Not as to “members of the Army.”
Certainly, as to Dick Cheney and Charlie McCarthy.
Nope, not remotely true. M4’s are very shortened versions of the older M16. We didn’t use M4’s when I was in the Army Infantry back in the early '90s, at least not widely (we generally used the M16A2), but you could shoot yourself in the head even with an M16A2. An M4 would make it downright easy.
The weapon in FMJ was neither an M4 nor an M16; it was, IIRC, an M1 Garand (which would indeed be difficult to shoot oneself in the head with, as it’s a Big Stinkin’ Gun). At the time FMJ is set (mid 1960’s), the Marines were using M14’s in combat in Vietnam (the M16 was introduced in 1967, IIRC). But in boot camp, they sometimes still used WWII- or Korean War-era M1 Garands.
