You're thinking of the M48A1 Spotter Tracer round, the 106mm recoiless had a semi automatic rifle attached to it. The ST's were used to range the target by tracing out and then when they hit would smoke. If the rounds were within killing range the gunner would fire the 106 mm round.
I believe the semi that was attached to the 106mm was the M8-C.
I see the world thru bloodshot eyes
Streets filled with blood from distant lies
The dogs of war never compromise,
No time for rearranging.