What load data are you using? I agree with the cautions to start below 90% of max and work up. Hodgdon lists 25.3gr as max for 55gr cup and core bullets. Are you trying to load these to 223 Remington max pressures or 5.56?
To me it seems very easy to start a 80%, which is 20.2 grains, and work up 5 rounds apiece in half grains until you get to your 24gr loading you have already loaded.
New gun, same ol' shot.