How effective is the 6.5 Grendel at long ranges? Let’s take a look at one of 
the top Grendel bullets, the 123 grain Lapua Scenar. The G1 BC is .527, and the G7 BC is .263. I used the online JBM Trajectory Calculator with default settings for temperature, air pressure, and humidity. The maximum effective range was taken to be the distance (in 50 yard increments) at which the bullet still has at least 1300 fps velocity and at least 500 ft lbs of energy. Elevation was calculated in inches with a 200 yard zero.
Velocities for different barrel lengths were taken from the AR15.com 6.5 Grendel FAQ page, rounded to the nearest 25 fps:
14.5″ barrel 2475 fps
16-in barrel 2500 fps
20-in barrel 2575 fps
24-in barrel 2650 fps
From my perspective, though, a 24-inch barrel is only practical for target shooting. A military sniper might use a 24-inch barrel, but not with a 123 grain bullet.
The maximum effective range, using the G7 BC (.263) provided on the Lapua website was as follows:
14.5-inch barrel at 2475 fps
750 yards at 1397.2 fps and 533.2 ft lbs (-185.1″)
16-inch barrel at 2500 fps
800 yards at 1354.3 fps and 500.9 ft lbs (-217.4″)
20-inch barrel at 2575 fps
850 yards at 1347.9 fps and 496.2 ft lbs (-241.5″)
24-inch barrel at 2650 fps
900 yards at 1340.7 fps and 490.9 ft lbs (-266.2″)
I lowered the threshold for terminal energy to at least 490 ft lbs, since the velocity was well above 1300 fps. But given that a 24-inch barrel is not very practical, the maximum effective range for the 6.5 Grendel with the 123 grain Scenar bullet is 750 to 850 yards.
I also did the same calculations using the G1 BC (.527), and the result was 50 to 100 yards greater range, supposedly. Since the G7 bullet form more closely approximates the Scenar bullet (and most modern boat-tail bullets), the G1 is overestimating the Grendel’s range. So when you see online discussions of the Grendel’s range, using a G1 BC, you should subtract at least 50 yards from that range.
Ballistics Charts:
6.5 ICS 