One thing to be aware of is that powders can vary in bulk density enough that one lot might be slightly compressed while the next might not. Western lists bulk densities for the Accurate and Ramshot lines and some of the Accurate powder bulk densities are ±5.6%, so you could have an 11.2% difference in fill with the same weight of two lots at the extreme ends of that range.
If you settle on a charge that seems pretty loose in the case, you can try magnum primers to better pressurize the extra empty space. Magnum or standard, whichever produces the lower velocity SD is generally the better choice, though you do need to back your charge down and work back up if you change the primer. It's a good idea to do with any component change, though brass brand doesn't usually make a difference in .223 Rem (it does in 300 Win Mag, for example, where different headstamps have significantly different internal volumes).
You can use your 2.400" for single-loaded rounds, but you'd have to check to see if it fit the magazine. Figure pressure to go up about 20% if the bullet is in full contact with the throat. Backing the charge down 10% will compensate for that with typical rifle powders. you can then experiment with different COL's to see if one has an accuracy sweet spot.
