My appoloies if I misunderstood, which it appears I have.
....but the reasoning would be the same, I think: With 5.56 carbines and ammo for same as common as dirt, why would anybody want to special order something that is harder (often meaning more expensive) to feed when it offers no advantage?
If there was a viable market, somebody would make this ......
Jeff Cooper wanted a Scout Rifle, and had several custom made for him, because there were no production models available ..... Custom work is expensive, but if that is what you want, and you have money and can find a 'Smith with the skills ....... it is possible you could come to an agreement.
TheGolden Rule of Tool Use: "If you don't know what you are doing, DON'T."