Advanced Marksmanship Variables that affect POI Questions

[paramil]

Ok so I know that for every ten degrees of difference (temperature) from
that which you got your zero, there is about a 1MOA change in POI (point of
impact). I have also been told there is also a change in POI for every so
many feet of elevation (from sea level) from that which you zeroed and the same applies with
barometric pressure.

Anyone care to fill in the blanks about how much shift in POI there is for
these variables?

Also I have seen most of this measured in MOA, what is the shift in MILS?

Also if I am missing anything other variables the data would be appreciated.

Weapon is a .308, 25 inch barrel. Thanks in advance.

 

[Barn Side]

Re: Variables that affect POI Questions

<span style="font-style: italic">A general rule is that when a rifle is zeroed, a 20 degree increase in temperature will raise the point of impact by 1 MOA. A 20 degree decrease in temperature will drop the bullet 1 MOA. The key is consistency in shooting environments or knowing how the changes effect your shots when the consistency is off.</span>

 

[Sterling Shooter]

Re: Variables that affect POI Questions

There are several factors which will effect zero: wind, and weather, as well as drag and gravity. Your first statement about a 10 degree change in temperature being worth 1 MOA, however, is not fact. Some would argue it's more like a 20 degree change in temperature which is needed to produce a 1 MOA change in impact; but, this too is not universally accepted as fact.

 

[gstaylorg]

Re: Variables that affect POI Questions

The effect of temp on POI is also largely dependent on the specific powder/load you're using. Although some powders are much more sensitive than others, most will exhibit a significant effect if the temp differential is great enough. Trying to pin a specific number to the change (such as 1 MOA per 10 degrees, which for me would represent a gross <span style="font-style: italic">overestimate</span>) will likely be an exercise in frustration. Much better to actually determine the change yourself with your specific setup and record it meticulously. Over time, you will likely generate more than enough data to put together a nice little graph or plot of change in POI vs temp that is specific for your setup. You can then print it, get it laminated at FedEx/Kinkos, and keep it in your rifle case or range bag.

Atmospheric conditions including elevation and humidity can also have a substantial effect on POI if the changes are significant. As an example, going from where I shoot in San Diego (elevation ~420 ft) to Raton, NM (elevation 6600 ft) required approximately 3.4 MOA less elevation at 1000 yd under comparable temp and humidity conditions. If you want to get some idea of how much these parameters can affect POI, go to JBM Ballistics (link below) and play around with different elevations/humidity/temps, etc. using your specific load, MV, zero, etc. You can change a single variable at a time and see the predicted effect. It's pretty easy to do and of itself is a useful exercise to familiarize yourself with the ballistic calculator.

http://www.jbmballistics.com/cgi-bin/jbmtraj_simp-5.1.cgi

 

[Lowlight]

Re: Variables that affect POI Questions

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: dkealty</div><div class="ubbcode-body"><span style="font-style: italic">A general rule is that when a rifle is zeroed, a 20 degree increase in temperature will raise the point of impact by 1 MOA. A 20 degree decrease in temperature will drop the bullet 1 MOA. The key is consistency in shooting environments or knowing how the changes effect your shots when the consistency is off.</span></div></div>

But where?

This "rule of thumb" never states where?

100 yards, no....

200, 300, doubtful,

So where does 20 degrees of temperature change cause a 1 MOA POI shift with a 308?

This "rule of thumb" was written so long ago, it's essentially invalid with modern bullets, barrels, and powders. They are far less temperature sensitive, our average muzzle velocities are better than 100fps more than they were 10 years ago, so how this applies is not something you can simply determine, plus it never said "where" ?

Generally speaking you can say, @ 1000 yards. But still it depends.

Short answer, rules written 20 years ago or better don't apply. In fact go back to those same text that were written and you'll find things like Humidity as being complete wrong. Problem is people simply keep spitting out these falsehoods as gospel, don't buy it.

You have to determine the answer on your own,

 

[Greg Langelius *]

Re: Variables that affect POI Questions

Yes. And that's just for starters.

All the pertinent factors apply whether or not we can detect and/or quantify them, and therein lies the core of the issue.

I am reminded of the line from the song that refers to the "Weather Chart Mozart". I reckon there are a lot of folks here who have looked upon the literature and concluded that the POI should be calculable down to very small increments.

Technically, that should be true.

But the means they propose and the rules they embrace are formulated with the assumption that the pertinent interactions proceed on a visibly large scale. If this were so, they would have a reasonable probability for success.

But the true arena of these interactions is taking place at least down at the molecular scale, where particles of air and humidity are interacting with themselves and with the immensely larger projectiles that are wending their ways through a very complex medium.

Mathematics break down on all but a very general level. You can detect and quantify <span style="font-style: italic">some</span> of the pertinent factors on a large scale. They can, in turn, provide some general predictions about probable POI's. <span style="font-style: italic">Probable POI's</span>...

In the end, you can define some limits regarding cones of dispersion, but precise POI's are not even guaranteed to be inside that cone, just likely to be.

You have to make that trial shot, and adjust, and understand that even the adjustment simply predicts a probability. Things are changing all the time. We don't get to see but a small bit of that change. The more we see, the longer it takes to input and compute that change. It becomes a race between deriving a result and it becoming obsolete before we can even read it, let alone implement it.

Gotta live in the real world, and that world is Murphy's world.

Greg

 

[Barn Side]

Re: Variables that affect POI Questions

Thanks for the elucidation.

 

[stranded]

Re: Variables that affect POI Questions

Brian Litz's new book (accuarcy and precision for long range shooting) really put into simple/graphic terms why I will never get all the variables right for a high confidence(90%) shot past 400yds when hunting in the field.I really think everyone should read this to get a practical view of the variables.