I don't know where you got your pressure information, or you rifle strength information, but they are both wrong.
The action strength and chamber wall thickness on a modern .223 chambered rifle are exactly the same as they are on the same rifle chambered in 5.56mm.
No difference atall.
No current commercial or military rifle cartridge is rated at 75,000 PSI, period.
The .223 Remington is rated by SAAMI at 55,000.
The M193 5.56 is rated by the military at 52,000 average, with a SD allowed of up to 58,000.
The M885 ball load is rated at 55,000 with a SD of up to 62,366.
But regardless of all that, SAAMI measures commercial load pressure differently then the military measures military ammo, so there is no exact way to compare the two.
The only 5.56 NATO load even close to 75,000 is the M197 Proof Load cartridge which is rated at 70,000 with a +/- allowance of 3,000.
.223 Remington Proof Loads would be about the same pressure.
According to the official NATO proofing guidelines the 5.56x45mm NATO case can handle up to 430 MPa (62,367 psi) piezo service pressure. In NATO regulated organizations every rifle cartridge combo has to be proofed at 125% of this maximum pressure to certify for service issue. This is equal to the C.I.P. maximum pressure guideline for the .223 Remington cartridge, that is the 5.56x45mm NATO parent cartridge.
I will concede that GI ammo is sometimes loaded a little hotter then some commercial. But sometimes, it isn't.
I will disagree that all 5.56 cases have less capacity then all commercial .223 cases.
I have a few thousand rounds of Remington commercial and LC GI brass in the basement that proves that not always true.
rc