Is it possible to MOD on MNC?

Fix'd that for ya.

It is actually a good thing though. The more cheaters in a game, the more popular it has become. Unless cheating is easy to do, like the patched money glitch in Full House Poker (but in this case, like MW2, it takes a JTagged XBox, which requires massive hardware and software knowledge, which is difficult).

Might be worth mentioning that you did say you were headshot as a sprinting cloaked assassin. While this is challenging the sniper has a enough data concerning the assassin's location to pull this off. The sprint pretty much nullifies your cloak. The little dust clouds coming up, the slightly more visible outline of your cloak, etc. provide enough of an indication as to where your body/head is so it's doable.