Alright for those of us that are new, and to some of those ol heads out there here is some enlightening information about that age old dispute between the .223 and 5.56mm NATO. There are some that say there is a difference but cant really put their finger on it, and the others that say there is no difference and they are the same round. Well take a peak at what Im about to throw down straight out of the Guns&Ammo: Book of the AR-15.
.223 Vs 5.56
I cant discuss ammunition without discussing barrels and chambers. Thats where the cartridge fits and fires, and thats where the bullet takes life. The first thing to know is that the 5.56x45mm NATO and .223 Remington are not the same. They look the same , but they can be loaded to very different standards. The short course is this: If your barrel is stamped .223 Remington, do not fire NATO ammunition; if its stamped 5.56 or NATO (among other designations which overly complicate this), then mil-spec NATO is fine to use, as is .223 Remington.
Manufacturers cant seem to agree on how to designate a rifle with a NATO-spec chamber, and some dont bother stamping them at all. Thats a problem, and it can have serious consequences. NATO-spec ammo fires much hotter than .223 Remington. As long as you know the difference, Ill refer to it all as .223 from here on.
There are essentially two natures of factory-loaded .223 ammo: mil-type and specialty. Not all the first is the same, to be sure, but its the second group that shows the widest variety. Those variations revolve around the bullet.
This article proceeds to go more into detail about types of box ammo and what you pay for is what you get (essentially). Pretty much white box ammo is garbage. Use good judgement on what you are going out to accomplish. 55 grain ammunitions are not going to be a big deal on paper targets within mid-range, grouping may not be that tight but it wil get the job done. The higher the grain, the better the velocity, the better the grouping at longer ranges.
Bottom line, know what your barrel can handle prior to firing it (dont take the dealers or private parties word for it, look at the barrel itself).