To answer your question about 'zomg'....
It's like 'pwn', is you type own enough, you are bound to hit the 'p' key instead od the 'o' key. That's how 'pwn' came about.
'zomg' comes from you missing the 'Shift' key and hitting the 'z' key. 'zomg' could be a simple mistake, but if somebody types it like 'zOMG' or 'ZOMG', they are probably being sarcastic, or don't know what it means and are typing it to be 'cool' as you will.
As for the sniper, I haven't a clue.