You don’t have to play in the AR playground very long before you hear or ask the question; “Does .223 vs. 5.56 matter?” The argument can reach near-religious proportions, with some on both sides vehemently arguing their cases. The deal is this: They do differ, and, yes, it does matter.
The differences really come into play with the starting path of the bullet and the unrifled portion of the chamber ahead of the rifling, called the leade. A narrower (less diameter) leade keeps the bullet from tipping as it moves forward, which enhances accuracy. A larger leade allows for more buildup and gunk and thus greater reliability.
At the end of the leade is the rifling. To prevent bullet mangling, the rifling has an angle on its front face to allow the bullet to slide in and smoothly begin rotating. The steeper this angle (within reason) the more quickly the bullet is controlled by the rifling and thus potentially delivering greater accuracy. However, the steeper the angle, the more the bullet “stalls” on engaging the rifling and the greater the pressure spike.
A shallower angle on the rifling—in effect, a longer ramp—also creates a longer effective leade, as the ogive of the bullet has to travel a greater distance before it engages shallow-angle rifling than it would with steep-angle rifling.
When the .223 was invented, it was meant as a benchrest and varmint round. As such, accuracy was paramount. Velocity was a good thing, but not if it meant losing accuracy. So the .223 was designed with a short leade and steep rifling engagement, which is fine for shooting varmints or targets.
When the Army was forced to adopt the M16, however, it tried to avoid adoption by moving the goalposts, insisting on 500 yard penetration performance. To get that, the designers had to boost pressure and velocity. To control pressure (and also to get full utility out of tracer ammo, which uses bullets nearly twice as long as typical full metal jacket ammo), they modified the shape of the leade and rifling angle. And later, taking advantage of the longer leade and gentler angle, ammo makers tuned the 5.56 round to maximum performance using that extra margin.
Today, the difference can be marked. The leade on a proper 5.56 chamber is twice or more than that of a .223 chamber, and the onset angle of the 5.56 rifling creates a ramp with four times the distance. Firing .223 in a .223 chamber, or a 5.56 chamber, is not a problem. But firing real-deal 5.56 ammo in a .223 chamber can be a big problem.
The SAAMI-spec maximum average pressure for the .223, measured at the middle of the case, is 55,000 psi. The NATO spec for 5.56 is determined by SAAMI’s European counterpart, CIP. CIP measures at the case mouth and lists the 5.56 pressure spec of 62,000 psi. Measured at the case middle as SAAMI does, it shows 60,000 psi—so either way it’s higher than the .223.
But the problem isn’t just pressure. That CIP pressure of 62,000 psi? It is measured in a 5.56 chamber. If we take the same round, which shows 60,000 psi per SAAMI (which is already 5,000 psi over the .223 max) and put it into a .223 chamber, things get ugly. The pressure spike piles onto an already over-pressure round.