A .338 Whisper has only lost 120 FPS after 500 yards? I highly doubt it.
Punch in the numbers in a ballistic calculator if you don't believe me. Remember that wind resistance grows exponentially with speed. So slower bullets generally lose less velocity over a given range, compared to the same bullet at a faster speed.
For the example I posted I used a calculated G1 BC of .734 for a 300 grain .338
Berger Bullets 300 grain hunting bullet has a listed G1 BC of .818
I used a temp of 65 degrees at sea level. A subsonic muzzle velocity of 1050fps. Here's what I came up with, with the lower BC of .734
Range: 500 yards
With everything else constant, using Berger's BC of .818
Feel free to try yourself, if you don't believe the numbers I posted. I used Handloads.com Ballistic calculator FWIW.