As far as I can see the only difference seems to be that the 5.56 develops more pressure than the .223 and the .223 is aboat .04mm larger at the neck than the 5.56. The 5.56 seems to be primarily the military round? It seems more sensible to me to go with the .223 and forget the 5.56. #FoGetAboutIt. Opinions?

Get an AR-15 (if that's really the question) that is chambered in.556 that way, you can shoot either. However, you can't do that the other way around. You can't shoot .556 out of a .223 rifle.

But the .223 is a tadbit less accurate than the 5.56 when shot through a 5.56? I'm learning.

Here you go. This explains it better than I can.
https://www.thetruthaboutguns.com/5-56-vs-223/

If a .223 is 5.66 mm it seems a bad idea to shoot it in a 5.56. What am I missing?

223 wylde. supposed to be accurate with both.

A distinction without a difference. If reloading manuals don't list them separately, why would I?

The difference at the neck is because of the mil spec brass. Dimensionally they are identical. The chambers however are not. The 5.56x45 chamber has a longer throat than the 223R and the Wylde is between the two. The 5.56x45 as a mil spec round is loaded a bit hotter than the 223R and can produce pressure spikes in the shorter throated chamber of the 223. Thats about it in a nutshell. I use a wylde chambered rifle and run pressures in the upper 5.56 range.

sounds like the talk in the 70s I heard about the .243 and 6mm. I saw a box of federal actually labeled for both.

330, you mean the 244 Remington and the 6 mm Remington, the 243 is a completely different cartridge and they are not interchangeable. The 244 and 6 mm are

.243. One can but shouldn't be fired out of the other

Gotcha. I was thinking the bullet was different sizes. Casing makes sense. I'm a slow learner, but I do learn slowly.

I always thought they were the same, military just likes to use metric and the civilian world "caliber" measures of a hundredths of an inch. Reminds me of when someone I knew was in the Army and "brought home" some MG ball 7.62. We used to hunt jackrabbit with our .308s--I know that's crazy but this is South Dakota-- and we used a 110 gr hollow point. Redailed a bit to shoot this MG ball stuff. Man, those loads were a lot hotter than our handloads pushing that 110 hp!!

The bullet is .224 in diameter for both. The case of the military stuff is just thicker walled. On a side note, this also makes for slightly less internal volume of the case. That also leads to higher pressures if loaded with the same amount of powder , same OAL and same bullet.
Thats why one should never mix case makes for accurate loads as there will be inconsistencies in pressures due to the different volume and neck tension. I sort my brass by head stamp.

Military ball ammo and milspec chambers are designed to work in all conceivable conditions on the battlefield. The ammo is usually full power with thicker cases. Chambers and throats a little looser.
Civilian ammo is designed for civilized conditions and the chambers are tighter for better accuracy.
The differences are minute, measured in hundredths on and inch, but when your holding 50-60 thousands of pressure in a chamber it easy to push the envelope.
Will a 5.56 m855 blow up a civilian M700 .223 chamber? Probably not but the increased wear can't be good.

JMO, but the end use of the rifle should determine your choice of ammo and chamber.

Varmint gun, .223
SHTF gun, 5.56

It is. I built a 223 wylde and it is awesome tack driver

You will have issues with a 223 round in full autos though I doubt that will be a concern to you. Found that out the hard way and can cause more than feed issues.

the external case dimensions are identical internal may be smaller in 5.56 as the case may be thicker

5.56 specifies a higher pressure , longer throat and often harder primers as most 5.56 guns have a floating firing pin

223rem will fire safely in 5.56x45 chambers how ever light for caliber bullets more common in 223 may not perform as well.
223 rem may also be loaded with softer primers
no gun user should chamber a round in anything but a safe direction to fire but in particular with the floating firing pin , the pin will strike and leave a mark on the primer should the firing pin be caught on debris in the firing pin channel it could cause a round to fire.
or you could have a bad trigger in a Rem and it could fire , just never chamber a round in an unsafe direction.

some reloading manuals like Hornady 10th edition do distinguish between 223rem and 5.56x45nato

223 wylde is an in-between chamber throat rated for the higher pressure of 5.56x45nato