G7 Ballistic Calculator Load Validation 300wm Hornady Superformance SST 180gr

[stevotary]

Went to range to get my data down to get a custom turret. I zero'd in at 200 yards. I chronographed my 300wm 180gr Hornady Superformance SST at an average 3055 fps(the ammo advertises 3130fps with a BC of .48).

I shot a mid range group at 500 yards that took 6MOA from Zero'd and a long range group that took 12MOA from zero'd.

When I went to validate actual field MOA adjustments in the G7 ballistic calculator, it generated a muzzle velocity of 3224fps,

I seem to be dead on at any range with in 700 yards with the new ballistic data plugged into G7 ballistic calculator, but when I attempted to shoot at 850 yards I was way way over target with the recommended 52 clicks.

What could be the issue here? Could the velocities really be 3224 fps even through my chronograph gave me a 15 shot average of 3055 fps and the ammo advertised 3130 fps? Any ideas of what I am missing here. I have all my conditions in good.

 

[gtita]

Here is a reply almost a year and half later!

1) Your chronograph readings may be correct. I experienced similar discrepancies between manufacturers' announced muzzle velocities (MV) and my own field measurements. Idem for the real ballistic coefficient (BC) that I always found to be lower than the manufacturer's. There are marketing reasons behind this.

2) You should also adjust your velocity readings to the real MV. That is, if you placed your chronograph at e.g. 3 yds from the muzzle, then you should correct the readings accordingly. You may be missing some 4 to 7 ft/s.

3) BC varies with bullet velocity as an effect of the drag coefficient, which increases with decreasing velocity until the latter becomes subsonic, then the drag coefficient suddenly decreases. For marketing reasons, manufacturers' announced BCs generally refer to the MV, and not to an "average" BC of a bullet trajectory, which would anyway be quite complex to calculate far every velocity achieved along it. As a result, any trajectory predicted on the basis of the manufacturer announced MV and BC does not reflect the observed bullet drop. The difference between the predicted and the real bullet drop increases with increasing target distances.

4) Atmospheric conditions should also be accounted for (humidity, temperature, pressure and altitude) and entered into the ballistic calculator.

5) Be sure that you convert the G1 into the appropriate G7 value, and vice versa.

Considering the previous points, here is what I would do:

(i) Adjust your bullet velocity to MV;
(ii) Tune the BC until you obtain the bullet drop at 200 yds in your ballistic calculator taking into account the atmospheric conditions.

OR

(i) Adjust your bullet velocity to MV;
(ii) Measure bullet velocity and drop at 100 yds (or more);
(iii) Tune the BC until you obtain the 100-yds velocity and bullet drop in your ballistic calculator taking into account the atmospheric conditions.

This procedure may help in readjusting your ballistic calculator predictions and bring them very much closer to your field observations. However, it is possible that more adjustments would be needed starting from target distances when velocity decreases below 1500 ft/s and even more below 1200 ft/s (subsonic). Of course, be sure to use the correct drag function for your bullet (e.g. G7).

Good luck!