Q. Are the 5.56 mm NATO rounds (and M193 types) and the .223 Remington really the same round?

A. The cartridge cases themselves are substantially identical with the exception that military and/or NATO specification rounds (those marked with the NATO "cross in circle" mark) may have thicker cases and are thus able to withstand stretching better when fired in loose chambered military weapons (done for reliability purposes). Commercial SAAMI specification .223 chambers have a much shorter throat, a smaller diameter bullet seat, and less freebore than the military chamber. When military specification ammunition is fired in a SAAMI specification chamber chamber pressure can increase dramatically--sometimes up to 10,000 psi. While this should not cause any problems in modern well made rifles it should give one pause. Conversely, SAAMI specification commercial ammunition when fired in a military chamber gives significantly lower pressures and velocities. See the drawing below for the details.

It must also be noted that all 5.56 x 45 mm NATO ammunition (such as US M855) requires a 1:9" twist or faster rifling to stabilize the long 62 gr ball bullet and a 1:7" twist to stabilize the extremely long tracer variation. The standard twist used for .223 Remington commercial chamberings is 1:12" or 1:14" neither of which will stabilize the NATO type ammunition. The NATO ball ammunition is easily identified by its green bullet tip.