Is it okay to use a spent shell as a snap cap to avoid any damage when dry firing? I really don't need a package of 6 snap caps if all I want to do is protect the barrel from an occasional dry fire. This would be for a Browning Buckmark Camper.

To an extent. You are actually protecting the firing pin from slamming into the steel breech face. Each time you fire the gun it will dent the rim after you have hit the same spot enough times it will thin the rim of the brass so that the firing pin is impacting the breech face.

 

A cheap snap cap is a box of those plastic drywall anchors in a #10(?) size.