Currently, this is my only non-5.56 rifle, a 9" .300 AAC Blackout setup.
Here is one I used to own: 7.5" Rock River 9mm upper on a Spike's lower.
Also had a 10.5" Rock River 9mm upper, but I sold that as well. Probably should have kept a 9mm upper, but I thought I didn't really need one.