Calculating likely error or dispersion?

[med358-boise]

Do any of you guys do calculations calculating expected shot error or dispersion and "tolerance stacking"?

Let me give an example, so I shoot a 284 Win with a Barnes 145 LRX @ 2925.

So at 600 yards with my typical environmentals (temp, altitude, humidity), I know that a 1 mph incorrect wind call is 2.5" of error, and at 4 mph an 1/8th of an error of in wind value is 1.25 inches, my .5 MOA rifle has an expected dispersion of 1.5 inches, and 1 crosshair width of error is 1.25 inches so if those stack in the same direction that is 6.25 inches of error so the likelihood of hitting a 10 inch target on the first shot could be pretty low.

Do any of you folks do similar calculations to establish rules of thumb for whatever "pre-shot" planning or shot taking decision purposes? I am not suggesting that this is run for every scenario

For me, the application determines who buys lunch? and there is of course the right to opt out of the bet but opting out to often subjects me to at least 10 minutes of ridicule about every other day for a week.

(edited for clarity)

 

[waveslayer]

To be frank, way over thinking it! Fundamentals are very important. Reading the wind is the largest factor there, if your ammo and gun are of sound foundation.

No, I do not.

I shoot enough and know my gear enough that all I worry about is the wind. I know my reloads are all single digits in SD, so 600 yards it's not an issue, my zero is confirmed, what's the wind doing. I bracket for the wind, it's hardly ever a constant speed, so I have to bracket.

Practice OSOK drills, one shot one kill. But using statistics is great but way over complicated

 

[LRNut]

Mike:

Your post is something I have discussed on LRH ad nauseum - particularly with shots past 1000 yards. At 1000 yards, trying to hit a 10" vital area means you have to call the wind within 1 mph to have a 100% chance of hitting, even if your rifle is 1/2 MOA. But even a bad wind call can still look good due to statistical distribution. For example, suppose your wind drift is 4" per mph of wind at 1000 yards. Your rifle is 1/2 MOA. Suppose the wind is left to right, but you underestimate it by 1 mph - your entire 5" group (in other words, the natural dispersion of the entire population) is shifted by 4" - some shots will be 6.5" from center, but some will be very close to center. It only gets worse past 1000 yards, and at some point, hitting a 10" vital area is impossible to do 100% of the time. When I see a LR shot of 1200 yards or more, I know that luck has a lot to do with it.

One thing almost no one talks about on LRH is time of flight - past 1000 yards, your TOF is going to be at least one second - hold your Kestrel into the wind and see how much it changes in one second.

Bryan Litz addresses this with his WEZ models. Besides wind, you have error in ranging (especially if you are trying to range an animal on a slope surrounded by bushes or trees), errors in your ability to hold, effects of velocity distribution, etc. The new Leica binos have a shot probability that I am sure uses this principle.

I have thought a lot about this. I shoot every day when I am at my CO place (will shoot at my 906 yard gong in a few minutes). What I have learned is the conditions are super important when it comes to me estimating my hit probability. First up is mirage - can I see it? Yesterday right after light, it was cloudy with a slight wind right to left. My Kestrel showed 1.3 mph on average. I held for a 2 mph wind (which once entered into my SIG Kilo10 resulted in a .16 MOA right hold (I have spin zeroed for 500). My shot was one MOA left, my first miss at that range in at least two weeks (I start every day with that gong). Yesterday I wasn't using my most accurate rifle, but even still, I missed that wind. Later in the day, I shot at my 763 yard gong. Same rifle. Held 1.5 MOA for wind and hit 2" right of my 3" aiming circle. The difference? I could see mirage.

Another thing I have learned: it is much easier to hit at LR in a 10 mph wind from 8 to 10 o'clock that the same wind from 11 to 1 o'clock. Full value wind for 8 to 10 is 91 to 100% - getting the speed correct is the most important. Full value for an 11 to 1 wind is +50% to - 50%, which means you better get the direction right. In that wind, 625 would be the farthest I would feel comfortable shooting, based on my experience shooting quite a bit in this condition. Another condition that gives me fits are gentle wind (less than 5 mph) that constantly shift left to right and right to left (I see this at my CO place a lot). To hit at long range, you have to shoot when the wind is moving in the direction of your wind call.

