I guess it's safe to say you are using 3/4" wood for the gun cabinet. 3/4" wood is considered 1" thick so all you would have to figure is the square footage of all your parts and I normally add 20% for waste for most woods and 50% for walnut if you cut around knots and sapwood.
Anyway the way you figure board feet is multiply the thickness of the wood (before surfacing) in inches times the width in inches times the length in feet and divide by 12.
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