What I am about to describe is somewhat brutal but it has an extremely good chance of helping you remove a seized rusted and other reasons that make screw threads virtually impossible to move. It does involve heat but in a very specific way applied to the joint pin its self. You will need a stick welder not a Mig or Tig just a standard coated electrode welder capable of providing a couple of hundred amps. You attach a carbon rod (the copper coated type used for ark brazing) to each output cable of the welder then apply the carbon rods to each side of the guns hinge pin then turn on the current. You will find that the steel pin will start to heat up because the steels electrical resistance is far higher than the copper wire of the welder leads plus the carbon rods, If you keep the connection on long enough you can get the joint pin red hot. This method has been used for many years for removing rusted or generally seized bolts nuts and studs but things do get hot so not for the feint hearted!
Batshit. You know as much about the SMAW aka "Stick" rod electrodes, polarity settings, carbon, copper and mild steel resistance to amperage flow, as old Neville Chamberlain knew about what made Hitler what he was- You can't do thus, as you describe, with the usual Lincoln Electric red "buzz box"transformer arc (not ARK-think Noah and the Bible with that spelling) welder most farmers and hobbyist metal workers here in the colonies have- why, might you ask, my ill-informed Limey- because the buzz box welders- fed with 220 single phase input current and a 10% duty cycle welding machine (except for the 75 amp setting-encircled on the front panel, by the way_ you need a 440 3 phase input transformer or rectified DC welder with a 100% duty cycle- with at least a 350 ampere output at 100% duty cycle- such as metal fabrication plants that tend to join heavy gauge steel with 7014 or 7018 AWS code rods, 3/16" to maybe 1/4" in dia. and run in the 1G or downhand position- If some beginner here who doesn't know this tries your suggested proceedure, he will not be pleased with his end results==