Here's their list of uses. I searched for "gun" and it wasn't in there. No results found. I assume this means they don't recommend it for use as a gun lube
http://www.wd40.com/uses-tips/category/sports-and-recreation/
Here are the gun uses they recommend.
# Displaces moisture from guns after cleaning
# Drives moisture and mud from duck guns
# Cleans and protects gun carriages
# Cleans and protects gun linkages
# Prevents rust on guns used in the field
# Prevents corrosion on field weapons
# Protects gun parts when being stored
# Cleans and protects bayonets
Removes camouflage tape from guns
# Anti-rust agent and lubricant for gun magazines
# Protects bore and exterior of gun barrel from corrosion
# Removes powder residue and metal fouling from gun barrels
Removes crud from antique guns
Spray on pump action on shotgun to prevent freezing during cold weather
Removes lead buildup on guns
Lubricates precision gun fittings
Lubricates pump-action guns