In carbine length there's not a lot of fully usable for marksmanship in 5.56, at least for my definition of marksmanship.
A 16 inches barrel is a minimum so I'll say a SR-15 may be nice. G36 and HK416 can make some serious damage too but are way more reliable.
My favorite for multi purpose still is the HK417 in 16 inches.
CQB ? What the hell do you think sidearms are made for ?
Tous les jours nous nous endormons plus instruits mais pas forcément moins bêtes...
I can do a CTRL+ATL+DEL on your gun !
Gun doc in Montreal on AEG, GBB, GBBR and Bolt rifle (gaz and spring)
Heavy custom builder (including painting)