There are others that will have more experience and probably better advice. However, after not playing these types of games and certainly not multi-player there are a few things I needed to learn. As an aside; These are the worst Sniper Scopes in the history of mankind and Dust is thousands of years in the future. :-P

I have used scopes, not military, but honestly I am not a great shot yet I have never seen a real scope act like they do in games. And the Sniper Scopes in Dust are pretty bad on the comparison stick. And we cannot go prone either. Nor lean on a railing or up against a wall.

Enough ranting, how about some helping?

First, if you are using an Assault Rifle (AR) then you get iron sights until you cough up the experience for Tactical AR. From what I hear, seeing as how I am not there yet. Iron sights take a little more time and some particular practice. Always sight on the target and always aim for the upper chest then up to the head. You can practice shooting at arbitrary things and see where the bullet hits versus where you were aiming. Shooting from the hip will get you noise, a few hits and then a lot of kick back. Then no more hits, or at least not enough to take them out. Once you get the feel for them they shouldn't make a big difference. If you are hitting the target their shields go down first, the fun part is next! Then you can start aiming at the head first.

Sniper Scopes need the Light Weapon Sharpshooter perk to high levels and the Sniper Rifle one as well. Proficiency gets you access to the next step in rifles but it takes awhile to get there. I have one character leveled up and the others are not. Frankly I get some pretty good WP from paying attention and not trying to force the rifle to do something it doesn't want to do.

Now to aim a Sniper rifle, squat down with triangle button. Take your time. Don't, don't, don't try to follow the target. Find a target that is moving in a line (hopefully away from you) and maybe even focused on somebody else. Then move ahead of them slightly and squeeze the trigger. It takes some practice during which you will get killed. A lot. More than that sometimes.

These cloned mercs move fast. They love to jump. Watch their patterns and as they cycle through them aim at where they will be. As they approach, pull the trigger. Repeat until your are good at it. This is the opposite of the above, wait for them to come into the site. It can be a lot easier. Snipers can see across the battle field (at least it seems to me). That means the buddy of the two mercs you just took down is now absolutely certain you need a brand new clone. Move.

There are good snipers here that will give more fundamental advice; find a good spot, cover, not open, clear exit path, don't obsess about one target. Take the shot and if you are noticed take a hike, out of sight and live to shoot again.

Read some of the mercs helping mercs posts here in the training ground.

The one that sank in for me, from reading several of them, is to try to flank everybody. I wish I would remember that but too many years of playing monster killer and charging in are still frustrating me. Don't take a Heavy head on or even where they can get a clear shot as they will take you out. Use cover, a lot of it, grenades and crouching are all good. Oddly enough this advice works for many situations.

This last piece of advice is via the British Navy. When in port, always top off the fuel and ammunition. Not doing this sank the Bismark. At least contributed to it. Once you expand your Light Weapon Capacity you can start having 5 rounds in the clip. Which you shouldn't need. After four you should be moving. Get out of sight when reloading and please don't stand up to do it (unless I am aiming at you, I need all the help I can get).

Have room for a Hive? Carry it. Toss it down in a secure location and don't run out of ammo. Most of my Sniper Fits carry Hives. They work very well for backing up a Heavy with a HMG and sometimes the squad. You both have full clips and everyone is focusing on the HMG while you can thin out the herd from above (or the side). And keep him from being flanked.

The last piece is be aware of your silhouette. That is how I find Snipers and with the last (great) scope on the AR you could easily pick them off. If you are standing up like a good stick man against the sky then, yes Martha, Santa has just given them a present. Your head. It is how I lose mine as well. We also show up on the Scope nice and clear with little red letters that explain the urgency of killing us.

Good Luck and practice, practice, practice.