I wouldnt worry about weapon upgrades period. I would make things a baseline even or even create a few weapons not seen in the series to add a bit of flavor. When you try and make things up gradable you begin to change the focus of the game for alot of players who will suddenly play the race to see who can build the biggest baddest weapon quickest.

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"The gun katas. Through analysis of thousands of recorded
gunfights, the Cleric has determined that the geometric
distribution of antagonists in any gun battle is a statistically
predictable element. The gun kata treats the gun as a total weapon,
each fluid position representing a maximum kill zone, inflicting
maximum damage on the maximum number of opponents while keeping the
defender clear of the statistically traditional trajectories of
return fire. By the rote mastery of this art, your firing
efficiency will rise by no less than 120%. The difference of a 63%
increase to lethal proficiency makes the master of the gun katas an
adversary not to be taken lightly." - Dupont, Equilibrium