What happens after you have upgraded a weapon fully in resident evil 4

What ends up happening is that you get access to the exclusive. The exclusive is basically your last and most expensive upgrade for your weapon. Now, each weapon is different. Try toying with it a bit to see which exclusive is really helpful to you. However, there is one gun that does not get an exclusive. That is the Killer 7. After you get rid of the outrageous prices of the tuning up, the Killer 7 will reach its max capacity. So, there's one weapon you don't have to worry about. Pretty much everything else has an exclusive. In fact, if you unlock the Handcannon from the Mercenaries and tune it up all the way and get the exclusive for it, the Handcannon will become an infinite gun.