Amos has a few secrets that elevate his work above that of your normal mundane guntech.
First, while other guntechs attempt to exorcise the demons that inhabit a broken gun (with varying levels of success, the end result is either a pissed off demon or an empty vessel of technology), Amos' technique involves channeling the powers of that demon. He sates each and every gundemon with his own blood, and after hours of devotion exalts them. That is why his customers report such amazing functionality. Amos is a guntech, but more than that he brokers a pact that forever binds user and gundemon that neither party can ever be compelled to break.
Second he uses milspec wiring and a heat gun on the heat shrink. Makes it all really neat and tidy.
I can't say I understand his business model of driving to BC for a single house call repair, just like I don't understand how Constantine from the Hellblazer comics makes any money traveling around the world pissing off demons for no particular reason either. But holy hell does he get results. I definitely trust Amos far above and beyond any mere profit/ego oriented gun "technician". He understands there is far more to gunteching that the teching. He hears the gundemons calling to him, and won't rest until they are sated.