I own 2 AR-15s, one of which I built myself, but I still do not understand the difference between .223 and 5.56 NATO. I thought they were, for our purposes, exactly the same.
( ignore 'N' - I'm pretty sure they're the same )
what ive been told anyways...
See here: http://www.thegunzone.com/556v223.html
Short version: If your upper is stamped “5.56,” you can shoot both. If it’s stamped “.223,” you risk catastrophic failure due to higher chamber pressure if you shoot 5.56 with it, esp. if it’s an older rifle.
Went to bed last night so maybe this got answered already- 5.56 Nato has a different “lede” or I would say freebore beyond the tip of the chambered round; a longer one which allows barrel pressures around 70,000 copper units. .223 Remington is smaller, rated for around 45-50 cup, don’t recall exactly. You can overpressure a .223 chamber with a NATO round, but not vice versa. You give up a slight amount of accuracy shooting .223 thru a 5.56 barrel. External cartridge dimensions are the same- I think NATO is thicker in the base internally.
Most commercial semi-autos are chambered 5.56 ( Mini-14, etc.) Some bolt actions are not so you have to be careful what you put thru them, especially overall cartridge length if reloading.
Because there are alot of AR shooters after super-accuracy (varminters, especially)they offer the .223 chambering.