There are at least two shops that have made a DI 9mm
CMMG is a delayed blowback.
Lone Wolf Dist. has a short stroke piston. Not sure if it was released yet.
As a general rule I think that if I am going to have a pistol caliber carbine it should use pistol mags. And, for me, that means either Glock mags or as a distant third choice, Beretta mags.
Rudy at Macon Armory said that when he did the 9mm, it was so close to the breach that parts of the jacket sheared off and went back down the gas tube, jamming up the bolt.
I have a .45 D/I AR… which was actually my first D/I AR (did get a 5.56mm after, and likely will have a second once I move). Really happy with it, especially in regards to the weight. Blowback just gets heavy for a pistol caliber carbine.