In a game with different weapons and armor that actually affect more than just player stats, how would you achieve such effect?

(These are just examples not concrete ideas ) For example we could have a handgun, uzi and then you have the graviton-gun. The first two would just shoot bullets, the third one does more than just shoot a simple projectile. It could allow the player to hold an enemy and drag it to use it as a meat shield. The player could also wear generic armor but at some point wears armor that can absorb projectiles. After absorbing enough projectiles you can shoot a giant blast.

All these weapons/armor have different "behaviors" that either just raise stats or actually add new mechanics.

In a simple case most guns would have similar properties and changing a few settings would create a new weapon (handgun shoots at an interval of x amount of seconds, lower this number and you have a machinegun). This obviously does not work if you intend to do more than just shoot projectiles.

I'm pretty much stuck on writing the interface structure. While weapons and armor have different purposes they should both be able to process certain effects that change or add mechanics in the game world.