Altitude vs. Barometric pressure

[coyotelatrans]

Yep that is why I rely on JBM's program and can inout different things, but I make a winter chart for drift/drop and a summer one and for my hunting needs it works quite well, thanks for the program JBM!

 

[deepandsilence]

Try Eskimo (JBM) works just fine for an old jar head!
deepandsilence

 

[daveyj]

As far as the bullet is concerned, the only thing it cares about is the local "absolute" pressure.

Whether you are at 2,000 feet or 7,000 feet, if the ABSOLUTE pressure is 26" inches, the flight/drop will be the same.

Grab your Kestral and get on with the business at hand.

.

So just to clarify for the umptenth time, my kestral is giving me ABSOLUTE pressure? If that is the case then it seems we are making this much too complicated. Or is it absolute only after I have recalibrated it? what If I am buried in the woods for a week and have no way of recalibrating before a shot?

forgive me, Im still confused

 

[deepandsilence]

Try Eskimo (JBM) works just fine for an old jar head!
deepandsilence

What are you trying to kill?
Paper?

 

[tmhue001]

i cant believe how bad my head hurts from reading this post! Im happy i did as im learning a lot.

 

[Bart B]

Buzz, higher BP actualy changes the bullets BC, thicker air makes more drag which lowers BC.
RR

I don't think a change in barometric pressure changes a bullet's BC, but it does have a slight effect on trajectory which is the same result.

For example, a 30 caliber 150-gr. spitzer boattail with an average BC of about .435 leaving at 2850 fps has only about 10 inches less drop at 1000 yards when the pressure drops from 30.00 to 29.00.

A 30 caliber 220-gr. spitzer boattail bullet with an average BC of about .620 leaving at 2650 has about 6 inches less drop for the same change in pressure for a 1000 yard shot.

Few people will notice the difference at that range, fewer still at half that far. Barometric pressure's not a significant influnce on a bullet's trajectory. Never has been, never will be.

 

[BryanLitz]

I don't think a change in barometric pressure changes a bullet's BC, but it does have a slight effect on trajectory which is the same result.

Well put. I agree and this can be a confusing point. Some shooters believe that the bullets' BC changes when atmospherics change, and that's one reason why you can never 'count on' a published BC (because it was established in different conditions and doesn't apply for your conditions). The fact is that published BC's are corrected to standard atmospheric conditions, and the ballistic programs count on this. When you use that BC in a ballistics program with non-standard atmospheric inputs, the program applies the effects of those atmospherics and the computed trajectory reflects the non-standard atmosphere.

You could do a lot of fancy math to figure out what the effective BC of the bullet is in your conditions, but that's just the long way around to get to the same thing.

You could do the math to figure out what your 'effective' BC is for a particular set of conditions, then leave the atmospheric inputs of the ballistic program at their default, standard values, but that would be self-defeating. Let the program do the math, that's what it's for.

Barometric pressure's not a significant influnce on a bullet's trajectory. Never has been, never will be.

On this point, I agree IF you stick to the same altitude above sea level. If you venture much to different altitudes, you will find the pressure has a drastic effect on trajectory. When I leave home (Michigan, near sea level) and travel to Raton, New Mexico (elevation 6000 ft+) for a long range rifle match, I use over 4 MOA (~45") less elevation at 1000 yards than I need back home. The difference in pressure is ~29.90 compared to ~24.00". If you stick to the same altitude, you'll never see changes in pressure this large but if you travel, don't underestimate the effects of pressure changes.

-Bryan

 

[Bart B]

Brian, your comments remind me of problems several people face. I've seen more than a few people with their 1000-yard zeros at the NRA range in Raton at 6600 feet elevation try to get on paper at Camp Perry's Viale range at 580 feet elevation in a 1000-yard match. Only to have their .308's putting bullets into the safety berm in front of the target. Helped a few get on target as I'd see the bullets kick up dirt in front of and below the target.

When a bunch of us were getting ready to shoot straight into a 25 - 30 mph headwind in a 600 yard match at Camp Pendleton, there was another interesting event. Several of the folks were commenting about that headwind changing the BC's of our bullets and they'd have to come up a few MOA to zero. A few disagreed with that premise. "Why else would the bullets strike lower?" was the comment often heard. These "believers" saw their first sighter go way high (2 or more MOA) after starting out with their sights set up that far above range zero, then were surprized to end up only 1/4th to 1/2 MOA higher than normal. Head and tail winds are often thought by shooters to cause large elevation changes to their sights for a given range.

 

[D.A.T.]

we use to lie to the m1a1 ballistic computer and input 29.92 B pressure,sure did shoot better

 

[Mikecr]

a 30 caliber 150-gr. spitzer boattail with an average BC of about .435 leaving at 2850 fps has only about 10 inches less drop at 1000 yards when the pressure drops from 30.00 to 29.00.
A 30 caliber 220-gr. spitzer boattail bullet with an average BC of about .620 leaving at 2650 has about 6 inches less drop for the same change in pressure for a 1000 yard shot.

Few people will notice the difference at that range, fewer still at half that far. Barometric pressure's not a significant influnce on a bullet's trajectory. Never has been, never will be.

This has to put you deeply in minority here. >1/2-1MOA of change would hardly be noticed?
Do you represent common thinking among competitors?

