They've been found to be more effective than the previous 9mm guns issued.
Which aircraft? In WWII armament ranged from zero (trainers, etc) to rifle-calibre machine guns, heavy machine guns, cannons (15 mm upwards), heavy guns (57 mm, 75 mm, 105 mm), rockets, torpedoes, bombs, mines...
The Mitsubishi A6M Zero had two 20 mm cannon and two 7.7 mm (.303 in) machine guns .
There are various types of ammunition. There are also particular guns that use the recently developed 30 mm ammunition. Among them are aircraft guns and armoured vehicle guns.
Size of the bullet
100 1 cm = 10 mm 10(1 cm) = 10(10 mm) 10cm = 100 mm
7.62 mm (or 30 caliber) and 5.56 mm (or 223 caliber) are common rifle cartridges. Pistols, machine guns, and artillery will of course be different from these.
1 cm = 10 mm → 5/10 cm = 5/10 × 10 mm = 5 mm
1 cm = 10 mm → 3.5 cm = 3.5 × 10 mm = 35 mm
10 m = 10000 mm
Multiply cm by 10 to get mm. Divide mm by 10 to get cm.
10 mm = 1 cm20 mm = 2 cm
Convert to the same units, then the comparison can be made: 1 in = 25.4 mm → 10 in = 10 × 25.4 mm = 254 mm 254 > 200 → 254 mm > 200 mm → 10 in > 200 mm 10 in is greater than 200 mm