Well Ken, even though the FM doesn't provide a visual reference, I was imagining that the grenadier, with prpoer hold-offs marked on his sling, could loosen it to X length, take a knee and with the weak leg's foot, step on the tape mark, elevate the barrel until the sling was 90 deg to the ground, mounted the buttstock to his shoulder, and fired the round.

I can see how that would best work for night firing only. Or am I getting it all clearly wrong with that imagery?