This reminds me of a story my brother told me. He was drafted in,I think 1968 for Nam. He told me that during basic training the DI would put the M16 between his (the DI)legs on his groin and fire it calling the M16 a toy.
To show the low recoil, that, or a different version was a common demonstration back then.
What branch of service?...............
Sounds more like the urban legend that went around in Marine Corps boot camp at that time. Supposedly, a boot wrote his mother that the DI had taken an errant recruit, put him in a steel dumpster, and shot a flamethrower at him.