Separate gun model for when the gun fires. It would be simple! Have two gun models instead of one. The primary gun model that you see while moving around, and a second for when the gun fires! Not only would it allow the guns to show muzzle flash while firing, it would also add more visual appeal and immersion to the game by showing bolt blowback and the likes.

Well why not just add an animation to the gun firing instead? Switching between two different models on the fly is completely in-efficient memory wise.

i don't thing a 4-bit blowback would look good, and muzzleflash isn't necessary.

i think muzzle flash is a good idea, but a whole animation is pointless

You're asking for a completely new model, a completely new animation, and for the two rifle models to be seamlessly combined.

How the hell is that simple?

Very simple thing it is just implementing them that would be hard.

Also, I don't think this is necessarily a 'coding suggestion' rather than a 'modeling suggestion.'