There is an very easy and cheap way of removing 100% of the rust without touching the metal. This is so simple and works so well, you won't believe it. Ever seen ancient armour or swords in a museum that have been cleaned to a dark grey patina? Method used by museum conservators is called electrolysis. You probably have everything you need to build one around the house.
You simply imerse the action in an electrolyte solution and attach it to a 12v source negative terminal.
Dishwasher crystals work like a charm for the solution and a tank can be made out of cardboard and tape with lined with a single piece of plastic cut from a garbage bag. 12v can be supplied by a battery or charger, just make sure that the gun is connected to the negative terminal. The positive can be attached to anything conductive you have handy (eg, welding rod, piece of steel or copper pipe) and laid next to it in the solution, make sure that the two don't touch.
Bubbles will rise from the action and the rust will turn to black slime. The action can be left in the rig without worry about harm to the parent metal. The electrolysis will get into every crack and crevice and zap the rust down to the molecular level. Even screw threads will be released.
Tis the same process that they use on ships canon retrieved from the ocean, sometimes they will run the rig for a year to get the thick crud off.
Scrub the black rust slime with a finger nail or dish brush and wash it away to leave a dark grey patina on the steel.
Englishman, does this methood attack only rust without harming the bluing or say a gun that had nickle plating.