards and you have a 7mph wind blowing from 9 to 3 o'clock for the first 200 yards, then a 4 mph cross wind blowing from 10 to 4 for the next 300 yards, then a 6mph wind blowing from 2 to 8 for the next 300 yards and then a 4 mph wind from 1 to 7 for the last 200 yards, approximately how much SD do you have and how much windage you add?

You see I've shot long ranges all over the US and these are type of conditions I normally face when shooting at a grand or more. I've never been lucky enough to have a zero calm day to shoot in. Without guessing, please tell us how you separate out only measure your SD from the wind? I'm not trying to be a jerk about this I'm just a realist who works on facts and not assumptions?

Since everyone seems to believe that SD equals roughly 6" at 1000 yards or there abouts +/- with there rounds, why not just sight your rifle in .5" left at 100 yards and be done with it?

In average conditions most here do not have the abilities to separate wind drift from SD. Myself included. However, I am lucky enough to live in an area where there is little to do in the winter except shoot. What does this have to do with SD? Well on really cold winter mornings, wind is often nill, zip, zilch ZERO!

This is how I have been able to do my own testing and research. I have been able to measure it by eliminating wind as a factor.

Yes I factor it into any 800+ yard shot. Wind or no wind.

Most shooters might feel the wind and compensate and shoot. Experiance has taught them to compensate in a way that works. They get the results they get through experiance regardless of their knowledge or lack thereof or compensation, or lack thereof of SD.

The bottom line is that if you have a system that works, with or without compensating for SD, dont worry about it. Shoot your system and be happy.

If you never have the ability to shoot in a windless day, you will learn how to hit with your rifle in windy conditions. When youre learning and experimenting, you may feel like you always hit just a bit farther to the right than you like. This sets up your subconscience to estimate differently. You begin to judge with your instincs (through experiance) and start to hit, never knowing "other" forces are out there.

My system relys on me first establishing how much windage I need for actual wind. Then I either add or subtract my personal SD. For my 300 RUM its about 8" for SD and coriolus. For my 308 winnie its about 11" SD and coriouls. Whether the wind seems concistent throught the coarse of the shot or inconcistent, I still estimate how much to compensate for. I will come up with a rough number for MPH taking into account the direction. If time permits, I will plug these numbers into the ballistic program (mine predicts SD) it either adds or subtracts what is needed for SD. My software uses a clock 12 Oclock all the way through to 11Oclock. It knows I am using a right hand twist and what my stability factor and time of flight is, puts all the numbers in and give me an answer. Yes there are times that wind is more tricky than that. An example is this fall I was with my cousin hunting moose. The shot was 600 yards. BTW I dont factor SD at 600 due to its smallish effects at that range. I could see cotton wood fluff in the air in the wind and thermals very erradic at different ranges. at 200 yards it was one way, at 400 it was another. In our face it was dead on. At the bull I have no idea. I called for him to shoot with 0 wind even though it was a solid 10+ MPH. Guess what? He hammered the bull in the shoulders. Sometimes you just have to use your instincts to guess the wind and add or subtract any SD should it have been a longer shot.

Also anybody who relys on a 6" SD figure is fooling themselves. It can be a bit less or it can be a bit more. It will depend on many factors. This is why the software I use uses stabilty factors and time of flight. Because it is very dependant on twist, velocity, bullet demensions, and atmoshperic conditions as well.

Bottome line is if youre shooting with a system that is working for you. Dont change it.

Some guys myself included are just really anal for lack of a better term. I just want to know ALL of the variables and decide when the time comes to factor it in or not for a given set of circumstances. I am more mathematical when setting up for a LR shot. Others are more instictuall. Is there a right or wrong? If youre hitting and I am hitting, then no. It is what is working for who at a given time in there LR endevors.

 

[JRu]

How do you separate out the effects of crosswinds,

You can only measure SD on dead calm days, or in very steady uniform winds. You really only need to measure it to the closest 1/2 MOA to be effective.

Yes a 1 mph wind would throw the measurement off, but then it would have to be a steady, average, uniform 1 mph wind over the whole range. A 1 mph breeze affecting 10% of the flight path somewhere around the middle is not going to throw the bullet off 1 or even 1/2 MOA.

