The 40mm M79 Grenade Launcher used by the US famously in Vietnam, and in conflicts after that, had a radar system made by the Radio Corporation of America that would give an audio que to denote target range and speed. Almost like a mix of a policeman’s speedometer and a range finder, on a grenade launcher. It apparently worked up to 500 meters.
My understanding is that it would be the former. So you can ensure your first round hits.
Today we’d use a laser range finder. But back then lasers were something scientists had in their laboratories not something soldiers took to the battlefield.