To add to my previous comments...
I'm pretty sure @DocGKR has stated that 6.5CM has issues with feeding reliability in a semi- or full-auto application (heavy use, dirty in the field), but there were people sending a lot of .260 Rem rounds downrange really close together in time with good success.
Another factor in the "failure to launch" of the mid-frame AR platform is likely the drawn-out military program to procure a mid-size frame service rifle with a 6.8 projectile. There are other discussions on this site, but my recollection is that the performance requirements can't be met with a conventional brass cartridge with less than about 40 grains of powder. (I've expressed my skepticism about Sig's new Cross magic ammo beans in the thread on that.) So a reasonable person would expect that if that .mil program goes forward eventually, it will establish the standard, whatever that turns out to be. Until we get past that gateway and know what that template is going to be, the risk of developing something and going to market only to have it obsoleted by the production efficiency associated with the volume of the .mil selection is a too-predictable way to lose a lot of money, at least for most people who are spending their own money. Hence, you see innovative niche builders who are gonna innovate because that's what they do, and you see large companies where the management is placing bets with other peoples' money.
My prediction is that if the military does something sensible and there is a way for the template to pull on some civvies and go commercial, it will happen in a big way. And if they go F-35 with it, the mid-size frame service rifle will continue to wander in no man's land, waiting for Godot, while work gets done with ARs and bolt guns.