Effects of atmospheric pressure on bullet drop at long range.

[BlackTail30]

What app is that. Looks super user friendly.

It's Hornady 4DOF. I really like this app especially if you're shooting Hornady's bullets. Easy to set up and use, and it's been spot on out to 800 yards (haven't had a chance to shoot further with it) for 6 different rifles.

 

[Hugnot]

When I input your data I come up with 10.06 MOA on your come ups so you'd dial 10. That's less than 1/4 MOA difference than your rangefinder called for. I think you just pulled the shot.
Guys want to get really wrapped around the axle on environmentals and I agree you should try to get them right but they'll rarely be the reason you miss unless you're talking extreme long range. Here's two screen shots of Hornady's ballistic calculator using your shot as an example. The first is your gun with your 590 yard shot and the environmentals on your hunt; 7500 feet, 45 Degrees, 23 mmHg, 10 degree shot angle. Come up is 10.06. The second is your shot but at sea level, 80 degrees, 29.92 mmHg, 10 degree shot angle. Come up is 10.49. That's a drastic change in environmentals and one no one would be that far off of and it's still less than 1/2 MOA at almost 600 yards.
View attachment 313583View attachment 313580

For all purposes equal practical use to what I got on my H 4DOF run on my desk top - just curious about the tiny differences in MOA^ and Windage <.. All other input the same except I used 2840 vs 2830 (should have given less MOA & wind values), same pressure inches of mercury, same degrees down, same everything else. Possibly your data run used a different version of the Hornady calculator - any BC values input? I used a 8 twist barrel and corrected MV to 2830. Not too much obsessed with numbers today!

 

[BlackTail30]

For all purposes equal to what I got on my H 4DOF run on my desk top - just curious about the tiny differences in MOA^ and Windage <.. All other input the same except I used 2840 vs 2830 (should have given less MOA & wind values), same pressure inches of mercury, same degrees down, same everything else. Possibly your data run used a different version of the Hornady calculator - any BC values input? I used a 8 twist barrel and corrected MV to 2830. View attachment 313602

Yeah I thought in his first post he said 2830 muzzle velocity so that would account for needing a little more dialup and I put a 1:8 twist as I thought that's how Chistiensens came in 300 PRC. I also just used the quick ballistic calculator, not the 4DOF. I'm guessing that if we input the same thing, we'd get the same result. My post was only to illustrate that 10.25 at 590 was not the reason for him hitting a foot high and neither were the environmentals input.

 

[Hugnot]

Yeah I thought in his first post he said 2830 muzzle velocity so that would account for needing a little more dialup and I put a 1:8 twist as I thought that's how Chistiensens came in 300 PRC. I also just used the quick ballistic calculator, not the 4DOF. I'm guessing that if we input the same thing, we'd get the same result. My post was only to illustrate that 10.25 at 590 was not the reason for him hitting a foot high and neither were the environmentals input.

Agree - thanks for the info.

 

[73driver]

"Density altitude and altitude is not the same" a quote on the StrelokPro page for inputing Density Altitude calculated by the app or you can input temperature, humidity and current barometric pressure (not corrected pressure) for DA calculation with values you provide. What the altitude at your location is not relevant, the temperature, barometric pressure and to a lesser degree humidity add up to density altitude and that's what matters.

 

[sturner]

You can set it up for density altitude
Try Geo Ballistics app

 

[Bucklowery]

I was concerned about all of this as I prepared for my Montana hunt as I was coming from sea level in Florida. I was hunting in similar conditions and elevation in Montana. I checked zero at 100 yards when I arrived and was dead on. I confirmed at 500 and matched to my app for the conditions. Which showed environmental conditions started changing around 400 but at 500 I was .5 moa less than sea level. So I did not worry about dope changing until 500 and beyond. I found as I was practicing and preparing for this hunt that I had much more variations from shooting stances than I did with environmental. I normally shoot with a bipod at home but decided to remove it for this hunt in the mountains with the varied terrain and shooting positions. My groups were much higher on the bidpod (as I usually let it free recoil) than holding the rifle or shooting across my pack and other shooting styles I practiced.

In short my shooting style entered greater variance than environmental



Thanks

Buck

 

[ChickenTikka]

with a bullet and calibre like that, it would take a hell of alot of atmospheric change and angle to send your bullet a foot high if everything is as you say. i wouldnt worry too much if you are shooting in similar condition to what you zeroed in. as the man above me has said its normally the shooter that has faultered, the main thing is, its dead.

 

[Hugnot]

"Density altitude and altitude is not the same" a quote on the StrelokPro page for inputing Density Altitude calculated by the app or you can input temperature, humidity and current barometric pressure (not corrected pressure) for DA calculation with values you provide. What the altitude at your location is not relevant, the temperature, barometric pressure and to a lesser degree humidity add up to density altitude and that's what matters.

Agree ^, the Berger ballistic calculator appears to limit environmental stuff to temperature & elevation (altitude) and it appears to use the values shown in the table below to derive trajectories. As shown by BlackTail30 and other observations environmental considerations are sort of negligible like less than 1/2 MOA at ranges of 600 yards using extreme comparisons (Rocky Mountains vs. south east USA). Light air would affect bullet stability by increasing Sg values making bullets fly better by reducing drag & flattening trajectories. The Hornady 4DOF calculator accepts all usual environmental items.

For pressure vs. elevation - values are in feet, inches of mercury and celsius & fahrenheit temps.

View attachment 313460

 

[Radman]

I have a question about the details of calculating bullet drop at longer ranges.

