Effects of atmospheric pressure on bullet drop at long range.

[Greyfox]

Temperature and pressure surely effect trajectory at the longer ranges(500-1000+yards). In my early LRH years, a bullet with a G7 of .300-.320 and 2975- 3000FPS velocity, I would apply 1-2 clicks/1000ft of elevation change compounded with 1-2 clicks/20F change in temperature. These adjustments were mentally calculated and applied from 500-1000 yards. With practice I could connect with a high level of constancy. For the past 10 years or so I've used a G7 Ballistic RF for fast and concise corrections. If necessary, I will tune/calibrate actual POI with velocity inputs out to 400-600 yards, and BC at the longer ranges. Given an accurate wind dope and a solid rest, first shot hit confidence is very high

 

[Hugnot]

OP did compensate for -10 degree down as shown by 2 different ballistic solutions that both produced 10 MOA elevation needed at 590 yards and the OP dialed 10.25 MOA. I think, up to 600 environmental factor differences negligible. I think, actual air density a combo of temp, humidity, barometric pressure more important than just altitude numbers. Pre GPS altitude was calculated by barometers calibrated at some base station where elevation was known.

 

[Tulsa Reiner]

Elevation, humidity and angle using cosine all reduce the yardage dialed for the shot.

To clarify the above: INCREASED elevation, LOWER humidity, and INCREASED angle reduce the yardage needed to dial for the shot.
These are all relative to where and when the scoped was adjusted before the hunt.

 

[Tulsa Reiner]

" Leika rangefinder said it was approximately 23.0 mm Hg."
I hope this is a typo in your question, because if this is what was input to your ballistic calculator, it is off by a factor or 25.4.
It probably should have been 23.0 INCHES. However, an error in this direction would have caused you to shoot low, so it isn't the explanation for your high shot.

 

[Pro2A]

Without running numbers 10.25 moa sounds like a lot. So I have to ask some dumb questions. What range were you zeroed at and did you confirm your zero by shooting it to make sure it was dead on? Did you shoot your rifle at long range prior to the hunt to verify your drops?

Assuming all those things were done then it may be as simple as how the rifle recoiled in the hunting situation vs the controlled practice before hunting.

Agree. Position building/recoil management/pulse-breath control in the field is very different, more challenging/critical than on the zero range.

 

[Teri Anne]

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?

I agree with you that it was in the end a lucky shot but the Bull went down which is what was the desired result. I also agree that temperature, atmospheric pressure and humidity all have an effect on the flight of a bullet as well as the temperature stability of the powder you are using. I do however believe that the issue involving the bullet hitting a foot high, I might say luckily and successfully was the angle of the shot. When shooting at either an upward or downward angle you have to aim low to hit your intended target area. The reason for this is that our range finder measures the distance from your eye to the target as a straight line at 600 yards at a 20 degree angle the actual distance from gun barrel to target. The net result if you plot it out is that instead of flying the ranged 590 yards the actual target is only, let's say for giggles in this case 490 yards away since the upward or downward angle decreases the actual distance from you to the target. Since the distance is considerably shorter the bullet has not fallen the require amount to hit the target at the shorter distance. This results in the bullet not falling the estimated drop at the given range, or in other words hitting high. This high shot varies with distance so I would not be surprised that your shot was indeed a foot high. As it is said, gravity will prevail and we all know that bullets drop due to gravity as the range increases. To read up on this in a better manner than I tried to explain here check out this article from Rifleshooter. https://www.rifleshootermag.com/editorial/hitting-a-high-or-low-angle-shot/83768
You will have to copy and paste the link into your browser. Very interesting as well as scientific readint.