With the safety on and the muzzle pointed in a safe direction, remove the trigger group.
After that I don't know.
I had a new 51 in the late 70's. I never did figure out how to keep it working. After it spit out the 2nd bolt handle, I drilled a hole in the third and attached a piece of nylon cord and tied it to the rib.
By all means remove the trigger and then work on it ... They are problematic to the point I actually gave one away by letting it in the gun rack as we left the club one dark and gloomy night ... I bought it for my wife and it was nothing but trouble to the point she didn't want to shoot anymore ... I gave her a Model 12, no more problem but she don't shoot anymore any way ... WPT ... (YAC) ...
Do as one person said above but put the rod down the barrel and keep your hands and face away from the end. Take a hammer and hit the rod with the barrel as it is pointed away from you. That should loosen the shell.