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]

Originally Posted by CatShooter

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]

Originally Posted by 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]

Originally Posted by Ridge Runner

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.

[bsl135]

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