Which way do you push the selector on a SKB model 500 shotgun for full and modified barrel?

This answer will be for more than a SKB 500. On any over/under or side by side there will be differences in the mechanism that makes them work. Therefore, place a cartridge in a barrel and in a safe place, pull the trigger. If the cartridge goes off, the feel the barrels and note which is warm. Note where the selector switch is and now you know which barrel is fired when the selector switch is in that position. In the other position, the opposite is true. If when you pull the trigger and the cartridge does not go off, you know that the selector switch is set for the barrel without the round in it. I'd check it with the above method just to make sure.

lcj