Thanks mando, you got me a lot closer to the solution. However, I still only had about 50% success of the collision taking place between the two objects. I then changed it to create the bomb at position returned by modelToWorld, and it seems to work every time. The idea here is that the bomb is created at the center of the collision object, rather than its position (which is probably just below the actual object). Here's the modified code:
_b2 = "computer" createVehicle _pos;
_b2 setPos _pos;
_b1 = "M_Stinger_AA" createVehicle (_b2 modelToWorld [0,0,0]);
~.2
deleteVehicle _b2;
deleteVehicle _b1;