The most common refrain is that 5.56 is significantly higher pressure than .223. But that difference is almost entirely due to different testing methods.

The .223 round is civilian, so it's measured by SAAMI. They say they it is 55kpsi.

The 5.56 is NATO, so they measure the pressure. They have been using a testing methodology called EPVAT. That has the pressure sensor in a different place than the SAAMI test. NATO's EPVAT gives the pressure at 62kpsi.

NATO has also recently started using a different testing method called SCATP. The SCATP test is designed to check pressure in the same way that the SAAMI test does it. The pressure of the 5.56 round tested by the SCATP method is 55kpsi.

So the exact same 5.56 round measured by EPVAT is 62kpsi, and the exact same 5.56 round measured by SCATP is 55kpsi. The difference is the testing methodology and how the pressure is measured. In the NATO test that is designed to be the same as the SAAMI test, 5.56 tests as 55kpsi and the .223 tests as 55kpsi.

There are differences between the two rounds, but the pressure is a lot closer than what most people will tell you.

I’m at work but I can elaborate once I get home this evening