An early version of the 520 had an inertia unlocking system for the slide to avoid the operator opening the action in the event of a hang fire.
I've only owned one such 520 with that feature. I don't know how long they were made like that.
If you dry fire, or have a mis-fire, you can not open the action as normal by mearly pumping the slide handle to the rear.
On these guns fitted with the inertia block, you had in this instance, to manually unlock the action using the slide release button.
If you fire a round, the recoil moves a weight inside the action fitted within the trigger group that swings a short distance and unlocks the slide so it can be cycled 'normally' by pumping the slide handle.
The 'weight' is approx. 5/8" sq. and sits on the left side of the action inside to the rear. It is barely visable looking in from the bottom along side the carrier on the left side.
I've had at least a dozen different 520's & 520A's. There are several changes to minor parts over the years of mfg.
The carrier springs will break (there are 2 styles of those), firing pins fatigue and break the tips off about 1/4" from the end occasionally, and the slide arm gets loose on the forearm allowing it to come unlatched from the bolt.
The last one is fixable, but many times someone else has decided to 'fix' it first and has twisted the slide arm out of the sheet metal forearm tube. They are rivited in place and occasionaly soldered also.
Ejectors wear out with extended use and the later type mainspring (torsion spring) sometimes looses it's power. The early guns use a leaf spring.
They're slick shooting shotguns and still available at bargain prices. I shot one Ranger 520A 12ga over 5K rounds before the firing pin went. It needed an ejector also somewhere in there.
It's since been converted to a slug gun for another owner and still sees use.
Still kicking myself for not grabbing a 20ga for $100 last year! Gotta be fast..
