The stock dp is wrong but what the P-38F carried was 150 20mm and 500 .50cal per machine gun. You can change it in the dp file. Look for
[GUNS.0]
gun.0=1250,0.86,-0.001,0.9399,0,0,1,1250
gun.1=1250,-0.2,-0.012,-0.151,0,0,1,1250
gun.2=1250,0.2,-0.0118,-0.85,0,0,1,1250
gun.3=1250,-0.86,0,0,0,0,1,1250
[GUNS.1]
gun.0=50,0,0,0,0,0,1,50
Change the [GUNS.0] number 1250 at the beginning and end of the string to 500
Change the [GUNS.1] number 50 at the beginning and end of the string to 150
Now you will have the proper ammo load.