Well that is essentially the pray and spray effect coming to the fore I guess. You are just trying to lay down as much fire as possible withou really caring over hitting anything. In the case of selective fire, you are attempting to hit something so there is an increased level of skill being exhibited, as you need to smoohly control the whole shot process as the weapon cycles underneath you - dealing with vibration, climb etc...
However I can see a number of minor rules tweaks which may help in this case.
Option #1 Single roll to hit but fired at a penalty. Not my preferred option really, as given the OQ penalty structure you be firing at a -25% penalty to hit.
Option #2 Single roll to hit but you then make a roll on the appropriate die number to see how many hit. e.g: 3 shot - d3 etc...
Option #3 Single roll to hit and they all hit.
Hope the above helps.
Rik-KM