I would say 40+ kills is ALOT with a death of 5 unless the teams are greatly uneven. Say 20+ lvls against 1-5 level noobs. I've 1 shotted many guys with my guns and they go...whoa...wtf? Mk 16 can 1 shot 1 kill you if it's a crit to the head.
The prob I have with 40 kills or more...is the amount of deaths. If they have 40 kills, 20 deaths... ok..although that match would have to be long.
You run out of ammo alot in this game and the games are usually not THAT long to get that amount of kills. I think the most I've EVER got was 24 kills with 4 deaths. Among the 4 deaths...was because I had no ammo and I ran into 2 guys at once. My largest kill streak is 13 I believe to date. I know guys with the sr-25 that can stay nice and far away and rake in the kills though.