E.O.,
The velocity tables published are not usually high precision. The manufacturer typically just takes a G1 ballistic coefficient for his bullet and uses that to calculate the drops in velocity and vertical trajectory for his charts and tables. The tables don't contain measured values. So if you want to match what they got, just use the G1 function. Moreover, point blank range is normally short enough that your result won't change it more than a yard switching between drag functions appropriate for the shape of the bullet. It's at very long ranges that drop differences due to the differences in drag function become an issue. MPBR will be under 300 yards in most instances, even with a 6" circle defining a point blank hit zone. Too short for a practical difference to appear.
For example, I looked at Winchester's 150 grain Ballistic Silvertip ammunition. It claims a muzzle velocity of 2810 fps. The table says the velocity at 500 yards is 1856 fps. When I put those two velocities and that spacing into
this calculator and select the G1 drag function, I get a G1 BC of 0.442. If I then plot a trajectory table using that single G1 BC and 2810 fps for MV, the velocities I get for 100, 200, 300, and 400 yards all match Winchester's table perfectly, which is how I know that the table was calculated and not measured. Because the G1 BC decreases with velocity for a boattail spitzer nose bullet, a measured velocity would be a few fps lower at 500 yards.
If I then put that 0.442 BC and muzzle velocity and bullet diameter and 150 grain bullet weight into
this other calculator and ask it to convert to the G7 BC, knowing that will match the shape of the bullet better, I get a G7 BC of 0.221. Knowing the G7 is actually for a secant ogive BT, I also get a G5 BC of 0.274, which is for a shorter boattail tangent ogive shape. The actual bullet will likely fall between the G5 and G7 somewhere in its real behavior.
What's the difference? At 300 yards, not much. The G1 gives 2211 fps at 300 yards, the G7 gives 2208 fps, the G5 gives 2196 fps. The G1 says the transit time will be 0.3613 seconds, the G7 says 0.3615 seconds, and the G5 says 0.3621 seconds. That's equivalent to 1.4 fps difference in average velocity between the G1 and G7 drag function velocity predictions, and 4 fps difference between the G1 and G5. Both differences are smaller than actual shot-to-shot variation is likely to be.
The difference between the Winchester muzzle velocity and the actual muzzle velocity from your particular gun is likely to be even larger that those velocity prediction difference. So none of those predicted velocity differences are of any practical significance. When I run the ±3 inch MPBR calculator for those three drag functions for the Winchester round, I get 277 yards, 277 yards, and 276 yards for G1, G7, and G5 trajectories, respectively. Again, you'll get more error from shot-to-shot variation, and possibly much more from your particular rifle not achieving the same MV as the velocity test barrel did.
If you want more precision, and want to take the time to fuss with it:
Use both G1 and GL for a round nose bullet and figure real trajectory will be about half way between their results.
Use G7 if the bullets are secant ogive boattail bullets, such as VLD's and many of Hornady's current offerings.
Use both G5 and G7 trajectories for tangent ogive boattail bullets and pick results about half way between.
Use G6 for tangent ogive flat base bullets.
Use G8 for secant ogive flat base bullets.
