Deleted member 1487
The Italian 6.5 wasn't too weak or used particularly old powders (in fact it was pretty damn innovative with it's triple base tube powder and initial use of progressive twist rifling) it's just that it used a very heavy round nosed bullet which was aerodynamically inefficient instead of using a lighter faster bullet. To some degree the Japanese 6.5 had the same issue, plus that of a relatively weak powder. The Swedish round used a LOT of powder and by 1941 a very well designed aerodynamic bullet, which was too powerful for a infantry rifle that would be used with automatic fire (though still less than that of the 7.62x51 NATO), but was ideal for bolt action weapons and MGs.Odd thoughts:
Curious why folk advocate 6.8mm rounds as modern ideals and dismiss the .270/280 which are about the same size. Also curious how the older 6.5mm rounds are either characterised as too powerful (Swedish) or too weak (Italian/Japanese) when the differences in the cases are a length of 55mm down to 50mm all with older powders.
The 6.8/.270 round was workable for automatic fire from an infantry rifle, but less than ideal, while the .280 was a bit too much even in it's lowest power form, while clearly the high powered versions were missing the point in terms of the original point of the round.
In the case of the US I think that without it being said the motivation was to keep costs down due to the budget cuts and huge surpluses from WW2 left over. Keeping to the same caliber minimized the cost of conversion as they modernized the 7.62 round and all the US obstinacy in that regard was less about the performance of the round and more about cost. At least that is my gut feeling having read about the 7.62 NATO development and adoption over the British rounds. The 'not made here' part was that the 'foreigners' were making a new standard that would cost too much to implement versus shortening and modernizing the 7.62 round and largely make all the left overs from WW2 useless and forcing a major retooling of production equipment. That was the rationale the Germans used with developing the 7.92 Kurz; the Germans got that the round was less than ideal, but given their investments in production equipment with a certain barrel diameter, bullet width, and cartridge casing width it only made sense to utilize what they already had than try to retool and create a new caliber out of the blue; the Italians ran into that problem during the war as they were converting away from 6.5mm rifles and the Japanese who tried to do the same when they were dissatisfied with the 6.5mm round's cover penetration in jungle fighting. It turns out that is really expensive and difficult to pull off in war time. The 5.56mm round later on at least benefited from using the existing .22 diameter bullet, for which there existed substantial machine tools in the civilian sector already (being based on the .222 Remington cartridge introduced in 1950 for the civilian market).The order of events is what do you want the bullet to do. Then what case etc. will make it do it and then what weapon will best fire it. In real life nations want it cheap. In Europe there is no NIH syndrome as long as you can manufacture it at home (excluding France) but there was, and is, a huge NIH syndrome in the USA.