Lots of good points raised here, but some interesting exceptions to the standardised 50 cal could be found. The mention of the P39 cannon doesn't convey what must have been a supply nightmare as some squadrons in the early days in New Guinea, equipped with a mixture of P39's & redirected p400's. That resulted in - 30 cal bmg in the wings, 50 cal in the nose, 20mm in the nose (p400) and 37mm in the p39. A logistics nightmare. And how about the modified field gun in the B25 (h?) On teh opposing side, The Zero packed a very interreesting licence-built 20mm Hispano that weighed much less than a 50 cal. I cleaned up one salvaged from a wreck in the sea and was impressed with its small size & light weight, part of the weight saving for the Zero. Low velocity, only 49 (I think ) rpg but very effective in the right hands, vide Saburo Sakai. Finding various often non-standard installations from both sides made for some interesting analyses.