I prefer a 40mm objective. 50s seem a little bulky for what I do, and it should do a good job for ya.
Their biggest enemy is heavy recoil. On a .223, it should do well.
18X sounds great, but I've found the cross hairs 'bounce around' quite a bit from my heart beat.
When that happens, I find myself chasing the bullseye. If it happens with you, I'd dial it back to about 8 or 9X.