Stevens shotguns were plain ole utility grade guns,but are made like tanks and will last forever if taken care of. I have a 520 12ga that I hunt with,it was made in the 1930's and will shoot with the best of guns. Most of these shotguns are Takedown models.If you slide the forearm back towards the reciever you will see that the magazine tube has ribs in it.Twist the tube to unlock the cams that hold the barrel assembly to the reciever,after it is loose slide the forearm back forward and wiggle it free from the bolt. After it is free you can hold the reciever end of the gun and push straight down on the barrel assembly and the two should come apart.These are neat old shotguns.
Value depends on the condition usually between $100-250
16ga shells should be available at just about any sporting goods store that sells ammo.16ga shells cost a little more than 12 or 20ga shells at most places.