The OP I think has gone with a .223. Good choice and plenty of cheap brass, great bullet choice and availability. It's not the one trick pony that some small calibre CF's are and as others have mentioned, its versatility and availability make it an obvious choice for many. I also like the mini-mauser actions of the CZ527 which is still amongst my favourite little 223 rifles to shoot.
The next consideration now that the variation in is barrel twist and barrel length. There's a few ways to approach this. If you need 400 yds for crow/rabbit and sensibly want to limit wind drift then there's two ways around this. Use of higher BC bullets or driving lighter and lower BC bullets at very high speeds. Sensibly, and as far as accuracy goes I reckon that 400 yds is about the limit for most lighter 223 bullets shot from a 1/12 barrel of average 22 to 24 inch length. Longer barrels here offer a distinct advantage in MV.
I started out with a slower twist 22 inch barrel and have changed that now for a 26 inch 1/8 twist which has opened up far more possibilities. That twist will shoot from 50grn to 80 grn bullets (with a few exceptions) very well indeed and given the choice of bullets, for crow I tend to use the higher BC ballistic tipped bullets which give significant advantages to wind drift. The 69gr tmk is devastating and is a great choice for 400yd shots where you can drive at close to 3000fps MV and they then retain more velocity at 400 yds than some lower BC 40gr bullets driven much harder. They're great on fox too. For rabbit, I tend to stick with soft points and to 300 yds found none better than the SGK 55gr spbt #1365. It provides almost target bullet accuracy and has accounted for deer (to 200 yds), and smaller game to ranges over 300 yds. I have had limited success with the flat base 60gr V-max and just can't get the things to shoot that well from my rifle which is a shame as they're cheap! Sub moa yes (I average about 0.75moa at 100 yds with them) but for 400 yds on small quarry I aim for 1/3moa or better, obviously prone using a bipod.