All of my confirmations have been counted as soon as the player gets the kill message. Fair enough I think... 
I don't know, that could be even worse!
Due to lag, I sometimes haven't gotten a kill message for quite a while after watching the guy explode or impact the ground. Like, 30 seconds (or more) delay. I'm probably one of the worst shots in the game when it comes to ground guns, but with a delay like that I may need to go through some of my films, ha!
But again, it's up to you on what your measuring... Are you trying to see how far someone can be away before you get the kill message? Or are you trying to see how far away someone can be accurate from the ground gun?