I would, but I only know the metric system myself :P. I'm fairly certain the M79's arming mechanism relies on rotations of the grenade per second, only becoming active after a certain speed of full cycles per second is reached and detonating when the rotational speed then falls below that amount, which makes it more or less impossible to fire fire at a range capable of damaging the user unless fired straight up into the air. Realistically I would suppose the minimum arming distance is around 180 Hammer units, but ours may be shorter or longer for balance purposes. However, I can assure you that the only duds lobbed from FAS's M79 are those which hit something before the shells arm, and otherwise they're fully liable to explode in an sizeable effulgence of fire and death the instant they hit, whether you're nailing some poor sap between the eyes from half a mile distant or unclogging your plumbing in a hurry.
"What wonderful things are these chemicals, spores, and strains! They give life, restore it, prolong it, torture it, and take it all in unison. They predate us and we are spawned of them, and yet such things can unmake us with curious ease. If such things are not god, then I have yet to know anything more deserving of the title!"
-Dr. Karov Icari, taken from Biopreparat development notes, 1993.