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I was reading an article stating that SAAMI standardized 223rem with a chamber pressure (nato epvat measure) of 52,000psi but that standardized m193, still epvat-measured, is a 55, 000psi round.
Does anyone know when that change occurred? Does it come from the various measurements methods used by SAAMI and the Army?
M193 5.56 pressure is driven by a need to meet a velocity spec, 3,200 FPS. You do realize that the AR15 round came first, Eugene Stoner developed it from the 222 Remington and called it the 222 Special then Remington released it as the .223. So as far as when around late 50's early 60's.
From my understandings, when the Army switched from IMR to ball powder, they increased the charge to keep the speed at 3,250ft/s, resulting in a higher chamber pressure.
But I wonder how was the pressure in the first army tdp.
M193 also needs a longer freebore than .223 does. Firing 5.56 ammo in .223 chamber can raise pressure.