[Neus]

The way Nitrogen Drop works is it checks the gun's power, whereas One Hit Wonder applies when calculating total power (IE, SWA). Perhaps I should update OHW's description to be more accurate but that's why you're getting the results you're seeing. Whether or not it should be changed is a Fu topic.

Basic attacks with guns work differently, because the actual SWA is calculated and divided as bonus damage. When you 'force' an attack with it via Shinken it doesn't apply. The only times it does are for skills specifically for guns where it uses the attack skill of the weapon, such as with Lead Storm.