yes the Italians were behind in improving their aircraft guns. Unfortunately, the Breda-SAFAT, however good it may have looked in 1935-39 only had reliability on it's side by 1940-41.
It weighed within 1kg of the American .50 cal and while it had a bit better rate of fire (700 vs 600 both unsynchronized) in 1939-40 that went away in 1941-42. It had a comparable MV up until 1940 but the bullets were around 3/4s the weight.
Sometimes, the low efficiency of a gun can be solved by installing more of them. A move towards 4 Breda HMGs per a fighter should've been done by the time Italians have the engines capable for 1000+ HP at 4 km. But indeed a working 20mm would've been my suggestion, these don't rely on excellent ballistics and RoF as much as HMGs once armor is in the picture.
Another, perhaps missed, opportunity for an European power might've been the cannon firing the 'small Solothurn' 20 mm cartridge, that was used in some anti-tank rifles of the 1930s. Rimless cartridge, neither too big nor too small, it should've provided the ballistics of the 'middle Oerlikon'( the FFL), in a weapon that is of a modest size & recoil.
The hoped-for engine mount did not work, gun over-heated and jammed. Which lead to single 7.8mm machine gun in each wing in some of the Jumo 109s. Germans then tried putting a single 20mm MG-FF in each wing. Huge increase in firepower for about 7 seconds. Germans were using about 500 rounds per machine gun for around a 30 second firing time (?). Some of the 109E-3s got 1000 rounds for each cowl gun which was bordering on ridiculous. Ideally they should have developed the belt feed for the MG-FF in the wings and paid for the weight gain by ditching over 1/2 the cowl gun ammo, not an equal trade but something.
Agreed.
The motor-cannon idea seem to be working with the Jumo 210s and DB 601Ns, but not with the 601As.
It took a long time for the Germans to fix the problem in the Bf 110. rear seater was given two extra drums per gun to switch out. Maybe they didn't fix it becasue they were working on the Me 210 with MG 151 guns?
They fixed it once the MG 151 became available. The belt-fed FF/FFM would've been and elegant solution for the 110, too. Carrying 4 such weapons (and no nose LMGs) would've allowed for an unprecedented firepower for 1939.
Seems like everybody (mostly) wanted the initial punch of 20mm guns but then were going to hunt cripples, strays/broken formations or get themselves out of trouble using just the machine guns after the cannon ran out of ammo.
Perhaps the target effect of the 20mm fire by a single or twin cannon at bombers was over-rated before the real shooting started, especially once the bombers gotten the self-sealing tanks and some armor? Maybe the expected hit rate didn't panned out - basically instead of, say, 10-12 rounds landing, just three-four did so in many cases before the fighter run out of the ammo? Dud rate being grater than expected? Too sensitive or too insensitive fuses, that either detonated the shell to early, or not detonating them upon contact on fabric, respectively?
Definitely the case for a) at least two cannons to be carried, and b) more shells per a cannon.
Also for pilot's markmanship training not being as good as it was hoped for? Again more cannons and more ammo will be needed, until these mistakes are accounted for and rectified.
A lot of planes were designed and built with very mismatched firing times for their guns. What the men in the field did (not filled the ammo boxes to capacity) changed at times. Trying to find the reasoning behind a lot of this is hard.
One of the reasons I don't like very much the MK 108, the US 37mm M4, as well as the FF/FFF before the light Mine shell earned it another 100 m/s. The very high MV looks great on paper, but it results with either too small the payload, or with too big a gun (looking at MK 101 & 103).
IMO, 700-800 m/s is the sweet area for ww2 - hit probability is still good, but the weapon is handy and practical, and can be made to fire fast enough (here the cannons are, again IMO, already very good at above 600 rd/min for the 20mm, and above 500 for the 30mm).
All of this is for air-to-air fighting.