My tought was that the weld beeing cooled down quickly from the surrounding metal and that way hardened.

I have welded with tig and it became hard as rock and also the mig gets harder but the ordinary mma
seem to heat up so much surrounding metal so it doesent get as hard. The torch also heats up the surroundings so i have at least never had any problems. But this is only on the added metal not the "old".

But to be fair i have never tought of using a weld on a rifle, but it could work.