I took the 123's in my 65SS out to 600 yesterday. With a density altitude of 7750 feet, the Labradar showed 3335 over 8 shots with SD of 7 and ES of 20. Kestrel vertical with .229 BC was off the target low. 2.5 mils was slightly higher than aim point and 2.4 mils was lower. G7 BC to match drop was .192. Obviously, I need to shoot it farther to verify but I won't be able to get to longer range before my elk hunt.
According to the Kestrel, at 500 yards the remaining energy is 1521. Velocity drops below 1800 after 825 (energy is 906 at that point). 500 is about as far as I want to shoot at an elk anyway so I'm good as long as the bullet works.
For comparison, the 124 HH at 3322 required a G7 BC of .245 at 600 to match actual drops.