Is there any reason that one can't or shouldn't fill in minor pits in a mauser mag box/bottom metal/underside of an action with a MIG welder? I always hear of using TIG or gas - is this just to use a better filler metal for more even bluing?

In my experience, careful MIG welding can be done without any undue temp buildup in the overall piece, so I'm not worried about annealing the material. These are small pits, all below the wood line. One little zap in each would fill them to overflowing, but it wouldn't be with an 'approved' high nickel filler metal. Would it matter if the gun was not going to be blued?

You can easily weld up pits on Mauser bottom metal. I also weld the numbers. You can even weld up the locking screw holes if you want by either using a copper plug in the action screw hole (the easy route) or clean up the hole when you are done. I've only had one disaster on a floorplate while welding on it. I was letting the gas flow after cutting the current and it erupted like Mt. St. Helen. Must have had an impurity in there somewhere. A mild steel filler rod will will usually blue with no problem. You'll need to experiment to see which one works best for you.

Some people weld on actions. I try to avoid it. If a floor plate warps, softens or blows a hole, you're not out much.

If you are working pits below the wood line, why would you care if they'll blue or not? For that matter, why fill them at all if they won't show?

while you can use mig in this case, unless you're a damn good welder, I would avoid it. Mig requires much more heat than tig does, hence warpage occurs in mig that tig does not have, also tig is much more controllable and you can use a filler which matches the alloy of your base material. The mig will leave a much greater puddle to grind off also. If you do mig, I would suggest use of a one of the heat blocking pastes around the area to reduce the area of warpage.

Right. Heat stop paste would always be used. Forgot to mention that. I would fill one hole, cool the entire piece, fill the next, etc - trying to avoid building up the heat in the whole thing. I have all the time in the world.

They are going to be below the wood line, so looks really don't matter. I don't care if they blue or not. I'm leaning towards a baked on finish anyway. I just would rather have smooth metal there to avoid any more moisture-trapping crevices to try and stop any future rusting.

If you are going to use a baked on finish why not blast out the rust and fill with silver or high force solder. Depending on the amount of heat required for baking of finish you may be abel to use something like steel bead. Just a thought.

James

If these are below the wood line and not visible and the chemical reaction that made them pits is arrested why bother? If it is above the wood line, surface grind.

I only TIG an action. MIG puts a lot o fheat in there and you have very little control when compared to TIG. That is the reason that TIG is used in gun work and MIG is not. Many people have learned these lessons the hard way