Nintaku:
How would you go about it?
I have less experience with FATE than other systems, but I love Borderlands, and love generators. So to offer that same enjoyment of wild, infinite un options, I'd probably try to make heavy use of random generators for weapon components. That way you could make hundreds in half a second. For instance, start compiling a list of mix-and-match properties and then combine a number of them based on rarity.
The main thing is that the choice of which weapon to use has to be interesting enough that new options are worth time to weigh while you never stick with one forever. Yet also, in a rpg, just shooting people endlessly gets old without appropriate character/story drama surrounding it.
Maybe the following:
Type: pistol, SMG, sniper, shotgun, launcher (determines which skill is used?)
Brand: Hyperion, Bandit, etc. (with some mechanical effect each)
Tons of extra Aspects: super scope, slow, corrosive, fire, lightning, slag, has melee blades, destroys cover, stabilized, explosive, powerful, knockback, launches grenades, launches swords, steals life, beast slayer, does all kinds of other crazy things. Perhaps these indicate free aspects/invokes that can be created on a hit or with a boost.
Making weapons as characters like Harrigan says is interesting if it doesn't become too unwieldy. If it's anything like the video game, characters may end up carrying around tons of different 'characters.'
Alternatively, weapons could make up most of a PC's normal stats (like skills) and therefore not take up too much extra room. PCs could then have personal/dramatic aspects, stunts related to class maybe, and maybe noncombat skills. Each weapon could then have it's power conveyed by giving its wielder a few skills like the following (maybe providing reason to move around battle zones): Long (2+ zones away), close (same zone or 1 zone away), melee (same zone), etc. I don't think PCs should have combat skills, since those levels (other than health) seem to be more modded by your gear than anything else.
Shields could offer aspects (pulse nova, lifesteal, etc), or varying ways of offering stress boxes or consequence slots. Usually FATE aims for fast combats, but in this game you may want more stress boxes to drag things out more, if there are enough interesting tactical choices to make combat itself fun. I'm thinking somewhat of the semi-tactical use of range bands and varying weapon effectiveness in 3:16 Carnage Among the Stars.