Most cool little Pocket or Boot Pistol.
Yep electrolysis would be the way to go but could prove detrimental to brass in combination with Steel.
You may want to try it low current, slow and sure and see what's going on.
12 VDC @ 2 or 3 hundred milliamps would be good.
The brass if any would be etched faster then the Iron or steel.
More Conductive!
Hell you may want to put it in a Lye bath first!
That would clear out some of the gunk. Brass being Copper, Zink, Lead, maybe tin would be attacted first by lye.
Red devel lye for the sink works most well.
If you start seeing brass, Flush it out and end that proccess.
Then your going to have to do your restoration by hand.