Given various factors, chronoing yourself from your own gun is the best option. I have seems different rifles with the same length of barrel be over 100 fps different in velocity with the same ammo in the same shooting session.
As for Hornady claiming their Black is 2600+ fps, you must have a very special lot. They only make 6.5 Grendel Hornady Black ammo in 123 gr. and my boxes from multiple purchases all say 2580 fps at the muzzle. Yes, that will be from a 24" test barrel. You can subtract 20-30 fps per inch difference and get some idea of the velocity difference (depending in your barrel), but without actually chronoing, you won't know.