Distra,
I'm sorry for the loss of your grandfather- I have to say he had good taste in shotguns. The Stevens 520 in its several guises (I have a 12 gauge Montgomery Ward Model 30- marked 520 myself) is a classic John Moses Browning design dating from 1903 (patent filed and manufacturing rights sold to Stevens). The first 520s hit the market early in 1904. It remained in production until 1932.
One of the genuinely neat features of this design is its takedown mechanism, a very solid and durable design far superior IMHO to any other I have handled. I spent years looking for a bargain priced Winchester Model 12 field grade takedown gun because I wanted a good pump shotgun I could pack in normal luggage for road travel. When I found a 520 languishing on the used rack at one of my FFL dealer's shops and took it down the first time, I quit looking for a Model 12 completely.
1) To take the gun down, first clear the magazine and chamber and put the safety ON.
2) Open the action all the way.
3) Using the ridges at the front of the magazine tube as a grip, turn the magazine tube clockwise. Watch the bushing at the base of the magazine tube as you turn the mag tube and you will see the wedges on the bushing backing out of the corresponding notches in the front of the receiver. Turn the magazine tube until it stops turning easily and the magazine tube bushing is completely withdrawn from the receiver.
4) Slide the barrel and magazine tube assembly straight down toward the bottom of the receiver. It may take 'wiggling' the barrel a bit to get it loose if it hasn't been taken down for a long time. The barrel/mag tube assembly will stop sliding down in the receiver when it bottoms out on the action bar after sliding down in the receiver about 3/16" or so. At this point the action bar will have disengaged from its notch in the bolt inside the receiver.
5) Slide the forearm all the way forward until it stops and the action bar is all the way out of the receiver. Then you can remove the barrel/mag tube assy the rest of the way out of the receiver by sliding it all the way down. With the barrel assy separated from the receiver, take care not to 'ding' the ridges on the barrel assy that mate into corresponding notches in the receiver.
Reassemble by reversing the above process.
That should be enough for normal cleaning- I'd be hesitant to go further into the receiver unless absolutely necessary, and if that were the case I'd let a gunsmith do it. If you need to get into the magazine tube for cleaning, use a properly fitted screwdriver to remove the single screw holding the magazine end piece to the barrel lug.
Note that removing this screw will free the end piece, which is under pressure of the magazine spring. Control the magazine end piece and magazine spring carefully. With the end piece removed the entire barrel/magazine assy can be disassembled for cleaning.
Take good care of it, it's a genuine keeper.
lpl/nc