The Zero had a twin 20mm cannon and twin 13.7mm MGs, which, imho, gave it a fire power advantage over both the Wildcat and the Hellcat’s six 0.50 Cal. The Hellcat was an entirely new airframe.
Number and caliber are important but you also need to know the rate of fire and velocity to make a final determination of pounds of steel on target and energy delivered on target per unit time.
Zeros had 7.7mm machine guns, not 13.7mm. Rifle caliber machine guns didn’t have near the punch of a .50 cal BMG, especially against studier US aircraft.
The Zero’s 20mm cannon is a heavy hitter but it has two major drawbacks. It has a slow rate of fire and a very limited ammo supply (60 rounds per gun IIRC). A great weapon against a big, slow bomber. Not so great against a small, nimble fighter.
the 20mm cannons had a slow rate of fire, which made them somewhat impractical in many situations.