Let me just say that I WOULD NOTICE

 

[mikebob]

This has to put you deeply in minority here. >1/2-1MOA of change would hardly be noticed?
Do you represent common thinking among competitors?

Let me just say that I WOULD NOTICE

+1

That would cause you to miss a little critter like a yote

 

[TexasHunter]

Whew - this thread is a brain strain that I messed with for years. I feel like I've had every single question and frustration raised here (except I never went down the complicated math stuff). I spent almost a decade trying to use Exbal on a Palm Pilot with a Kestral 3500 to predict my long range ballistics. I chronographed everything. I entered sight in conditions and field conditions and on and on and on. Bottom line is it never really worked. I'm sure it was some sort of error on my part but it drove me crazy. I was mostly shooting a .308 out to 1000 yds by the way.

Without getting too technical, let me take a stab at what I think everyone is trying to figure out. The key issue for bullet ballistics is DA (density altitude) which is a function of absolute pressure (also referred to as station pressure), temperature and humidity. What this single number tells you (or more accurately, your ballistics calculator) is the "effective" altitude / density of the air that your bullet is flying thru. This is what determines how much it drops at distance.

So to cut to the chase, the Kestral 4500 has the feature to display DA directly. It measures all the variables and reduces them to a single number - Density Altitude. Then you put that single number into the Ballistic FTE JBM calculator on the iPhone / iPad / iTouch after you've configured it to take DA and not all the separate variables (altitude, pressure, temperature, humidity). And voila, your come ups will be dead on for a while. Why only a while? How long is a while? Usually no more than an hour. On most days, things change as the day warms up and weather moves above you. When we shot BJs match in OK back in Sept, the actual field altitude was about 1200' above sea level (this from memory) confirmed with my GPS (also on the iPhone). At around 0800 the actual DA was about 2100' and by the time we finished the 18 stages at about 1600 the DA was near 5000'. With a .308 168 AMax at 2750 fps - THAT'S 2 MOA AT 1000 YDS!!! 20 INCHES!!! THAT'S A MISS!!! Oh and let me add one other important variable. We were shooting east all day. As the earth rotates underneath the bullet during it's 1.6 second flight time to 1000 yds, the target falls away from where it was when you sent the round. How much? .3 MOA!! Another 3 inches which may not seem like a lot but consider the size of your target. If it's only 6", that's another miss. So turn on Coriolis and enter your latitude and azimuth of fire. This is the solution I have finally settled on and it really works. We've confined it out to a mile on numerous days. It really works well and is very simple and quick to deploy.

Good Shooting to all.
Dale

 

[Scot E]

Whew - this thread is a brain strain that I messed with for years. I feel like I've had every single question and frustration raised here (except I never went down the complicated math stuff). I spent almost a decade trying to use Exbal on a Palm Pilot with a Kestral 3500 to predict my long range ballistics. I chronographed everything. I entered sight in conditions and field conditions and on and on and on. Bottom line is it never really worked. I'm sure it was some sort of error on my part but it drove me crazy. I was mostly shooting a .308 out to 1000 yds by the way.

Without getting too technical, let me take a stab at what I think everyone is trying to figure out. The key issue for bullet ballistics is DA (density altitude) which is a function of absolute pressure (also referred to as station pressure), temperature and humidity. What this single number tells you (or more accurately, your ballistics calculator) is the "effective" altitude / density of the air that your bullet is flying thru. This is what determines how much it drops at distance.

So to cut to the chase, the Kestral 4500 has the feature to display DA directly. It measures all the variables and reduces them to a single number - Density Altitude. Then you put that single number into the Ballistic FTE JBM calculator on the iPhone / iPad / iTouch after you've configured it to take DA and not all the separate variables (altitude, pressure, temperature, humidity). And voila, your come ups will be dead on for a while. Why only a while? How long is a while? Usually no more than an hour. On most days, things change as the day warms up and weather moves above you. When we shot BJs match in OK back in Sept, the actual field altitude was about 1200' above sea level (this from memory) confirmed with my GPS (also on the iPhone). At around 0800 the actual DA was about 2100' and by the time we finished the 18 stages at about 1600 the DA was near 5000'. With a .308 168 AMax at 2750 fps - THAT'S 2 MOA AT 1000 YDS!!! 20 INCHES!!! THAT'S A MISS!!! Oh and let me add one other important variable. We were shooting east all day. As the earth rotates underneath the bullet during it's 1.6 second flight time to 1000 yds, the target falls away from where it was when you sent the round. How much? .3 MOA!! Another 3 inches which may not seem like a lot but consider the size of your target. If it's only 6", that's another miss. So turn on Coriolis and enter your latitude and azimuth of fire. This is the solution I have finally settled on and it really works. We've confined it out to a mile on numerous days. It really works well and is very simple and quick to deploy.

Good Shooting to all.
Dale

It seems you should have been able to get the same come-ups with a Kestrel 3500 entering the station pressure, temp, and humidity as you did with the Kestrel 4500 outputting DA, which basically just combines the readings into one output.

Was that not the case? Maybe it was exbal settings and now FTE is working correctly?

 

[Loner]

There's an app for that. It's called DenAlt and it's free, if you don't have a kestrel. I like
DA for the simple reason I can't screw up or forget an entry, it's one number.