This may answer some of your questions.
http://www.ak-47.net/ammo/ss109.txtParamilitary 5.56mm ammunition should not be fired in commercial
rifles with chambers reamed to SAAMI specifications.
The SAAMI chamber used in sporting firearms has a smaller diameter bullet seat, a shorter throat and less free bore than military chambers. Chamber pressures can rise dramatically when military ammunition is fired in them.
SAAMI specifications for commercial .223 ammunition specify an average
chamber pressure of 52,000 CUP. When military ammunition is fired in the
SAAMI chamber, pressures may rise to 55,000 or 60,000 CUP!
U.S. manufactured paramilitary ammunition intended for civilian
consumption is purposely loaded down to SAAMI pressure levels and bullets
are often seated deeper to accommodate the SAAMI chamber. Ammunition
certified to meet Mil-C-9963F (the military specification for M193 Ball) is
only available to U.S. government agencies and private security firms under
contract to provide security for U.S. government installations.