The 520-520A shotguns are great ole guns. The 520 was the double hump,and the 520A was a single hump receiver.These guns were built to last and I enjoy hunting with my 520A,it is also a Westernfield with a factory Poly-choke.
You can shoot just about any 2 3/4" shell out of it. If you don't already know,if you dry fire the gun,and try to function the pump forearm and it will not work,push forward on the forearm to release the bolt,and then pull it backwards-These are recoil operated bolts per the design,and when dry fired the bolt stays locked in position.
Like Lee said just takedown the receiver/barrel assembly and spray or soak the receiver,and then use a compressor to blow it out to clean it,unless something is broke there's no need in trying to disassemble the receiver to clean it.
Clean it up,and go enjoy shooting it,these are great shotguns.