Well... and that was what I said.... trigger which every time he attacks add the special effect (gunfrie, contruct what ever name is) to weapon then destroy effect after lets say 0.10 sec... you could also use a dummy unit in with a movement height that mach the gun height and locust ability wich is created everytime unit attacks at position of units faceing position (A trigger like that, dont have WEU open so cant check the correct trigger, but something similar to the above.
Did also said that if the gunfire what ever it is doesnt mach the other attached model (machine gun, he could also insted of a dummy unit, edit the gunfire in a model editor changing its place so it would fit the machine gun attachment even if attached to same attachment point...
Hope this makes it clear what I meant...