I think for the next COD (COD 9) you should add customizable guns, a system where there is body, trigger, barrel, and default sight (maybe stock for style) The body is the part which defines reload time most of the mobility and ammo per mag (chain) The trigger would define if it is semi-auto bolt-action (for snipers, maybe battle rifles) burst or full-auto this would define some of the kick. The trigger is what defines most of the gun. It defines some of the mobility, some of the kick, fire rate, range, and damage. The default sights is just picking some comfortable iron sights.
This can also be used to edit default guns ex: I really like this burst shot rifle! I would like if it was full-auto though... by editing the gun you would sacrifice some of the rifles kick for it being full-auto. All pieces of default guns should be included in the parts list, as to mix and match guns however you please.