Is torpedo a common noun or a proper noun?

The noun torpedo is a common noun, a Word for any torpedo of any kind.

A proper noun is the name of a specific person, place, thing, or a title; for example:

• Torpedo, PA 16340
• Torpedo Road, Stafford Township, NJ
• Sierra Nevada Torpedo (beverage), Sierra Nevada Brewing Co.
• "Torpedo Run", 1958 movie with Glenn Ford and Ernest Borgnine
• "Torpedo", a novel by Ray Verola