Dale-

I'm going to be pretty frank here.
I would likely trust one one-thousandth of one percent of US welders to work on my gear.

There are a lot of welders out there who can get certified, pass ultrasound weld tests, properly manufacture mining equipment, and perform all sorts of other 'sticking metal together' tasks without endangering themselves or others.

Welding gun parts is another story entirely.
Most guys out there welding (on) gun parts don't really qualify as far as I've seen and have been seeing since I was a boy.

What you want to find is a shop where the welders instinctively scrutinize the parts, where the guys notice right off the bat, before they even get within arm's reach of the piece that say, there's a rib soldered to that barrel, or even further that the solder that's holding that rib (or sight ramp or whatever the hell) could be soft solder or it could be cupric-metal high temperature silver solder -- they might even be able to tell that from a couple feet off.
You want the guy who can tell the difference between a casting and a forging. You want the guy who will be able to tell what parts of the gun will need pre-heating before the weld, heat treating after the weld, special welding wire (and have it or access to it) if necessary, and will know to ask when he doesn't know.
This guy's going to want to know the wall thickness of the barrel tube or receiver cross section or the kind of work the part does so he can leave you with the proper hardness and surface finish.
He'll know when to tell you to forget it and join the parts in a different fashion.

The guy I just described is most likely to be either a very successful bicycle framebuilder,
or a medical/biotech/refinery/process piping welder (and a very good one at that), an old boilermaker who's been around for a while...
You want welders who clearly understand the grave nature of liability and their responsibility as a welder. Guys who regularly walk away from their last job knowing someone's life is at risk if the work they just did fails -- and sleep well at night knowing it won't.

And you're not going to be able to walk into the shop were the 'tig guy' who's going to be doing the welding on your gun just pulled a couple of feet of wire off the spool out of the mig gun.
There's a lot of different welding wire to choose from out there. For instance I have wire here that cools to the hardness of the tip of a drill bit or saw blade, I use that wire for cutter repairs. There are countless other welding rod alloys with countless different properties.
He's not going to listen to a thing you have to say about what you think he needs to do while welding up your --

Exactly what is it you need welded?

--Tinker