The tip about finding out where the centre of the screen when not scoped is a really good one. One thing I would suggest is not to get obsessed with headshots. I think Bungie did an interview with kjhovey where he talked about the value of body shots. They still do a ton of damage, especially with a high impact sniper. If your enemy is already weak, you may not need a headshot. I would also recommend finding a sniper you like using and sticking with it. Since Weyloran's March dropped for me I always use it in PvP. Only does 250ish to the head (low impact) but it handles great, very little recoil and good rate of fire.