Only one of those saw significant ground combat and the others did so to have anti-material rounds. What was the new cartridge Italy adopted? The 8mm anti-material round?
Not sure about the "anti-material" terminology in this timeframe, and given that these rounds were intended for use in perfectly ordinary MMGs.
Apparently one of the motivators for the Swedish project was increased effectiveness beyond 2.4km range (within this range they reckoned the characteristics of the MG and its tactical use were more significant than the cartridge), while the 8mm gave good effectiveness from 2.4-3.6km as well as better effect against aircraft. Sweden obviously was lucky enough to stay out of the war, I think this only saw active use in vehicle MGs in the congo
The norwegian motivation was also a desire for better effective range (especially with tracer) and better anti-aircraft effect. All of the their Colt M/29 (Browning M1917s) were supposed to be converted but the Germans scuppered that plan, along with any opportunity for significant ground combat. Ammunition was apparently produced up to 1944 and finally used up during the sixties.
Not sure what the Italians were up to or how widely they used it, I'm going go guess they had exactly the same interests as the scandinavians since it has basically the exact same spec and they did the exact same thing of converting their standard MMGs for it.
The japanese cartridges always give me a headache, I can never remember all the different 7.7mm flavours and who used them for what but again, the general story seemed to be that bigger is better.
Then why did they develop the T10/23 MG?
An excellent question which presumably the people involved asked themselves whenever the programme got back-burnered. Presumably someone thought it was a good idea at one time, just like the ever-growing more-like-.30cal-with-every-iteration .276.
An even better question is, how much effort went into the program and how come it and all the other good ideas that weren't .30-06 Brownings up in the trashcan? As far as I know the only serious interwar firearms programs were the M1 Garand and the M2 SMG. During the war there was the M1 Carbine and the M3 SMG. Everything else amounted to a bit of fiddling with prototypes.
I understand that 7.35 was chosen as it allowed the 6.5 barrel to be bored out and re rifled to 7.35.
Indeed, I've seen many references to it being an economy measure, worn-out gain-twist 6.5 barrels could be rebored to 7.35 and save the cost of buying new barrels (in 6.5 or 7.35). Now whether those references are accurate is another question. Similarly I've seen claims the low power was not due to any special cleverness, but due to the case being full up with propellant and they couldn't get more oomph without new propellant they couldn't afford. But never any really convincing references one way or the other