Russia has long experimented with a 6mm cartridge dating back to 1913; unfortunately, or fortunately, the Russian government saw it as not worth changing calibers at that time. Fast forward to the 1980s with the 6×49 Unified hypervelocity cartridge, then again around 2015 with a reportedly reduced-velocity 6×49 for assault rifles. Once again, Russia is investigating a 6mm cartridge, this time for the future Russian assault rifle and possibly light machine guns (LMG). At least these are the ones we know about.