I've read, but never had adequately confirmed, that the 3 round burst mechanism absolutely destroys the semi-auto ergonomics. Specifically, each pull has increasing weight until the 3rd resets it back to the first pull's weight.
As I understand it, 6.8 SPC requires new magazines but the rest of the form factor is the same, so no new pouches etc. I've just read that .300 AAC can use the same magazines, although it was claimed in Wikipedia it was designed to essentially be a 7.62x33mm clone, with of course subsonic provisions.
I gather that if you really want to do it right, you want something in-between a full power battle rifle cartridge and an assault rifle one, like the interwar .276 Pedersen or post-WWII .280_British (https://en.wikipedia.org/wiki/.280_British). But that would require replacing too much stuff today, we really should have done it in the '50s when we had the money, everyone else needed to change their service rifles, and everyone but the Germans needed to procure General Purpose Machine Guns. But we just went with a nearly pointless shorted .30-06 with a beefed up rim for better semi- and full auto reliability, got everyone else in the West to adopt it, then screwed them over with the M16/5.56x45 debacle.
6mm rounds seemed to be common in pre-War II era: Swedish Mauser, Arisaka, Carcano (aka "JFK assassination rifle"). Interestingly enough, the Fedorov Avtomat -- the first select fire rifle -- used the 6.5 mm Arisaka round (from the captured Japanese stockpiles after war of 1905).
As I understand it, 6.8 SPC requires new magazines but the rest of the form factor is the same, so no new pouches etc. I've just read that .300 AAC can use the same magazines, although it was claimed in Wikipedia it was designed to essentially be a 7.62x33mm clone, with of course subsonic provisions.
I gather that if you really want to do it right, you want something in-between a full power battle rifle cartridge and an assault rifle one, like the interwar .276 Pedersen or post-WWII .280_British (https://en.wikipedia.org/wiki/.280_British). But that would require replacing too much stuff today, we really should have done it in the '50s when we had the money, everyone else needed to change their service rifles, and everyone but the Germans needed to procure General Purpose Machine Guns. But we just went with a nearly pointless shorted .30-06 with a beefed up rim for better semi- and full auto reliability, got everyone else in the West to adopt it, then screwed them over with the M16/5.56x45 debacle.