First question: Practice. There's a Sniper/Pistole Lab on right now so get in there. Just don't expect to headshot people who have been picking up the sniper from day one. With playing you'll get an idea of where people will be on the maps, keep this area in the center of your screen so you don't have to swivel your gun to meet them too much. Play Co-op, it helps with your aim for sure.
People like the G-Mal because once you catch the hang of it, you can be super effective with it.
sniping takes a lot of effort... but it seems for some people it can just be really easy to pick up....like me, i seem to always do well sniping, but i don't practice, i just seem to be able to do it...
I've got one: What does the G-Mal Accuracy mod actually do? Lowered recoil? Larger scope?
I think it's supposed to lower recoil, which helps at long range, but I've never noticed any difference in it.
And mine: Does explosive buddy reward you with any explosive kill you get? Like Hammer, RPG etc. or from grenades only? And is it possible to stack the grenades (same way like clusters in co-op) even if you have only one grenade slot?