I use 1 and 3 on numberpad. Works nicely.

OK, I did some testing with a mission (Operation Aspis - Part 1) where I could switch between different soldiers with varying loadouts and I found:

1. It generally seems that soldiers can look further over their left shoulder than their right. Using Alpha 1-1-3 Autorifleman as an example as his back is relatively clear. Maybe this is explained by the fact that he's holding the rifle in his right hand but shouldn't he be able to be able to rotate his upper body slightly so that he can look as far in both directions? Looking left I can pretty much see my six, except for a small blindspot but looking right it's limited to maybe 4-5'o'clock. I can also do some pretty weird stuff looking left, like look back at my own chest and radio as if my head was on a stalk but that doesn't bother me as it doesn't affect gameplay.

2. Crouching does cause the soldier to lean forward a bit/hunch his back, so that his harness somewhat blocks his six but not unreasonably and his shoulders are still not in the way, although that varies depending on his load-out of course.

3. Moving forward whilst crouched causes the soldier to lower his body almost parallel to the ground which does of course result in him looking at his shoulders when turning his head. I guess this is realistic and necessary to maintain balance although I wonder if IRL he couldn't just raise his head a bit to see better.

4. Lowering the weapon also lowers the shoulders, providing a clearer view over the shoulders.

5. Moving forward with the weapon lowered actually results in the shoulders moving around more and them (or any attachments) blocking the view more than when moving forward with the weapon raised (I don't mean combat stance which obviously makes him go a lot slower). This probably makes sense though as although he doesn't appear to be moving any faster, having the weapon raised forces him to keep his upper body more stable.

6. Soldiers seem to have a tendency to sway their upper body quite a bit (idle anim?) whilst I'm looking over my shoulders, which causes them to get in the way and block the view intermittently.

So it's probable that on the MP missions (which is all I've been playing lately) that I encountered this issue on I was either wearing a backpack, was moving forward or possibly crouched and moving forward.

So the only issue after further testing seems to be 1. It seems that when looking over my left shoulder, whatever the furthest point my left eye can see should be roughly the furthest point my right eye can see when looking over my right shoulder (this certainly seems to be true for me IRL with little or no upper body twisting, although as I say this difference may be explained in Arma by the fact that he's holding a weapon in his right hand, even when it's lowered, or due to the weird head-stalk thing allowing him to see further behind him when looking left. Not as big an issue as I previously thought then but at least I've learnt something about the way stances and movement affect my view

It may be that, as suggested, there's something wrong with my FOV, as other people have said they can see directly behind, turning in either direction (backpack permitting) which doesn't seem to be the case for me. I did notice that a house looked smaller when it was in the centre of my view and got bigger when it was at the edges, which seemed rather weird!

I do wish that upper-body rotation would be added so that we can aim down our weapons more to our left or right whilst moving forward then we can at present, as it doesn't seem unreasonable to want to have weapons covering our 2 and 10 o'clocks without having to face in that direction and crab-walk sideways but that's a different topic and I only mention it because upper-body rotation could make the freelook feel a bit more realistic as well, although I accept that it might not be worth the effort if we can already see as far as we could IRL anyway.

Link to forum thread:
http://forums.bistudio.com/showthread.php?170042-Allow-player-to-look-over-shoulder