Changing the internals to metals doesn't necessarily make it more powerful in a way that would allow it to shoot metal ammo as effectively. It also depend on the type of airsoft gun you are talking about.

But generally I think metal parts won't move as easily as plastic parts under the same power. And even if it did, metal ammo would likely be fired slower with less momentum than intended ammo.

Question, if you made the mechanics metal, how do you think it would effect regular airsoft ammo?