Not every weapon add-on needs to be (or perhaps even should be) an aspect. Take scopes for example, all they really do is extend the rifle's accurate range. Someone who can't hit a target at 50' without a scope isn't going to be any better at 50' with a scope. Probably be worse.
Some things do make sense as aspects - Polymer Construction for example. It's only going to matter in a limited number of circumstances...and only hiding from detection systems seems immediately applicable to the game. I suppose lack of rust / corrosion might become a factor but can't see it being common.
One method of representing crafting unique weapons (weapons with aspects) is re-skinning the maneuver mechanics and using thaumaturgy's duration. For a gunsmith adding laser sights to a pistol: roll relevant craft / repair / gun skill, need at least three shifts per use per scene, add shifts for duration (next time it needs calibration, using thaumaturgy duration). You might or might not require the weapon smith to have a stunt depending on much you want this in the game.
I too like this idea but I will have to examine it in detail a bit more and talk to the GM of the game as well as the players in the game I run then test it out and see how it works.
A lot of the mod/aspects I'm looking at are based on the design of the individual weapon.
Some Examples; EAA Witness 10mm Semi-Auto; Polymer Construction, Laser Sight, Reliable
Dan Wesson PPC .357 Revolver; Reliable (Revolvers rarely jam), Intimidatin' Manner (It's a big and ugly gun), Limited Ammo (revolver)
AA-12 Auto Shotgun; Full Auto-Fire. Scary as hell (Yep)
Kriss Vector Sub Machine Gun; Full Auto-Fire, Laser Sight, Silencer, High Tech (more vulnerable to hexing?)
Sniper Rifle; Long-Range, Scope, Silencer
Barrett XM-109 25mmm; Anti-Material (Bonus to hit vehicles), Long Range, Scope
Note: I don't actually expect the character to use any of these but wanted to cover a spectrum of possible examples.