 

[med358-boise]

Mike:

Your post is something I have discussed on LRH ad nauseum - particularly with shots past 1000 yards. At 1000 yards, trying to hit a 10" vital area means you have to call the wind within 1 mph to have a 100% chance of hitting, even if your rifle is 1/2 MOA. But even a bad wind call can still look good due to statistical distribution. For example, suppose your wind drift is 4" per mph of wind at 1000 yards. Your rifle is 1/2 MOA. Suppose the wind is left to right, but you underestimate it by 1 mph - your entire 5" group (in other words, the natural dispersion of the entire population) is shifted by 4" - some shots will be 6.5" from center, but some will be very close to center. It only gets worse past 1000 yards, and at some point, hitting a 10" vital area is impossible to do 100% of the time. When I see a LR shot of 1200 yards or more, I know that luck has a lot to do with it.

Bryan Litz addresses this with his WEZ models. Besides wind, you have error in ranging (especially if you are trying to range an animal on a slope surrounded by bushes or trees), errors in your ability to hold, effects of velocity distribution, etc. The new Leica binos have a shot probability that I am sure uses this principle.

I have thought a lot about this. I shoot every day when I am at my CO place (will shoot at my 906 yard gong in a few minutes). What I have learned is the conditions are super important when it comes to me estimating my hit probability. First up is mirage - can I see it? Yesterday right after light, it was cloudy with a slight wind right to left. My Kestrel showed 1.3 mph on average. I held for a 2 mph wind (which once entered into my SIG Kilo10 resulted in a .16 MOA right hold (I have spin zeroed for 500). My shot was one MOA right, my first miss at that range in at least two weeks (I start every day with that gong). Yesterday I wasn't using my most accurate rifle, but even still, I missed that wind. Later in the day, I shot at my 763 yard gong. Same rifle. Held 1.5 MOA for wind and hit 2" right of my 3" aiming circle. The difference? I could see mirage.

Another thing I have learned: it is much easier to hit at LR in a 10 mph wind from 8 to 10 o'clock that the same wind from 11 to 1 o'clock. In that wind, 625 would be the farthest I would feel comfortable shooting, based on my experience shooting quite a bit in this condition.

LR Nut (AAZW),

Thanks for the response. My OP didn't mean to suggest I do this for every shot (so I edited it). It is very much along the lines of what you are describing.

I suspect a good percentage of folks do this to one degree or another they may not just do it as formally or with as many variables or with multiple calculations. It may be as simple running a few calcs to know that (for example) given my load,s a 12 mph means I need to limit shot to 600 yards (just an example) because of the likelihood of variability in the wind an its impact on the shot and intended target size.

I don't think it was on this forum but I remember a Poster once describing to a "younger Post" the choice between longish range shots and he asked the youngster "Do you know how I knew what shot I could make?" and when the youngster couldn't answer the older guy told the youngster "Experience, I just know" and many people chimed in with "that's right when you have experience you just know" etc. Well, thru a set of back and forths (which he found rather annoying ) and him denying a set of rules of thumb, I demonstrated to him through his own words that his "experience, I just know" was really a database in his head of what variables and the values of those variables that impact makeability.

Take care,

Mike

 

[LRNut]

Here is that shot at 906 that I just took. Cold and clear, could see mirage boiling up but no drift left or right. SIG Kilo 10 said 20..5 MOA elev and .5 hold left. Held that. This is with my Savage Lapua, the only factory rifle I shoot past 600 yards - my least accurate rifle.

I have an older iPhone mounted on a spotting scope that I use to record my shots, so this video is me taking a video with my new iPhone while I played it back on my old one. Note the time of flight - not inconsequential.

To answer your last post, I "just knew I could make this one" - not the confidence I had yesterday.