.50cals are already overperforming on damage. Plus, they always have far more ammo capacity than 20mms.
Every single 109 has, AT MOST, 200rds of 20mm. On a SINGLE cannon.
La-5s and 7s get 340-390rds of 20mm across two cannons.
The completely overkill J6K has 1200rds of 20mm spread across 6 cannons.
A Ki-61-I otsu or Ki-44-II hei, both with 4x 12.7s, have 1000rds on board.
Now let’s look at the US and their .50s.
All P-47Ds get 3400rds across 8 guns, the N gets even more.
P-51s get 1880rds on 6 guns across most models, Corsairs get 2350rds for the same guns.
P-51C and Bearcat have 1250-1260rds, with the same burst mass and gun count as for the two 12.7mm-armed japanese planes above.
The P-47 gets a higher burst mass than everything I mentioned, except the J6K. It is better than most Fw190 models (only 2 or 3 are better, and those are even worse aircraft than the P-47), or a 109 G2 WITH gunpods. And you get that as low as 3.7.
The P-38 has a very good 20mm and you can fly it as low as 3.0. Due to how unreliable it was, not many other US aircraft got a 20mm cannon at all. The US also has what is probably the best armed plane at 3.7, the P-51A with its 4x 20mm cannons. Unlike comparable aircraft from other nations, it is low enough where its speed is relevant.
At 5.7, the F4U-4B has more 20mm ammo (and arguably better guns) than anything else I could find on a fighter near its BR and with comparable ammo load. N1K2s, J2Ms, Fw190s, Tempests… only the N1K2s get close with 900rds, but the cannons have much lower RoF and the plane itself is quite bad.