Well, I was gunning away at some skulks with my lmg. And then suddenly my crosshair disappeared. Later in the game I got a shotgun and it had a crosshair. I dropped it and pick up an lmg during a firefight and that lmg had a crosshair. So any idea why my crosshair disappeared suddenly? I only have this problem with the lmg.

Type 'crosshair 1' in console.

check your rates. That happens to me sometimes too on some servers with alot of loss and a low updaterate