And maybe not even all that
This is much ado about nothing. The 5.56×45 is the NATO-ized version of the .223. The differences are in the chambers (see attached reamer spec comparison from AR15barrels.com). They are not, however, in the external brass dimensions. That is because the .223 was developed for the military. It was merely commercialized as .223 Remington, and is not a case of a commercial load being adapted to military use.
The military chambers are more generous to accommodate specialty ammo and ensure full-auto feeding. This means once-fired military brass is typically blown out larger than the exact same ammo would be if once-fired in a SAAMI spec chamber. Brass is springy and is not returned 100% to original size by a resizing die. By how much it fails to return to size depends what size was when it went into the die? Indeed, as Sierra's manual points out, some brass can be stretched so extremely that it simply cannot be made to fit a smaller chamber by using any standard sizing die; not even a small base die. We've had that situation that pop up in a few threads here.
5.56×45 and .223 drawings from QuickLOAD. I'll let you pick out which is which?
The above explains why .223 and 5.56×45 new cases both fit each others chambers. It explains why the same sizing dies work for both if the cases are fired for the first time in the same chamber. It explains why small base .223 dies usually cure problems with resizing once-fired brass that has been stretched excessively in a NATO spec chamber, not to mention stretched by extraction in a machine gun. Frankly, I don't know how a 5.56 die set would be any different from a .223 die set unless small base .223 sizing dies would be its default? Lee doesn't currently make a small base .223 sizing die, so maybe that's what they had in mind for a custom set?
A 5.56×45 chamber has a longer throat, but it's only 0.025" longer than the standard .223 chamber. I don't believe I've ever seen a .223 seater die that couldn't be adjusted for that 0.025" of extra length a NATO throat has, so that part of a 5.56 die set should be no different.
Then there is the pressure issue. I don't really believe it. Take a look at the top group of cartridges in the table below. These are cartridges that, like the .223, were originally specified by SAAMI at 52,000 CUP Maximum Average Pressure (MAP) for ten rounds of a test load fired in a test gun. Now look at the newer PSI MAP specs for them in the second column. The new specs for those cartridges average 62,000 PSI. Next look at the .223's new spec in PSI. See how much smaller the difference is than for the others? I don't know if that is the result of the anomalous behavior of the two measurement systems or if the pressure spec was reduced by SAAMI between the changes? I suspect the former, but have no solid way to tell without asking SAAMI? CUP by copper crusher and PSI by non-conformal piezo transducer have a pretty shaky relationship.
Now go one step further and look at the CIP specs for the two rounds at the bottom. Their conversion from old CUP standards to new PSI standards is much closer to what you'd expect to see based on the SAAMI average conversion for other 52,000 CUP cartridges. The CIP locates their pressure test ports differently, and perhaps that corrects the difference issue for their copper crusher and piezo transducer readings?
So, I believe this pressure issue is mostly created by the anomalous nature of the measuring systems. CIP pressure port location is known to read about 2,000 psi (not 20,000 psi, as some claim) higher than SAAMI port locations with the same transducer type. That is likely why the CIP crusher number is about 1,700 CUP higher than the SAAMI crusher number, though that would be an unusually tight tracking of different crusher numbers if it were. Certainly the European NATO countries are using the European piezo transducer arrangement to load NATO ammo. But since we developed the cartridge and provided them with the reference loads, I expect it is just how the same ammo reads in their gear. So it probably reads 52,000 CUP on SAAMI's gear where it reads 53,664 CUP on theirs.
So, we have very close agreement between SAAMI and CIP CUP standards. We have the fact we sent them the round to copy, not the other way around. We have the fact that the SAAMI PSI MAP is so much lower than those for other cartridges they have converted the standard for from the same CUP MAP numbers that it is likely one of the anomalous scatter points that appear using these measuring method. Taken together, it suggests the 5.56 NATO pressure difference is more instrumentation artifact than real.
The one remaining pressure difference that may be legitimately disguised by all those differing pressure readings seems to me to be that NATO pressure tests are run in a NATO chamber with its longer throat and bigger diameter at the base, while SAAMI's are done with the shorter throat. That will be an issue if you try to shoot NATO specialty ammo with an ogive far forward enough in the chamber to get close to or touch the SAAMI throat. But if you are talking about ball ammo, the difference is unlikely to be much. Certainly the .223 Match chamber put in my AR by Compass Lake has never had any issue with surplus ball ammo, which it digests just as well as anything else.
There has been enough surplus 5.56 sold and fired in SAAMI spec rifles that if gun damage were a normal outcome, we'd all know about it. Not to mention that rifles chambered for .308 and other 60KPSI+ American chamberings are also chambered in .223. The guns can clearly take the higher pressure. Only the brass has the potential to be a weak link.
Code:
SAAMI MAP specs for various cartridges:
52,000 CUP 60,000 PSI 243 win
52,000 CUP 60,000 PSI 308 win
52,000 CUP 61,000 PSI 7mm SE vH
52,000 CUP 61,000 PSI 7x50 R
52,000 CUP 65,000 PSI 270 win
52,000 CUP 65,000 PSI 6mm rem
52,000 CUP 62,000 PSI Average of all above
SAAMI MAP spec for .223:
52,000 CUP 55,000 PSI 223 rem
CIP MAP spec for .223:
3,700 bar(crusher) 4,300 bar (piezo) 223 rem
conversion:
53,664 CUP 62,336 PSI 223 rem
For the OP: Note the chamber differences do make brass once fired in NATO chambers sometimes hard to resize for use in SAAMI chambers. Buy a small base die and use it at least the first time you resize it. That fixes the problem most of the time, though not always. Get a case gauge and use that to sort out what the small base die has correct fixed and what it has not. Try re-resizing the oversize stuff in the small base dies, leaving it pressed into the die for a count of five. Do that twice. It often gets the last couple of thousands of interference out.
Once you have fired the brass in your gun's own chamber, it likely will resize fine with a standard die, but again, not always. If not, continue using the small base die if the brass seems to be OK otherwise. If it shows pressure signs, though, even with fairly moderate starting loads, it has likely been permanently damaged by over-stretching. We had a fellow recently whose once-fired .308 brass stuck in chambers even when loaded significantly below listed manual starting loads. At the same time, new Lake City or IMI brass gave him no problems with loads above the same manual starting load level.
