Tactical Operations: Advanced Rules pg 100 says for Rapid-Fire Machine Guns, "Each time the weapon is rapid-fired, roll 1D6 to determine the heat generated. This number also becomes the damage inflicted by the weapon if it hits (it is possible for a burst to inflict only 1 point of damage). Each rapid-fire burst uses a number of rounds equal to the damage it inflicted x3".
The Piranha carries 12 Machine Guns and 200 rounds of Ammo for those Machine Guns. If I Rapid-Fire the Machine Guns and somehow manage to roll a '6' for Damage/Heat for all 12 of them, that would 12x18 (216 total) Shots used.
In the unlikely event this actually happens, besides effectively turning the Piranha in to a Stone Burner from Dune Messiah, are those 16 extra Shots just ignored for Ammo expenditure, or would several of the Machine Guns have to halve their Damage reduced as a result or one simply not actually fire at all?