if it's the nes version, pay me no mind, but if it's the gba version, I can help you.
Each person has a certain level with each weapon type, the more you use a certain weapon, the higher your level with that specific type is. Each time you go up a level with a weapon, the damage inflicted with each is multipplied. (ex: base damage is 100, with a 5 hit, you can hit 500, but with a level 6 weapon, you can now do 6oo damage............make any sense?