Effects of atmospheric pressure on bullet drop at long range.

[richhymas]

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?

 

[motrapper]

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

 

[waveslayer]

Just like motrapper said. Elevation, angle, humidity, pressure, temp. Your powder you were using will change when all those factors change.
Have you trued your Ballistics calculator? Know the temp stability of your powder, what does your Velocity do with the temp changes? Angled shooting is a big factor too.. just like bow hunting.

 

[RockyMtnMT]

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.

 

[Gamesniper19]

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?

First question - did you true your dope at that elevation prior to your shot? Electronics are not full proof and while we all use it, truing your dope at known distance and in known environments matters. Case in point in nearly the exact same environment:

My hunting buddy and I were hunting Montana at 7000 ft and in 40 degree temps this year. He is shooting a 30 Nos with a 210 berger. We arrived at the location after his flight and started to shoot. Before we did we checked his torque on all bolts and mounts. We laid prone with a bag and an atlas bipod to ensure a good rest in the field. First, zero at 100 to ensure he is on. Shot a great sub moa 3 shot group. Then we moved to 500 and input all the data - Velocity had a SD of 9 with magnetospeed (yes i travel with it), BC, temps, humidity etc, etc...drop calc on the Kestrel 5700 stated 7 1/4 MOA. First shot at 500 high, second shot high. Corrected 1/2 MOA down, still high burning the edge of the 6 inch plate. Dial 1/2 MOA down to a final 6 1/4 MOA center hit. He is not comfortable shooting more than about 600 so we stopped.

Conclusion. True your dope. Know for sure before you hunt. Angles matter and so does everything else. While we could not in the field verify why there was such a diff in dope, the results told us exactly what we needed to do. Could have been something we missed, could have been that the scope tracking isn't perfect (it happens so I verify all my scopes on a tracking board) but the electronics were wrong. DOPE means Data on previous engagements. Know what your rifle will do before you go in the field and it will give you complete confidence in your shots. He harvested a nice mule deer and a nice elk and never had a doubt when he shot. nice feeling

 

[Reelamin]

I have seen a couple threads along these lines. All I can add is I have never had that much dope change at that distance with those amounts of external changes.

 

[Hugnot]

The Hornady 4DOF Ballistic Calculator might work. Your bullet, the .30 225 gr. Hornady ELDM is in the library of bullets.

Variables - temperature, humidity and elevation - may be entered into the process to come up with a firing solution among others like shooting angle.

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

A write-up of the process;

Overview - Hornady Manufacturing, Inc

Overview The Hornady® 4DOF® (Four Degrees Of Freedom) Ballistic Calculator determines trajectory solutions based on projectile Drag Coefficient (not Ballistic Coefficient), combined with exact physical modeling of the projectile and its mass and ae...

www.hornady.com

A quick firing solution using the H 4DOF process with your data:

Humidity, twist rate, wind speed, and pre-set zero not provided so I provided them as seen in screen-shot

These data were not provided for any financial gain and only for personal use.

I now do most of my shooting with a 7.7 twist .22-.250 using the 75 grain .224 ELDM at about 3,200 fps. My shooting & the H4DOF calculations are real close. Whenever possible, I compare actual results with H4DOF data. I would guess that most shooters don't have much time at ranges having up or down hill targets. Lots of shots at rodents, thus lots of comparisons with the H4DOF process & real stuff.

I remember taking a shot at a real big rockchuck at just under 850 yards using a 6.5-06, with a 140 Sierra GK at just under 3,000 fps. The shot was from the bottom of a coulee to almost the top of a big cliff, like up 7 degrees. My scope was a 14X 1 1/2 Unertl and I had a printed firing solution table. I took the shot and my spotter said about 18 inches over the rodent, TOF was about .5 sec & I was barely able to spot hit at bottom of field (14 pound rifle). The rodent moved over 2-3 feet & I was able to shoot again with success. I went to school for a real long time & took lots of hours of math and the obligatory physics & chem. courses. After that I was a gofer for some real smart guys.

 

[Sportshooter5]

I use the "iStrelok" app on my mobil it will use current weather and slope degrees (Providing you enter the degrees) Reasonably priced. Great app. And worth looking at.

 

[richhymas]

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.

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.

 

[richhymas]

The Hornady 4DOF Ballistic Calculator might work. Your bullet, the .30 225 gr. Hornady ELDM is in the library of bullets.

Variables - temperature, humidity and elevation - may be entered into the process to come up with a firing solution among others like shooting angle.

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

View attachment 313460

A write-up of the process;

Overview - Hornady Manufacturing, Inc

Overview The Hornady® 4DOF® (Four Degrees Of Freedom) Ballistic Calculator determines trajectory solutions based on projectile Drag Coefficient (not Ballistic Coefficient), combined with exact physical modeling of the projectile and its mass and ae...

www.hornady.com

A quick firing solution using the H 4DOF process with your data:

View attachment 313499

Humidity, twist rate, wind speed, and pre-set zero not provided so I provided them as seen in screen-shot

These data were not provided for any financial gain and only for personal use.

I now do most of my shooting with a 7.7 twist .22-.250 using the 75 grain .224 ELDM at about 3,200 fps. My shooting & the H4DOF calculations are real close. Whenever possible, I compare actual results with H4DOF data. I would guess that most shooters don't have much time at ranges having up or down hill targets. Lots of shots at rodents, thus lots of comparisons with the H4DOF process & real stuff.

I remember taking a shot at a real big rockchuck at just under 850 yards using a 6.5-06, with a 140 Sierra GK at just under 3,000 fps. The shot was from the bottom of a coulee to almost the top of a big cliff, like up 7 degrees. My scope was a 14X 1 1/2 Unertl and I had a printed firing solution table. I took the shot and my spotter said about 18 inches over the rodent, TOF was about .5 sec & I was barely able to spot hit at bottom of field (14 pound rifle). The rodent moved over 2-3 feet & I was able to shoot again with success. I went to school for a real long time & took lots of hours of math and the obligatory physics & chem. courses. After that I was a gofer for some real smart guys.

Thanks. My gun is zeroed at 100 yards and MV is 2840 FPS so your dope sheet with that should be darn close to 10-10.25 MOA.

 

[Hugnot]

We live just SW of Missoula & 3 years ago a huge herd of elks could be seen on the hills just east of our house, ranges were 1,500 to 1 mile out. There were so many elks & they kept moving about so I could not count them. I'm guessing 50 - 75 elks - never seen anything like it.

 

[richhymas]

View attachment 313551

We live just SW of Missoula & 3 years ago a huge herd of elks could be seen on the hills just east of our house, ranges were 1,500 to 1 mile out. There were so many elks & they kept moving about so I could not count them. I'm guessing 50 - 75 elks - never seen anything like it.

Thanks. Glad to see the updated chart. My dope didn't account for the shot angle which probably accounts for the 0.25 MOA difference between my phone table and your calculations. Glad they are so close. Thanks for posting. Lots to learn and lots to practice. I suspect that differences in shot setup etc accounts for the high shot.

 

[BlackTail30]

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.

 

[richhymas]

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

What app is that. Looks super user friendly.