BC at high elevation?

[Franklink]

Curious if anyone has measured any actual BCs at high elevation? 

I live at 5600feet and need to use a BC of 0.05 in Strelok to get my field target gun to match the ballistics programs predicted pois. I realize that is much higher than most people see. 

This is a .177 shooting at 19.2fpe with a LW poly barrel.

I know actual poi in 1 yard increments from 10-20 yards and 5 yard increments from there on out to 55. 

I know I've got the velocity right. I only own one chronograph or I'd measure actual BC myself.

I've got to fudge the scope height slightly higher than actual to get numbers to match, even with 0.05 for a BC. If I fudge scope height lower than actual, the predicted pois are even worse.

I'm just using pellet weight listed on the tin. I imagine there are higher and lower weighted pellets in the tin but assuming the 10.34 is an approximate average. 

The field target gun is great and I don't NEED the numbers to match Strelok as I already know its actual trajectory for field target ranges. I do however, like to use Strelok for shooting prairie dogs, ground squirrels etc at longer ranges after ranging with a laser rangefinder. 

I recently picked up a Veteran in .22 and, while I haven't had a chance to know the gun as closely as I do my FT gun, the trend of needing to use higher BCs than most people see seems to be continuing with the Veteran.

So, the big question here is if I need to use higher BCs (when hunting with different airguns at greater distances) than most often quoted online due to my higher altitude. 

 

[mmahoney]

I'm not an authority on it, but anytime I measure the BC of my pellets here at 4550 feet, they are always higher than recorded values. I just assumed that Strelok made an adjustment when I enter the elevation into the program to compensate, but you know what they say about "assuming".

 

[Franklink]

That's the kind of info I was looking for-good to know it might be an actual thing.

And, you just reminded me, yes, I have 5600 feet plugged in as the elevation.

 

[Macros]

I also live at around 5500 feet altitude and find the same thing. For some reason using the strelok density altitude and the chairgun location don't seem to correct for anything. My BC is nearly always 0.05 or above to get ballistics programs to work. 

I know one of the smaller south african airgun youtubers who lives not too far from me and at the same altitude (AirGunnerSA) tested the bc of 18grain JSB out of his daystate wolverine and found them to be around 0.055.

 

[ackuric]

IDK...I have actually measured my BC using 2 chronographs @ 5k+ altitude and the BC was what chairgun had. The only change with ballistics and altitude should be caused by AIR DENSITY...which does NOT effect Ballistic Coefficient, but does effect trajectory. 



Denser air = more drop. If I change my elevation in chairgun the trajectory / drop changes..but the BC does NOT...



Within external ballistics, when the sectional density of a projectile is divided by its coefficient of form (form factor in commercial small arms jargon); it yields the projectile's ballistic coefficient.



Sorry but you're either entering information incorrectly or are mistaken. But the ballistic coefficient of a projectile doesn't change at different altitudes..just air density.

 

[Franklink]

So, please help me understand.

My dumbed down idea of BC has always been that it is a number assigned to a projectile that basically quantifies drag. So, I've thought that factors like barrels that impart more shallow rifling grooves (smooth twist and polygonal) essentially give the pellet a slightly better BC since the pellet doesn't have as pronounced drag-inducing "fins" of rifling cut into it. To better wrap my brain around it, I've kinda formed the ideas that low humidity (fewer water molecules in the air) and high altitude (fewer air molecules) can give a projectile a slightly better BC, again, reducing "drag" the pellet experiences as it makes its way to where it's going.

I thought that BC was highly variable, due to the above listed factors and probably many others. Is it a constant? 

It sounds like you're making the case that my modification of the BC in the ballistics program simply accounts for changing air density. So, the ballistics program's elevation change function appears to not account for actual POI of a known weight projectile traveling at a known speed at known distances.

The air density should be a function of altitude (when shooting here at home that is a constant) and temperature right? I have tweaked the temp settings enough times in various different shooting sessions to know they don't change the trajectory enough to match what my gun is doing.

Hoping to generate some discussion on this as I would really like to understand it better. Also hoping that I don't come across as confrontational. 

 

[Macros]

Reduced air density reduces drag on the projectile. That is the only way that pellet drop can change based on altitude. Therefore it must affect the bc, which is a measure of drag.

Regardless, whether ballistic coefficient 'technically' changes or not (although from everything I've ever read, this is not the case, and from experience, everyone I know who has tested with dual chronographs in my location has significantly higher bc than chairgun gives) would appear moot, especially as chairgun clearly does not correctly account for the degree of change and the only other way to simulate the change is to increase the bc value.

When I go to sea level the chairgun bc works perfectly with no other adjustments, so there is nothing wrong with my other data.

Interesting link regarding this stuff:

http://www.camdenrgc.com/techie/ballistic_effects_of_altitude.htm

 

[JimNM]

FWIW, altitude density is a function of pressure and temperature and humidity. You will note about %3 change per 1000 feet as you rise above sea level at a given humidity, temperature.