Could be a geographic thing but over here in Finland at around 300ft elevation, surrounded by forest, dead calm is not an anomaly.

trigger control, shooter induced error and group margin error?

If people are able to precisely determine their 1000yd zeros even with "trigger control, shooter induced error and group margin error" , they should be able to see if the group center is 6 to 12" off to the side. We're not talking about 1" right or left, that's just insignificant. I just need to know if my SD is around 0, 1/2 or 1 MOA at 1k, I'll turn that value into wind speed and I should be close enough for field shooting just adding that left to right wind value at ranges over 600 or so.

Since everyone seems to believe that SD equals roughly 6" at 1000 yards or there abouts +/- with there rounds, why not just sight your rifle in .5" left at 100 yards and be done with it?

Some do and it works just fine.

 

[Pdvdh]

Folks, nobody has been able to explain how you separate just the spin drift from all of the shooting variables I've mentioned in my above post. How do you separate out the effects of crosswinds, trigger control, shooter induced error and group margin error?

Spin drift is a fact of life and physics I will agree. Ya'll say you can measure it, but how? Explain how you separate out and measure only the SD from all of the above variables namely wind. I'm sure finding a dead calm day with an absolute zero wind is more of an anomaly than an everyday occurrence.

It's really pretty simple. After confirming your rifle is dead on at 200 to 300 yds, shoot a group on a windless day at 1000 yds. Measure how far the group is to the right of POA. Subtract Coriolis drift and your left with spindrift. If you're 1000 yd group measures 12 or more inches, don't worry about measuring your spindrift, because your outfit isn't fit for 1000 yd shots anyway. But if you shoot a 0.7 moa group or less at 1000 yds and the center of that group is 11-12" right, then subtract Coriolis drift and then you'll have your spindrift.

Yes, if it's windy out, the wind will affect the bullet's POI more than spindrift, but are you trying to convince us we should ignore 6-10" spindrift @ 1000 yds because the wind might be blowing? Would you ignore a known, constant 90 degree 1 mph wind on a 1000 yd shot? That's about what spindrift amounts to. Once you've figured out how much spindrift your rifle/bullet/load combination produces, it's guaranteed to be present on each and every shot. I've spent the time to determine my spindrift, and know that I know how much drift I experience @ 1000 yds, I'd feel pretty foolish ignoring it.

We don't all live in wind alley. But even if I did, there's no reason for me not to allow for the 9" of spindrift on each and every shot. In the wind conditions you described, most won't be shooting at anything other than targets at 1000 yds. Your example is along the lines of, "if you're shooting through a tornado, is spindrift really an issue?" The answer is no, not if I don't shoot. But every single time I do shoot, spindrift is an issue, about 9" worth at 1000 yds.

 

[Jeff In TX]

. Your example is along the lines of, "if you're shooting through a tornado, is spindrift really an issue?" The answer is no, not if I don't shoot. But every single time I do shoot, spindrift is an issue, about 9" worth at 1000 yds.

Tornado's ... my example is considered a calm day in Texas. Texas has more wind generating turbines (windmills) than all of the other states with Kalifornia a close second. There is no such thing as a dead calm day here, not even in first thing in the morning. Our winds normally push 6 to 10 mph on any given day, though the day after Christmas we had sustained winds of 35mph and one measured wind gust of 73mph.

Well at least we all agree and now can understand each others perspectives. I don't and never have lived or shot in a location where there is zero wind but in Alaska and few rocky mountain locations it happens. I can see how you can factor in SD, just as I now believe you can understand why I think it's insane to try to factor in.

 

[KQguy]

I think if someone wants to accurately measure spin drift from their rifle,the ideal distance would be 500-600 yds.I think measuring spin drift from 1000 + yds. introduces too many other variables.Although the results from 500 yds. won't be as pronounced,I think it will be more accurate.It's kind of like load developing from 1000 yds,how could you determine a good load when you have all those variables working against you?

