If it is a slide action, it is more than likely a Wards gun made by Savage/Stevens, model 29A, circa mid 1960's.
It seems that there may be a few other rifles that would fit your description if it is not a slide action. It seems that there were some bolt action Mossbergs with this name also. I am pretty sure that the oct barrel is a pump though, so it should be the mod 29A Savage.
Gunparts has an online schematic available for viewing.