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.
I've talked to professional ballisticians, guys who use million-dollar labs to measure ammo for their ammo manufacturing bosses. They have reported some instances of 5.56 ammo demonstrating peak pressures at or above 75,000 psi in .223-chambered pressure barrels. That is the pressure of the proof load gun makers test each rifle with before it's shipped.
Almost all the "generic" ammo you shoot is not 5.56. Oh, it says ".223 Remington/5.56" on it, but it isn't really 5.56. The high-volume, low-cost ammunition that most of us use is not loaded right to the red line. I've chronographed enough of it to know that much of it falls 100 to 200 fps short of full-book 5.56 spec. That right there is enough to make it no big deal because the peak pressure of the .223 load is sufficiently less than that of a true 5.56 that the artificially induced spike still falls below the pressure ceiling.
However, you can have a serious problem if the variables stack up against you in a range session. Rifles get hot when you shoot them. They also get hot in the summer. So on a hot day you're shooting 5.56-spec ammo through your .223-chambered rifle. The summer sun beats down- note that black rifles left in the sun can easily reach 140 degrees even before they are fired- and pressures rise.
Let's make it worse: Your 5.56 ammo is at the top of the allowed pressure and at the bottom of the allowed brass hardness. Your rifle suddenly stops working. What happened? The heat increased the already maximum-made-excessive pressure, and on extracting a fired case, the pressure had expanded the case enough for a primer to fall out of the primer pocket and into your rifle.
How can you check for it and solve it? First, you can't depend on the markings on the barrel. Some makers can be counted on to actually make 5.56 chambers in their barrels. However, many rifle makers do not make their own barrels. They buy barrels, and the barrel makers have been known to ship barrels marked "5.56" that were actually given .223 chambers because .223 chambers tend to be more accurate, and everyone insists on m.o.a. AR-15s these days..
Are there makers you can count on, ones who will supply the proper chamber? Yes. Based on measurements I've done over the years, CMMG, Colt, Daniel Defense, LMT, Noveske and Stag deliver chambers that match what's stamped on the barrel.
Everyone else? Unless you know who makes their barrels, you have to check them, and for that you need a leade/throat gauge. I recommend the .223/5.56 Gage from Michiguns (m-guns.com). The gauge is simple. It is ground to just over the minimum specs of a 5.56 leade/throat. Drop it in, and if it drops free, you have a 5.56 leade. If it sticks (it is hardened steel, don't pound it in) you have a .223 leade.
Okay, so you have a .223 chamber, but you wanted a 5.56. If the rifle is still brand-new, you can send it back. However, the maker probably only has more barrels from the same maker, and you may not get a 5.56. So you need a specialized reamer. One that cuts the leade and the leade only becauseYou don't want your headspace changed.
The various reamer makers will be happy to supply you with a 5.56-spec finish reamer. You just have to be aware that a finish reamer will also ream the shoulder if you aren't careful. You may go in attempting to make a 5.56 throat and end up creating excessive headspace. Michigun makes a reamer that does not cut on the shoulder at all. When you feel it stop cutting, you are safely done. It also makes a leade longer even than that of 5.56, by a small margin.
What if you have a chromed barrel and don't want to cut the chrome? Stick with a chromed .223 chamber. But if you really want a 5.56 leade, yes, the reamer will remove chrome. However, the area being cut is the area where the chrome is blasted off first, so if you've put more than a few hundred rounds down your barrel, there is probably not much chrome left there, anyway.
Having a .223 chamber in your AR is a greater concern than just the social ostracism of having a rifle that is not mil-spec. However, it is something you can test and fix if needed. Me, I'm checking all of my rifles, and those that don't pass the test will get corrected.
What about you? Will you change if your current AR isn't what you wanted or expected?