Re: stupid question....
The "for sure" fact is that a SAAMI .223 chamber will have too short of throat/leade to safely shoot military 5.56mm M193 ball ammo*. Another example of an ammo maker rushing the "military equivalent" to market before Uncle Sugar's engineers and beancounters are done tweaking the system.
Bullet jams in the bore before firing, and without the jump that round was designed for, pressures spike a bit past proof levels, IIRC the pressure detail.
I've asked several places on-line and still have not heard anything beyond rumor about whether the M855 ammo has a similar length to ogive. However, since M193 in a A2 barrel doesn't spike pressure, I'm reasonably confident to my own satisfaction that M855 would probably have the same problem in a SAAMI chamber...never mind that the typical 1:12 twist starts keyholing the bullets somewhere beyond 50 yards.
There is a bit of cross-talk on the ammo chamber pressure issue which leaves me a bit uncertain. The modern convention is to list PSI as the number derived from piezoelectric transducer readings (favored because they give pressure over time, not just peak), and CUP (copper units of pressure) for numbers obtained from the older copper crusher method. Problem is, before PZ came in, all CUPs were listed as PSI, and people reading the older (1960s-70s) military sources tend to interpret the pressure PSI figures as PZ instead of CUP.
PZ tends to give higher number values than CUP, for the very same ammo/barrel combinations.
So, some people say SAAMI ammo with PZ pressure numbers runs higher pressure than military ammo with CUP numbers. Others say military runs hotter, but I suspect most of them are incompletely regurgitating the M193 in SAAMI throat problem. I just gave up and load my ammo and back off if velocities are much above what they are supposed to be, regardless of any lack of "pressure signs".
My sources have been SAAMI publications and military manuals, cross-referenced with having read every issue of The American Rifleman going back to a couple years before I was born. Dad is a Life Member and saved 'em all.
*Though perhaps it would work after the throat is burned out...
