When you fire a 5.56 M855 cartridge in a military chamber with its longer throat the chamber pressure is 55,000 psi.
When you fire a civilian .223 cartridge in a civilian chamber with a short throat the pressure is still 55,000 psi.
That sounds reasonable, within the limits of measurement precision.
But you are confounding two variables, throat length and cartridge load. That needlessly confuses the issue.
A simpler way of stating it is, for any given chamber, 5.56 loads will produce higher pressure and velocity than 223 loads.
Throat length is a separate variable. Shorter throats produce more pressure and longer ones produce less, all other factors equal.
If the Lucky Gunner author had been just a trifle more clever he could have done a balanced experimental design and cleanly separated those two variables.
And I do think the issue is a bit overblown. When I was working on an M855 problem at Lake City, we ran thousands of rounds heated to 150-160F, with the rifles at the same temperature. We weren't testing MV or pressure, but those temperatures had to be producing some serious out of limit conditions. We had no problems. I suspect that the SAAMI spec may be pretty conservative compared with other cartridges.
