Goose slayer,
Are you having your rifle built on an existing 223 action?
If so, and you want fast hand loads, ask your smith to cut your chamber to 223 Ackley and use light for calibre bullets.
If not choose a different cartridge or calibre, remember depending on boltface diameter, you can have anything you want when building up a rifle.
Think of this though, speed is not everything, the faster you launch a bullet the more it is effected by drag and the quicker it slows down, especially light bullets but hey, if thats what you want go for it!
Some more info on your build and use of your rifle like rost495 suggests would be great.
Ian.