Rifle bullets are usually smaller diameter but the cartridge is a lot longer therefore a lot more powder can fit in them. Of course this makes the high velocities that Robo was talking about. Just check out any of the ammo makers web sites and you'll see what I mean.
Shooting pistol cartridges through rifles goes way back to the old west when it was good to have a side arm and rifle using the same ammo. It's a lot of fun shooting these rifles as the recoil is usually mild and they are accurate.
I've never shot a CX4 but have the AR. The AR uses a .223 bullet but again it's a true rifle cartridge and so packs more powder. It's more affective at long range and there's quite a debate because of it's small diameter but it's a pretty good killer. Not nearly as good as the .308 was but that's another debate for another thread!