Granted, they don't compare to a 7.62x54R (I use a 100 year old Moisin Nagant 7.62 x 54R with all matching serial numbers and high bluing!!! HEH HEH), but they do kill. M16's use .223's. The small bullet size is made up for the by the high velocity.
I think people here think a .223 is merely a slightly larger bullet than a .22 . It is a slightly larger bullet PLUS a big cartridge (compared to a .22) loaded with higer velocity powder.
A .223 is not a "manly" bullet, but it is very deadly.