Need help with "true velocity" for a drop chart
I'm trying to figure out a drop chart and need to know what the true velocity of my load is. I am trying to do this without a chronograph ( I have a Oehler 35 but I do not trust its numbers).
I'm shooting a 7mm RM. loaded with 180 grain Bergers (stated BC is .672), zeroed at 200 yards. Went shooting this morning to recheck my numbers as the ones I got last week gave me two vastly different calculated true velocities and I figured I must have done something wrong. I ended up with very similar results today. Conditions Wind none, temp 65, pressure 25.20, elevation 4950.
465 yards 5.1" high (5 shot group) using the G7 program the velocity is calculated at 2915. I then move to 753 yards and shoot a group 2.8" low the calculated velocity is 2686. Nearly 250 fps different. Same load, same gun, same shooter. I checked the distances with two different range finders multiple times. These data are very close to last weeks numbers so they are reproducible but they close are far numbers do not correlate. Any thoughts?