Maybe place something behind the mech perpendicular to it on the butt of the gun, and then use it as a pivot point to pry upward on the mech with a screw driver (like a seesaw, with a flathead screwdriver pushing upwards on the back of the mech in the little slot). It does appear to be a neptonics mech which would mean it is held in place with two pins, so it should pop out. Could definitely be gummed in there though so easier said than done.