The ones at Walmart said they were .223. I know that 5.56 should feed but I've heard that 5.56 has more powder and one should use caution because the higher pressures might not sit well with a barrel rated for .223.
Is there any merit to that debate?
Here is how it actually works.
The .223 Remington Cartridge has
its unique specifications.
The .556 NATO specifications are different,
but not by a large margin.
The main difference is in the "length" of
the chamber. The NATO spec for the .556 is
actually .0034" longer than the .223 Rem. spec.
Since the allowable chamber length in the NATO
spec is about a .005" window, the upper limit of the
.223 Rem. spec overlaps the lower limit of the
allowable NATO chamber length spec.
All .223 Rem ammo made to spec will
fire in a NATO chambered rifle.
All ammo made to the upper allowable NATO spec may
not work in the .223 Rem. chambers that
are at the lower allowable limit of the .223 Rem spec.,
They will be .001, to .002" too long to chamber.
NATO increased the length so that ammo made from
all over the world would have a better chance of functioning
in the NATO rifles. If a round is slightly short, it still goes
bang and runs the gun, if it is too long, it won't chamber and
the guns jams and or stops running.
The allowable pressure spec is slightly higher for NATO
than SAMMI allows for the .223 Rem. ammo, particularly
the ammo made for arctic warfare conditions.