"...on the assumption I use it..." Even if you're not shooting a lot of .223, you'll have to work up a load anyway. Same as you did for your .40 S&W. Buying ammo in bulk when you don't know how well it shoots out of your rifle isn't a good idea or investment. Buy one box, if you can, and try it, before you buy in bulk.
Your current press matters too though. If you have a Dillon or other progressive for your .40 S&W, you'll want to consider how much it'll cost to change calibres. A Dillon RL 550B Caliber Conversion Kit for .223 runs $40.95 plus dies. Regular dies will do. Dillon carbide rifle dies are mucho expensive. $133.95 for .223 carbide dies.
Mind you, I suspect that, depending on what rifle you have, 3 gun shoots might catch you're eye. One rifle, one shotgun, one pistol. You can't have too much fun shooting.