Thanks for the replies. These factor's make sense: firing unit has low or small profile, is in command, stealth value in present, is in a concealed position, has not previously fired or at least been observed to have fired from that position, and I would add is not or has not moved in sight of the enemy. But the last vehicle I lost was KOed by an AFV hunting forward. To me this falls outside reasonable parameters. In addition I find it interesting the all respondents have been victims of unsighted fire but no one claimed they had purposely fired in this way, let alone mastered the technique.