 

[Pdvdh]

My understanding is that spindrift increases at an increasing rate with distance. I offer the following example with fabricated numbers simply to illustrate this. If you see 3" of spindrift at 600 yards, you might experience 8-9" at 1200 yds. Exactly how much more spindrift you'll experience in the 2nd 600 yds compared to the 1st 600 yds the bullet travels will remain unknown until you shoot at that distance on a calm day and then measure the additional right-ward drift. Your suggestion may get you to within a couple inches at 1000 yds, but I don't see how you'll ever really know unless you just wait for a still day and then shoot at the maximum yardages you intend to shoot at game.

Additionally, I've never noted enough spindrift at 600 yds over the past 20 years to cause me to question my equipment. Once I stepped back to 1000 yds, it became so obvious I started examining my scope mount systems for flaws. Indeed I found and corrected a problem with the alignment of one scope, which was mounted pointing off to the right on my 7mm Rem Mag. After I corrected that alignment issue and was able to shoot again on a windless day, I learned that between spindrift and Coriolis drift, the gun was still printing about 12.5" to the right at 1000 yds. I was shooting at 60.4 Degrees latitude (Alaska) and Coriolis drift accounted for ~4.5" of the 12.5" of my total measured right-ward drift at that range.

 

[Michael Eichele]

Has anybody else tried the (stability factor *2) * (time of flight) ^ 2 equasion?

 

[Pdvdh]

Has anybody else tried the (stability factor *2) * (time of flight) ^ 2 equation?

Not me. I didn't learn enough about spindrift to realize I needed to allow for it until the past 3 months. Is this your suggested equation?

[(2 x stability factor)(time of flight)] Squared = Estimated Spindrift

If you'll provide me with the equation to determine my stability factor, I could run both of my bullets (7mm Berger 168 VLD and 300 Berger 210 VLD) through and tell you how the math shakes out compared to measured spindrift from my two rifles.

 

[Eaglet]

Not me. I didn't learn enough about spindrift to realize I needed to allow for it until the past 3 months. Is this your suggested equation?

[(2 x stability factor)(time of flight)] Squared = Estimated Spindrift

If you'll provide me with the equation to determine my stability factor, I could run both of my bullets (7mm Berger 168 VLD and 300 Berger 210 VLD) through and tell you how the math shakes out compared to measured spindrift from my two rifles.

Phorwath, I think you have to square the time of flight first and then multiply it by twice the stability factor.

 

[Pdvdh]

Thanks Eaglet,

I now see how that could be the case. I'll wait to see if meichele replies before I attempt to run a calculation.

 

[KQguy]

What I was trying to say about testing for spin drift from5-600 yds. was assuming you would use Exbal's spin drift calculations to figure out the rest.Exbal aslo recomends testing from 5-600 yds.

 

[Michael Eichele]

Not me. I didn't learn enough about spindrift to realize I needed to allow for it until the past 3 months. Is this your suggested equation?

[(2 x stability factor)(time of flight)] Squared = Estimated Spindrift

If you'll provide me with the equation to determine my stability factor, I could run both of my bullets (7mm Berger 168 VLD and 300 Berger 210 VLD) through and tell you how the math shakes out compared to measured spindrift from my two rifles.

The calculations I use are very long and complicated. I have them written into a program that I use to calculate SF.

If you reply with bullet lengths, diameters, twists AND velocity I can shoot you a very very close SF.

If you have access to MS EXCEL you can copy and paste the formula into any cell for the results.


=(1.500*2)*(1.3101)^2

The number 1.500 you will replace with your actual SF. And 1.3103 will be replaced with your actual flight time at any given range.

 

[Pdvdh]

meichele,

Now I understand the mathematical equation. Here's the specs for the SF calculations. If you provide a SF for these two bullet rifle combinations, I'll calculate the spindrift estimates. The units of estimated spindrift for this equation are in inches?

7mm 168 gr Berger VLD info:
Length= 1.452"
Diameter= 0.284"
9 1/4" twist barrel
MV= 3056fps

308 210gr Berger VLD info:
Length= 1.479"
Diameter= 0.308
10" twist barrel
MV= 2890 fps