I will preface my question my sharing an experience I had last month hunting elk. I shot an elk in the dark timber at 590 yards. The shot was at approximately 7500 feet elevation, 10 degree down hill shot, temperature was approximately 45 degrees F, and my Leika rangefinder said it was approximately 23.0 mm Hg. I was shooting factory 225 ELD-M out of my 300 PRC in Christianson Ridgeline at 2830 FPS. I doped my scope 10.25 MOA, put the crosshairs on the mid crease and let her fly. The animal was quartered away facing right standing on a steep hill so the head was up and away. He was looking back over his left shoulder so his head was almost directly above his shoulder. When I pulled the trigger I heard the distinct "smack" and the animal dropped in his tracks. Upon retrieving him the bullet struck the base of skull which had to have been nearly a foot high. I felt like I was more lucky than good that day.

My question is this: if I use the atmospheric pressure from a kestrel, or advanced rangefinder, is that the only piece of information needed to account for temperature, elevation, and humidity? In other words, does anyone know if the "black box" is imputing temperature, humidity and elevation to calculate the pressure, or are all days points needed to accurately predict bullet drop? Equations or references would be more helpful than opinion here as I am sure the answer is known. Any physics or chemistry majors out there?

While everything matters, most of my 2-cents worth is on the 10 degree angle!

 

[JiminAZ]

Setting all the ballistic conversation aside, one thing I've noticed is that I have a tendency when excited/under pressure to push into the gun ever so slightly when firing that first shot. Makes it deviate high if the front support is firm.

Tell myself to get my form right and the shot is right on with my dope.

 

[PatriotHenry]

I have a question about the details of calculating bullet drop at longer ranges.

I will preface my question my sharing an experience I had last month hunting elk. I shot an elk in the dark timber at 590 yards. The shot was at approximately 7500 feet elevation, 10 degree down hill shot, temperature was approximately 45 degrees F, and my Leika rangefinder said it was approximately 23.0 mm Hg. I was shooting factory 225 ELD-M out of my 300 PRC in Christianson Ridgeline at 2830 FPS. I doped my scope 10.25 MOA, put the crosshairs on the mid crease and let her fly. The animal was quartered away facing right standing on a steep hill so the head was up and away. He was looking back over his left shoulder so his head was almost directly above his shoulder. When I pulled the trigger I heard the distinct "smack" and the animal dropped in his tracks. Upon retrieving him the bullet struck the base of skull which had to have been nearly a foot high. I felt like I was more lucky than good that day.

My question is this: if I use the atmospheric pressure from a kestrel, or advanced rangefinder, is that the only piece of information needed to account for temperature, elevation, and humidity? In other words, does anyone know if the "black box" is imputing temperature, humidity and elevation to calculate the pressure, or are all days points needed to accurately predict bullet drop? Equations or references would be more helpful than opinion here as I am sure the answer is known. Any physics or chemistry majors out there?

Did you compensate for angle? If not that is a big part of the high hit. My 2 cents

 

[vancewalker007]

concerned about all of this as I prepared for my Montana hunt as I was coming from sea level in Florida. I was hunting in similar conditions and elevation in Montana. I checked zero at 100 yards when I arrived and was dead

To answer the initial questions, the gun was zeroed at 100 yards, initial muzzle velocity was obtained at about 50 degrees F with lab radar. Pressure was around 25 mm Hg at the range. I confirmed the dope sheet out to 480 yards on steel. (7.0 MOA at 480 yards). I typically shoot at the range using front and back sand bags.

My Leika Geovid gave me an atmospheric pressure of 23 mm Hg on the mountain at 7500 ft elevation. I used the Ballsitic AE app on my phone and I changed the elevation and pressure to match. The shot angle was mild. 10 degrees or a bit less. I did use my pack as a front bag shooting prone with no back support other than my left hand.

Given the angle in the field I suppose the recoil of the gun may have lifted the barrel or the pack I shot off may have been more (springy) than the sand bags causing slight lift compared to the bench.

I guess this experience just highlights how subtle field differences really can affect the outcome when hunting and the more we practice in the field (shooting rocks etc) the more we identify the effects of field conditions on long range shooting. A high country horseback elk hunt is much different than long range "field" elk on private property.

So glad you got your elk.

Its obvious you shot high. I think the .25-.5 MOA error you had from using line-of-sight range instead of true-ballistic range plus your rifle's accuracy factor (I'd assume 1 MOA for the purpose of this) you could be dealing with 3-4 inches of height error. Maybe more if your ES and SD are over 20fps.

You were shooting off a setup that was different from your practice setup in particular you had no rear bag. You're an American, so am I ;-), we tend to aim high as a group to begin with (that's an assumption). Lets say this could cause another 2-3 inches or error.

So worst case in my example you may have induced around 7 inches of error plus aiming a little high you could easily be inducing 1.2 MOA of error at 590ish yards.

I don't think you enviro data effected your shot very much compared to the other possible errors. Another subtle effect can be wind across broken terrain. If there were any up drafts between you and the elk that could have added to the error.

What to do to avoid this in future:
1. Practice shoot long range off your field set ups at angles. Its hard if you live in flat land. Maybe you could find a deer stand to shoot out of?
2. Get yourself a set of light weight shooting bags so you can anchor the rear of your rifle - We use Phoenix Shooting bags.
3. Make sure you are always using true ballistic range, at all times, for doping a shot.

Man I gotta go shoot some steel.

 

[Plinker147]

Angle is the biggest influencing factor in your scenario. All the other inputs wont amount to that big of difference at that distance.

Atmospheric pressure wont make a big difference unless you get into extreme conditions that you didn't account for or you get into longer distances like1K. At 590yds not much.

Also the type of rest doesn't make a difference. It doesn't matter if its sand bags, bipods, rocks, back packs or tripods those alone will not change your POI. However how the shooter applies pressure or doesn't can change POI, meaning its all shooter induced. Its always best to practice in the positions you expect to shoot in while hunting.

I've had a "good miss like yours" and it was all shooter induced