I dont have the numbers in front of me but I do have Military rounds and 223.
The Military has changed the type and weight of bullet it has used over the years but they all must fit in the magazine with some room to spare. The COAL of 57mm. If you take a 62 grn SS109 or M193 for example and the velocity it was made to be 3250 fps the pressure 52000psi. The COAL is 57mm.
According to NATO the proof pressure was 62366 psi *125% =77958 psi
If the lead or Free bore was longer it would mean a very long jump to the lands.
I can load 73 grn at least in a Howa 1500. It is easy to fire a heavier longer bullet faster in a 223. The accepted standard pressure is 60000psi proof 60000*125% = 75000psi
The Military is known to save every dime they can and cut corners at every turn. Private enterprise covers its arse and over engineers things to protect themselves against idiots.
I have not seen a difference in either weight or water capacity in 5.56 Vs 223 cases that is large enough to support any of the claims that abound on the internet. There are differences between brands Cheap VS expensive. Even in high end expensive cases there are tolerances.
This is an interesting video
https://www.youtube.com/watch?v=BTgGhIcR_Mk Custom target 223' are now shooting 80 and 90 grn bullets granted no one gives a toss about SAAMI as that is just irrelevant. There are also 90 grn Hunting Rounds that are made for 223.