How do you find information on a model 100 slug barrel single shot 12-gauge shotgun?

Model types can change depending on the manufacture of the firearm. The question was originally placed in Western Field. Assuming this is who the gun was made by, it may have been made for Savage or Western Auto. To get find out more, you will need to gather some more information from your piece. Some places you may want to check are Modern Gun Values or Gun Trader.