Easy enough answer: five rounds are easy to load and unload at the beginning and end of each watch. Five rounds are sufficient to stop an intruder or at least prevent anyone from taking your weapon from you. Guards aren’t supposed to engage in extended firefights, particularly since most of them are brand- new PFCs straight out of basic training.
Guards are supposed to be the tripwire who notifies the Corporal of the Guard that he has a problem and then the guard force turns out like gangbusters.
Back in the early days of my Marine Corps career, I was a guard for an ammunition depot in the hills of Camp Pendleton. At another post further in the ammo dump, a new lieutenant is the Officer of the Day and he decides that he’s going to sneak up on the guard at that post. The guard is surprised and opens fire on the lieutenant with his M-14. The lieutenant pulls his .45 and the fight is on. Luckily for all concerned, nobody was hit and they only had five rounds each, so the damage was limited.
Five rounds is just right, it turns out.
I was the company armorer for a Finance Company in Germany. Army regulations required that any remote pay activities have armed guards. However, USAEUR (IIRC) regs required that no loose rounds were allowed in arms rooms.
If it made sense, it wouldn’t be the Army.
Hm, I seem to remember something from the Army:
General Order #1Yeah, I know the Marines have different general orders — but I'd bet that [or functionally similar] is in them.
I will guard everything within the limits of my post and quit my post only when properly relieved.