175 LRX trued BC?

[bbraden]

I went to the mountains this weekend and took my 300 Win Mag out to 500 yards. Altitude was 8,600'. Barnes publishes a .508 BC for the 175 LRX. I used my Labradar to take velocity readings at the muzzle and at 25 yard increments out to 100 yards. Muzzle velocity was 3000-3015 fps. After fiddling with the BC input on my Bullet Drop app, the BC that made the velocities correspond to the Labradar's reported velocities was .398! That's a huge difference!

I put the new BC in the app, held over the newly computed amount, and shot about a 5" group at 500 yards, pretty close to centered which tells me the .398 BC is about right.

Has anyone else trued BC on the 175 LRX and seen this sort of discrepancy?

 

[cohunt]

applied ballistics shows the G1 as .482 and the G7 as .247
but MV twist rate changes BC

 

[bbraden]

.482 would be substantially better. My Kestrel showed 45% humidity, trending downwards, 22in-HG barometric pressure and 65 degrees temperature, trending up. If those numbers were wrong, could it make the BC that different?