You're loving dense.
For some of the attachments, yes, there would have to be new datablocks. For example, FMJ, incendiary ammo, and stuff like that.
You wouldn't be creating the datablocks in-game. You could create one FMJ type bullet and then there, datablock. Shoot it out of any gun you want.
But surely, why are we arguing, this is obviously not going to get done, so why are we disputing over how it would be done if it were in fact going to be done.
Too overly complicated and in most cases, this mod would require the cooperation of every modder making a weapon, which I know won't happen.
Summary: No, the guns are fine how they are. This is Blockland, go build something.