Would that have helped B-17s protect themselves from fighters? Many of the fighters they were up against had 20 mm cannon. I wonder to if the 8th Air Force shouldn't have been more cautious about sending bombers, deep into Germany, until they had fighter protection. I guess while the raid to destroy ball bearings, went very bad. It was found out after the war, that damage was done that delayed the ME 262. Otherwise the Allies would have had more problems with that aircraft. But, the P-51 was so effective, had they waited for it, losses would have been less.
20mm cannon would have helped to a degree. When it comes to weapons calibre bigger is usually better. The B17 was the most heavily armed bomber in the world at the time, and the USAAF thought that this heavy defence plus flying in box formation with bombers supporting each other would allow daylight attacks with minimum casualties. They were wrong. As for waiting for the P51? hindsight is a wonderful thing.
I wonder if it was even considered. The 20mm cannon would have been bigger, but also heavier. Remember the gunners had to be able to lift and wield their weapons. And the ammo would have been correspondingly heavier. So consider that factor. Also consider that each gunner would have to be retrained, if for no other reason to handle stoppages. That means re-vamping the training programmes and schools. And the 20mm, while it had more punch, fired slower. When incoming fighters streaked across the sky, and the gunners were trying to puzzle the problem of where to place their shots so as to intersect the arc of the cometting fighters, would it really be in their interest to fire a slower weapon?
Obviously 20mms' (Hispano) would have been superior to the said .50cal (Browning/Colt) had they reliable to some extent equally. The same time the US unable to produce an effective and reliable 20mm guns, as far as my knowledge goes. Not only this, compatible range finders also had to install.
Some US fighters had 20 mm cannon. I believe the P-38, some Hellcats and Corsairs. But, I have read that the US did have production problems with 20 mm weapons. A licence agreement with Bofors, was hammered out. So, producing a 40 mm in the US was not a problem. Why the 20 mm would be a problem?
Well, I think it was due to the beaurocratic tug of war. British developed two types belt feed system(Hispano-Suiza) replacing 60 rounds drum and later modified the gun with a little shorter chamber. Next they sent it to US for comparing, but the US did not agree to change their version(design of chamber). USAAC, meanwhile had as good as a stock of 40 million rounds for M2s' but the guns remained unsuitable to install. Two types of malfunctions that never been tried to solve are the 'half-strike' of firing pin and another one is that the outboard guns 'freezed' at high alts.
