I've got a couple of 5.56. A 10.5", and an 11.5".
For something different, I went with 7.62x39 with a 10.5" barrel. I chose that chambering for the availability and cost of ammo (a couple years ago). Also since I have a couple rifles in that chambering.
The 7.62x39 is nice. No regrets on that choice. 124gr does around 2150-2200 fps.
50 yard sight in,
Churched-up with some Magpul furniture,