[4mer Liberal]

It’s essentially the same caliber as a 22 LR. Different bullet mass and powder charge is all.

[Lurkinanloomin]

Yep, a very fast .22 is all it is.

The battle rifles of WW II used .30-06, 7.62x54r, 7.7x58mm and 7.92x57mm.

All of those are far more powerful than 5.56mm.

[IndispensableDestiny]

It’s essentially the same caliber as a 22 LR. Different bullet mass and powder charge is all.

Not quite. A .22LR uses a lead or copper plated heeled bullet whose diameter outside the shell is .223 inches. The bullet is usually round nosed, hollow point, or nearly flat (match). "Heeled" means the exposed part of the bullet is the same diameter as the case. It's narrower at the bottom to fit in the case.

The .223 Remington and 5.56x45mm NATO use a .224 diameter bullet which is copper jacketed. The bullet may be a soft point, fully jacketed, match, hollow point, or plastic tipped.

FWIW, the .22WMR also uses a .224 bullet.