[mgtfguy]

Time, oil, heat, patience and a VICE may be your friend.
1 Often a stubborn fastener will not feel like it is moving apply torque and STOP when you feel the tool starting to slip or before. Repeat several times a day for a few days (that is the patience part) the fastener will not feel like it is doing anything but often magically moves on the second or third day. (I restore 70 year old cars that have fasteners made of unobtanium, that haven't budged in decades. The hardest parts are the smallest, instruments etc. Where your strength and leverage far exceeds the yield strength of the part)
2 Clamp the offending nut, or bolt head in a vice and rotate the rest of the part (eg the gun) pay attention to what direction you are rotating it. This can be confusing.
3 oil heat and time, apply all three. Heat cycles can often loosen a badly stuck part the heating and cooling make the parts move in relation to each other. Repeat the heat cool cycle thing a half dozen times before attempting to loosen part.

Are you SURE that this is not a left hand thread part? are you SURE you are turning it the right direction? (yes I know both are obvious but ......)

The gun is not ruined, a good gunsmith should be able to fix it if you don't have any luck.

And know when to quit....