Yeah it's prolly coming from the netcode or something. I mean when I use it and hit I get the flow of hit signals on target - if I die I'll see in kill cam that idd I hit him a lot but damage (blood on edges of screen) seem to show after those half dozen shots. When I use rapid fire though, things look weird; the rate of fire is really increased but the kill cam shows the same thing happening in the same amount of time instead of the amount of bullets. It's really weird.
And once I am shot by a K-Volt, things are happening exactly how you mentioned. Granted I'm often not on full energy but sometimes I am and, well, it just seems to be 3-4 shots.
Maybe the energy drain is affected by the damage multiplier from where you are hitting. I guess I should attempt going for the head again but the pellet travelling velocity and its recoil just makes it a gamble everytime. :/