Velocity of 9mm at sea level vs mile high

[Lock-n-load]

Just wondering what the effect on velocity a 9mm round has at sea level compared to mile high  (wyoming, colorado) does the bullet go faster with the thinner air resistance? And at what altitude are the velocities on ammo boxes decided at? 🤔🤔

[tdogg]

Well I live at 7200ft (in AZ ) and routinely chrono slightly faster (maybe a 1-3 PF higher) at lower elevations.  I'd attribute that to a rise in temperature and differences in chrono equipment.  I have a cheap Caldwell unit and when I go to majors they all mostly use Lab Radar units now.  Maybe the Lab Radar units are consistent enough that if I got one it would be more consistent to pick out shifts in air density?

The data off a typical light based chrono is not that precise.  Shot angle across the sensors can have a pretty significant effect on output.

Cheers,
Toby

[Joe L]

The velocity measured close to the muzzle won't show any difference due to air density.  The velocity measured 100-200 yards down range will be different due to the difference in drag on the bullet due to the air density change.  The velocity will decrease slower at altitude.  You can try a ballistics calculator on your phone to see exactly what the difference is. 

From what I remember from my last trip to New Mexico, the effect was on the order of 4"-6" at 100 yards zero shift for my 9mm pistols using 135 PF JHP ammo.  Starting with a pistol zeroed at 800 ft ASL, then going to 6000 ft ASL, the group will be 4"-6" high at altitude, so I rezero to lower the impacts, then when I get home, I have to crank up the elevation again for the 800 ft range. 

Joe

[tdogg]

I'm actually not sure why LocknLoad posted his question but at USPSA competitions we aren't actually that concerned with ballistic performance as we are with muzzle velocity or power factor.  It could mean the difference between major and minor scoring (or shooting for no score).

It brings up a good point though that I hadn't though of with the Lab Radar equipment.  Where does it actually measure the bullet velocity and how does it account for the distance measured vs the muzzle velocity?  With a standard chrono, you know the distance to the muzzle and can estimate the bullet BC to get a muzzle velocity.  Do you do the same thing with the Lab Radar?  I know it spits out the muzzle velocity but what distance does it use to convert to MV?  It must all be internal and setup prior to using.

Cheers,
Toby

[Lock-n-load]

I'm actually not sure why LocknLoad posted his question but at USPSA competitions we aren't actually that concerned with ballistic performance as we are with muzzle velocity or power factor.  It could mean the difference between major and minor scoring (or shooting for no score).

It brings up a good point though that I hadn't though of with the Lab Radar equipment.  Where does it actually measure the bullet velocity and how does it account for the distance measured vs the muzzle velocity?  With a standard chrono, you know the distance to the muzzle and can estimate the bullet BC to get a muzzle velocity.  Do you do the same thing with the Lab Radar?  I know it spits out the muzzle velocity but what distance does it use to convert to MV?  It must all be internal and setup prior to using.

Toby I was just thinking one day , since I live in Wyoming and just spent 2 weeks in the Florida keys. So one day on Duvall St. (Margarita-ville ) I thought hmmmm ….. I wonder,..🤔🤔🤣🤣

[Lock-n-load]

Vacation time was great by the way, just beat the hurricane by a couple weeks!🌊🌊😄

[Togmaster]

The real question is what is the air speed velocity of an unladen swallow?

[M1A4ME]

I always thought that tale of the Air Force F86 pilot (McConnell) was a great story about altitude, thinner air and bullet trajectory.

He went to Korea and started shooting down N. Korean and Chinese piloted Migs.  He was wearing them out and running his numbers up.  NO one could figure out how he was shooting them down so quickly and so often.  He'd figured out that it wasn't WW2 and the jets where flying and fighting at much higher altitudes.  Even though the F86 Sabre jets used the same six Browning .50 caliber machine guns as most of the US WW2 fighters at the altitudes he was shooting down Migs the bullets had more range/speed and he didn't have to spend as much time getting close to them to get a good shot because he started shooting farther away.

Darn shame to survive Korea (he was also in WW2 but not as a pilot) and then get killed testing a new airplane after coming back to the US.  I know six .50's isn't a 9MM but the reasons are the same.  Thinner air.

Most of us never have to worry about shooting as far off as Joe does.  If we can shoot good enough groups to notice the drop we adjust by "Kentucky elevation/windage", not by making adjustments to the screws on the dot sight.  Good thing, too.  I can't remember my cellphone number, let alone how many clicks up/left, etc. to adjust for a change in altitude or distance to the target.

[Lock-n-load]

Ya that was more on the lines of what I was thinking. A large caliber like a 50 at long ranges.
Not much of an issue fo pistol competition, but if you were doing like a mile shot with a 50 cal sniper rifle it could make a difference. But I’m sure those guys have charts and graphs for that.. or for now days apps🤔