@cetra
Yeah, I'm personally fond of the idea of crafting items of power. Wait until you guys start investigating the 4 kids I sent you after at the end of our last session.
BTW, If you are still serious about making magic guns, I've pretty much decided that as long as you don't create it with any mortal magic, it's not a first law violation. The Laws of Magic only govern mortal spell casters and magic. If the gun fired fae magic and wasn't wielded by a wizard, it would be a grey area the wardens would have to sort out. That being said, one of our party did get drafted into the wardens at the end of our last campaign, so I can imagine how that will go.
Also, interchangeable cylinders would be a nuisance to carry around from a practical standpoint. Just come up with a design for a gun and give it command words or something to change powers.
@everyone else
I would allow a PC to craft an Item of Power, given the right story-based opporunity and circumstances.
However, I wouldn't apply the thaumaturgy rules to the process.
Getting an Item of Power, assuming the story has provided the opportunity, is mechanically as simple as paying the Refresh. I don't believe a player should be able to produce a powerful artifact just because they can manage the right amount of Shifts, but likewise, if it's fitting that a character should be able to forge a powerful artifact as part of the story, then I don't believe the rules should be the last obstacle in that process.
I'm with worldmaker here. I'd let enchanted items go under thaumaturgy rules, but not items of power. My group has brought up new wardens being swordless several times. If we can agree on something, I might be inclined to let them come up with a replacement design, but as we've already discussed, it would take place over the course of several campaigns.