The uncon duration doesn't seem to differ from calibre to calibre. I have been knocked uncon by .380 and my character has woken up within a few seconds. Sometimes they wake up as soon as they hit the ground. The same can be said for .308 and 5.56. Also, at times, there has been a strange 3-5 second delay between being hit and going uncon. The only noticeable difference I have seen has been shotguns. They seem to cause an extended uncon state compared to other bullets.