L/R Elevation Help ?

[alton9]

Shooting a .22-250 ,9 twist Brux, 75gr. A-Max , about 3200 fps , Nightforce scope. At 800yds , I find some days I need 16.25 moa other days I need 15.75 moa and today I only needed 15.25 moa (same loads) .Why are my come ups not consistent? Is there a way to predict which days will need more or less ? Maybe based on environmental conditions? Out to 600 yds I find this setup to be very consistent and repeatable , but I'm just now starting to shoot further & could use some advice. Thanks in advance,
Alton9

 

[jonbearman]

Wind and mirage are the culprits.When shooting that far,you should be using wind flags or at least surveyors tape on stakes to try and judge the calm times. There is an interesting article written by gale mcmillan on the home page that will describe mirage better and teach you how to overcome it.

 

[fdshuster]

alton9: Sky conditions will also effect vertical. Overcast, or a cloud in front of the sun, and the shots will go low.

Bright sun, blue, clear sky & shots will go high.

I had this pointed out to me several years ago when I was getting a high group of shots, and a low group, same ammo. Was going nuts until another shooter made me aware of the sky conditions.

After that I paid attention to the sky conditions & each time a shot went low, a cloud was in front of the sun. Cloud moves on, full bright sun, shots went high.

Not saying this is your problem, just something to consider.

 

[M14AMU]

alton9: Sky conditions will also effect vertical. Overcast, or a cloud in front of the sun, and the shots will go low.

Bright sun, blue, clear sky & shots will go high.

I had this pointed out to me several years ago when I was getting a high group of shots, and a low group, same ammo. Was going nuts until another shooter made me aware of the sky conditions.

After that I paid attention to the sky conditions & each time a shot went low, a cloud was in front of the sun. Cloud moves on, full bright sun, shots went high.

Not saying this is your problem, just something to consider.

fdshuster

That's just REVERSE in my opinion with post sights on a rifle. When I was shooting with the AMU many years ago at Ft. Benning on the Service Rifle Squad....when bright light was on the face of the target the 'glare' off the target face made the bull to appear larger and if a normal zero was used under this condition the 'glare' would make the 6 o/clock position in one's vision to appear lower than it actually was and the rule was "lights up...sights up!" If the bull appeared somewhat darker than normal...the tendency for a shooter using a 6 o/clock hold would unknowlingly 'hold closer' and the group would impact higher!

 

[alton9]

Well there may be something to that, it was very overcast today . Anybody know how this works? So far I've seen 1 moa difference @ 800 , a coyote isn't that forgiving.Thanks again,
Alton9

 

[M14AMU]

It's nothing uncommon! If you start keeping a data book and pay attention to the temp....you'll find out temp has everything to do with zero variation!

 

[Win94ae]

Barometric Pressure, mirage and temperature; one or all.

 

[Keith Glasscock]

All of them, but also include the head or tailwind vectors. They can be a real eye-opener.

Almost forgot, you can also get vertical from orthographic lift (wind blowing over bumpy land). For that matter, very slight changes in the cheek weld and grip pressure can change your zero.

 

[Nomad47]

Ammo sitting in the sun or a hot chamber can easily have a considerable increase in muzzle velocity. When you have a 30 degree increase in air temp (less drag on the bullet) AND an increase in muzzle velocity, it is easy to have a 1.5 MOA POI change at 1000 yards.

 

[kentuckytroutbum]

In addition to the other factors that have been mentioned, I think atmospheric conditions really come in to play once you get out to 500 yds and more: temperature, humidity, barometric pressure, etc.

On the Daily Bulletin of this website, updated URLs for the JBM Ballistic Programs have been posted for downloading. Over the past 5 years, I've found that JBM has been very accurate in predicting downrange ballistics. Download the URLs and then start "playing" with temp, humidity, pressure, etc, and see the results for 500 yds, and out further in 100 yd. increments. I think that you will be surprised at the changes in MOA settings.

Bill

 

[M14AMU]

RH....is really a NON-ISSUE!! ;D

 

[DOGCAPPER]

Look up ALTITUDE DENSITY . It is a combination of all atmospheric conditions..It strongly reflects temperature...If you are seriously going after long range shooting buy a KESTREL and learn to use it.LT

 

[alton9]

Ok , I searched altitude density on this site & found an article in which the author says it's really barometric pressure that matters? He seemed to be talking about actual altitude rather than a reflection of all atmospheric conditions? What will a Kestrel tell me & how do I put that info to use? Sorry for my ignorance , but thanks for the help.
Alton9

 

[brian427cobra]

Its Density Altitude not the other way around.

Here is how to make DA cards for free
http://www.arcanamavens.com/LBSFiles/Shooting/Downloads/DA/

Here is how to calculate DA without a Kestral 4000, but I highly recommend buying a meter that does give DA readings
http://www.arcanamavens.com/LBSFiles/Shooting/Downloads/ManualDA/


I don't know if shifting DA is the reason for the changes in the OP(original poster) question, but I have seen DA change from -2000 in the morning to 3500 in the afternoon to 0 in the evening, all at the same place on our planet, from morning to midday that was over a MRAD difference at 1000 yards.

 

[alton9]

Okay, maybe I'm starting to catch on.. Density altitude is an expression of how thick or thin the air is & takes into account temp , actual elevation , and barometric pressure ? So if I've got this right , does a higher density altitude number translate into higher bullet impact ( less scope elev. adj.)? Or do I have this backwards? Thanks for your patience...I'm really trying to get this.

 

[brian427cobra]

Your spot on, the higher the DA equals less elevation required to hit your target.

 

[alton9]

Thanks Brian , I guess a Kestrel is in my future. Coyote killing has gone high tech!!!

 

[Killshot]

Think of Density Altitude (D.A.) this way...

Air at higher altitudes is "thinner", thinner air has less resistance upon a bullet allowing it to fly flatter for a longer distance.

When atmospheric conditions (a combination of the actual height above sea-level, air temperature, humidity level and barometric pressure) create a D.A. of say 500, the bullet will feel much more resistance than if fired in a D.A. of 2,000.
Like shooting at a higher altitude, the higher the D.A. is the flatter the trajectory will be.

The Field Precision shooters are more aware of D.A. given their need to range and dope targets at unknown yardages.
But a good log book will show you how much elevation corrections change when the weather is different at various time of the year.