AFAIK, you need to use a TIG welder. I asked my brother, who is currently a regional supervisor for "customer satisfaction" for Mazda USA (read, super-mechanic) and used to run his own shop and still works on cars, including doing some welding now and then, on a regular basis. He's been welding since he was 12 and is pretty proficient. He said that even with a TIG welder he'd be uncomfortable doing the job for me, as there would be a fairly good chance of warping the gas cylinder unless you've done quite a few to get the hang of it. I also spoke with a gun smith in my area that used to do these out of his shop, he said they weren't too hard once you got the technique down, but expect to ruin a few learning how. He used a barrel stub to hold everything in place while he did the welding to reduce the possiblility of warping the gas cylinder.