Uhhhh... Why? You have the crit slots open for the U/AC, and the regular AC/5 still burst fires just like the U/AC5.. What's the point accept to deny yourself the option of the Ultra fire?
Currently there's no reason. Later they'll merge the CAC/5 and the CLB5X and allow the swapping of ammo types. I'd still choose 6x UAC/5 though, personally.