WesWorld

Hmmmm. I have to say the dates on the Atlantean rifles are a puzzler. Seemingly, Atlantis went from a VERY short cartridge in 1914 (a 7 x 45mm, almost surely less capable than the 6 x 50mm Carcano) to a .30-06 class cartridge in 1925 (7.62 x 60mm), then shortened it a hair to a 7.62 x 57 in 1932. I'm puzzled by the two very close in capability cartridges (the x 60mm and the x57mm), and the early short cartridge is rather odd as well (most military cartridges of that day were at LEAST 50mm long, and the pre-WWI trend was for more powerful cartridges).

Still a work in progress, dates are likely fudged.

Ah, OK. In that case, a more usual breakdown might be:
7.62 x 57mm - 189(1-3)
7.62 x 60mm - 190(6-10)
7 x 45mm - 1925 or later

(Whether Atlantis would feel any impetus towards the shorter cartridge would probably depend on whether the Atlanteans were engaged in much trench warfare during the war, that was the prime mover for most countries examination of shorter, less powerful rounds in between the wars. If they weren't engaged in too much of this kind of fighting, the impetus might not be there, and certainly there's no real difficulty in producing a semi-automatic rifle for a 7.62 x 60mm, the US .30-06 is a 7.62 x 63mm round).

Never really went in depth with Atlantean involvement in the great war but I assumed their involvement in Europe was late, most fighting occuring in Turkey and the Salonika front. Not sure of the level of trench warfare by that stage.

I assumed the sorter cartridge would have been older, obviously a flawed assumption!

Historically, the way cartridge development went was that the first military cartridge rifles, firing black powder cartridges, were fairly long (had to be, to hold enough powder to make the heavy bullets move). When smokeless powder was invented, the cartridges shrank in bore size, but not a lot in length, because the infantry was expected to engage it's foes at long range (800+ m) and the machinegun hadn't really been incorporated into the infantry yet (they were a support weapon, at best, and might not be available at all). The Spanish-American and Boer Wars encouraged this thinking, showing countries that shorter, less powerful cartridges were inferior (though some of the inferiority was more due to the rifle than the cartridge). During WWI, on the Western Front anyway, it was found that the long range of the pre-war was really not needed when there were MGs around, the MGs could deal with the targets at long range and a less powerful cartridge would be easer to shoot for the less trained conscripts who filled out the armies. Historically, nothing came of this knowledge until WWII was rolling, though at least some of the combatant nations did studies and the US almost adopted an intermediate cartridge (the .276 Pedersen, which was rejected for the M-1 Garand only when the top General of the Army (Douglas MacArthur) said the Garand should be redesigned for the older .30-06).

So I'd assume then that the Atlantean 7 x 45mm would be an aboration? Then again didn't the Turks use quite a few MG's as per German doctrine? Perhaps a weapon for reserve units?

If the 7 x 45mm IS an early weapon for Atlantis, it would definitely be an aberration, and likely due for replacement by the time of WWI (as was the .303 British, the outbreak of war prevented it from being replaced by the .276 British, which was a good deal more powerful). Turkey, to the best of my knowledge, might have liked to have more MGs, per doctrine, but didn't have the money for a lot of them. Certainly, if the 7 x 45mm was in service prior to WWI and a replacement rifle became available, it would make sense for the older rifle to be stored and issued, when necessary, to lower-priority units.

Acctually I was following the premise that the 7 x 45mm would be a 1925ish weapon.

A 7 x 45 mm in 1925 is less of an aberration: historically such cartridges were thought about, but not implemented due to lack of funding (GB, Germany) or inertia (the US).

If thats the case what would be the motivator for Atlantis to persue a lighter cartridge then? Perhaps a cheap weapon to equip its African and South American confederate states?

A couple things: first, the larger, more powerful cartridges were found to be unneeded in relatively close-ranged trench warfare (even if the Atlanteans didn't experience this too much, their French allies certainly did); second, lower recoil makes the smaller cartridge easier to handle for less trained troops (the professional armies of the pre-WWI era don't care, but the conscript armies of the war did).

Putting a brake on this sort of development will be the squad LMG: if the squad's armed with less powerful rifles, the Atlantean army's faced with the dilemma of having a single squad having to carry multiple calibers of ammunition, and the riflemen of the squad being unable to reload the LMG.

Perhaps a 7x45mm LMG should be developed, or simply drop the 7x45mm completely? Export design perhaps?

The 7 x 45mm LMG is a possibility, but it won't have the range the armies of the day expect in a machinegun (MGs were expected to be effective out to 1200+ meters). An export cartridge is also a possibility.