Government Boy may drop in and tell us the current pressure ratings for military ammunition. Since day one, all the 5.56 had was velocity. If you search the early literature, the round was supposed to be 50,000 cup, then it got bumped up to 52,000 cup, to get the velocity up, and the Army has continued to bump the velocity, and pressure of the round, particularly since the introduction of the M4, which has a shorter barrel than an M16. Government Boy may be able to tell us the current pressure of the latest round, but we can't find that out on our own, as the Army has taken the latest ammunition specs behind their super secret firewalls. Rumor is, its hot!, hot!, hot!
The military makes their ammunition to shoot in their rifles. They have modified their rifles so the things will function with super hot military rounds. If you stick a military round in your favorite boom stick and it goes Kaboom, it is not their fault. It is yours. And since you are stuck using civilian rifles, I would be very cautious about using military ammunition. Especially if it is old.