[Snakedriver]

Either will work. The average shooter will not be able to tell the difference between .223 Rem. and 5.56mm NATO rounds. The latter produces a little more pressure & velocity, but not enough to notice.

I don't use any steel cased ammo in my RRA AR rifle, but it's a personal choice. Your LAR-15 should be able to digest it fine. Cost wise, using the cheapest ammo for range fun makes sense, but I like to shoot the stuff I would use every day for defensive purposes.

Enjoy your RRA rifle, it's a good choice.