Drill stem using isn't pipe at all, but seamless tubing and generally of the carbon steel such as 4140 or 4130. That is gun barrel classes of steels. Drill pipe may or may not be be seamless.
Welders can be the best in the world in their particular field, but know nothing about some other field of welding. Oilfield welders might have the skills to weld aircraft frames but not the knowledge specific to that field.
Of the different types of welding done, I would say the oildfield or pipeline welder would have to the best knowledge for what we are doing if you applied the correct engineering critera. Go to the Safe loads and cannons plans sticky at the top of our board and look for a link to the N-SSA safety rules, They have the criteria for minumum stnadards for cannons contruction. I suspect you are within the those guidlines, but you should check.
If you have used those carbon steel alloy's and the welder has followed the proper preheat process, before welding, then I don't think you double load hurt any thing. I would keep an eye on your gun for signs of damage for the overload.
How about posting some pictures