I should clarify this is only when they are on duty performing law enforcement or security duty. Except for a few exceptions, weapons and ammo are turned in to the armory at the end of shift.
My son (former 0311, Grunt, USMC) was hired as a supplementary gate guard at JEBR (Anchorage, Ak)
He carried an issue M9 and 3 spare mags of live ammo..
The pay was good, but the PTB decided the use of DoD Police was a better choice. Now he guards the local airport.
They only carried 5 rounds when I was in the AF. State-side, ‘81-85.