You can build a 9mm carbine. I have done three. I prefer the New Frontiers Armory side charging design. They are Glock comparable. I also build this as an NFA SBR.
If you are building, you have to tune them to cycle properly, between BCG mass and buffer spring. But once running, they are great, and yes, you get a pistol and carbine in same caliber with compatible mags.
Interesting. Being able to use the same mags would be a big benefit. I mostly own Sig so I’ll have to see what options there are for such.