For the mini I put together I used a two-piece Flint Hill Manufacturing aluminum welding jig made for a full size Uzi. It can also be used for a mini if you remove the ejection port protrusion. Once I removed the protrusion carefully with a belt sander from front part of the jig, it just slides right into the mini center section and front receiver part that you will be using. To weld the rear receiver piece onto the center section you can use the back part of the aluminum jig which is much shorter. In order to get the jig piece back out of the newly welded receiver, make sure you haven’t installed the feed ramp or ejector yet.
One thing I have found helpful when TIG butt welding two pieces of steel together is to add a third piece (a scrap) to the end of the intended weld line. That way, instead of blowing out the end of the weld you just continue right onto the piece of scrap. Then cut off the scrap with a dremel cutting disc and you have a nice square receiver edge.
I have an Eastwood Elite 200 Amp digital TIG welder which does have a pulse feature. I found that 60 amps penetrated all the way through to the aluminum jig. One good pass is all it took without having to do any remedial welding on the inside.
