How do you disassemble a 3 Marlin model 120 magnum pump shotgun?

First and foremost as with any weapon make sure 100% that is unloaded both chamber and magazine.

-retract the slide/pump half way and unscrew the magazine tube cap and remove the barrel.

-unscrew the buttpad from the stock and have an extended socket drive handy w/a 7/16th socket and remove the screw that hold the stock to the action.

-punch out 2 drift pins for the trigger guard assembly and pull up on the trigger guard to remove from action.

- remove the positive shell stop and the spring shell stops. these are recessed in to the receiver sides and are held in by the two drift pins.

-then make sure the slide is locked in the forward position and look in the action from the under side and pinch the shell lifter ends inward with a flat head screw driver and make sure both are off from the holding post that are fixed into the receiver.

-this will then allow you to remove the entire bolt assembly and shell lifter by pulling it forward and the slide action will come off the magazine tube.

These are the firearm specific ways from dissassembling the Marlin model 120 pump action further parts brake down follow all other similar shotgun makes/models.