Ok, this was most likely been covered by hundreds of posts, but why does S&W say in their manual to never cross over 5.56 ammo in a barrel marked 223 rem but doesn't say a thing about 223 rem being able to be used safely in a barrei marked 5.56 nato? Think maybe it has something to do with morons with lawyers?

__________________
15T,15-22,5946 & some not S&W

5.56nato chambers have a longer throat than 223rem chambers.
5.56nato in 223rem chamber can lead to bullet hitting rifling when chambered before firing and cause high pressure potentially catastrophic.

same thing with 7.62nato/7.62x51 and 308win.

some differences in cartridge specs and pressure.

Also 5.56 ammo is loaded hotter and has a bit thicker brass case to accomidate use in various NATO weapons, from rifles to SAWs.
Besides the leade (longer chamber as MrApathy said) there is also a very slight difference in shoulder angle.
Ad it all up and it causes a possible over pressure when 5.56 is used in a .223 chambered firearm.
A notable exception are rifles chambered in .223 Wylde which was designed to accomidate both cartriges safely.
And yes, it's fine to shoot .223 out of a 5.56 chambered firearm, just not the other way around.

__________________
Hold my beer and watch this!

Thanks for the info on the .308 and the .223 Wylde, I was refering to the lack of information in the S&W manual about being able to use .223 in a 5.56 barrel

__________________
15T,15-22,5946 & some not S&W