kinda iffy in my opinion if they're behind you with a gun to your head don't do it as truth be told when a guns to the back of your head your not about to do a 180 and kill them are you, unless your james bond of course. If they're to your right or left i think it's up to you but in the end there's no rules on it so i don't think we really have an awnser.
I'm quite certain someone (not a cop) involved in a situation like this had been punished for turning 180 to react against someone with a gun to their back. It's ultimately poor roleplay and just escalates it immediately to an exchange of fire as opposed to what could be an RP situation.
@Vladic Ka- I'm pretty sure it was you I saw pass such a ruling, if you could confirm/deny