@Danny - the whole "if you can hear them" thing confuses me, so I wasn't talking about that. I was just saying that if you can hear them, and you are far enough away from the weapon, then yeah your going to see the rounds impact before you hear them. Same thing applies to thunder and lightning.

@Farhad - The ammo switching mechanism adds hardly any weight, just a couple pounds maybe. The weapon system I am referring to uses 2 smaller ammo bins and two feed chutes rather than one large bin and one feed chute... so really the only thing your adding is the aluminum feed chute which weight practically nothing. 2 round selection, same weapon, so the same fail safes. Room is really the only issue you would have, as that aircraft is more than powerful enough to handle a few pounds extra weight. Being able to choose between HE and DU actually adds a significant amount of functionality in the benefit's of what the rounds were designed for. HE is great against troops and wheeled vehicles, while DU is great against armored vehicles. HE is practically worthless against an armored target, while DU is very inefficient against troops and light vehicles.

On the ground there is no need to consider the weight added to munition switching mechanization.

However for fixed wing aircraft there is considerable weight, spacing and redundancy fail safe considerations, it would add little functionality while adding more weight to a very heavy aircraft already.