The one point that I want to make is to not trust the statistics. I constantly see playerse use weapons and not know what they are doing, such as those who use the RL but never use the right-click feature. There are many, I'm sure, who use the shotgun but don't use the secondary fire. This does not mean that the shotgun isn't too powerful, for those who know how to use it. The stats don't always tell the whole story.
"I don´t like the idea of having to stand still while using the nex. You would be easy prey for the Mortar or RL.
Well, that's sort of the idea. You have to have some sort of downside to using a weapon to create balance. If you are going to establish a weapon as a sniper weapon, that weapon would be both powerful and accurate for long distance. Such a weapon needs to have compromises or it will throw the game off.
I suggestion splitting the Nex into two weapons so that you could have a weapon similar to the existing Nex that wasn't as powerful, but you could also have a good sniper weapon that made you more vulnerable. It is possible that the solution provided by others might have a similar effect. I'm talking about the idea where the Nex is more powerful at distances, but less powerful close up. Personally, I don't think that covers all of the bases. I think the "close up" fighting should reload quicker. I think the accuracy when sniping should improve considerably if you are staying completely still.
For example, I sometimes play Ghost Recon Advanced Warfighter (GRAW) on the XBox 360 at my brother's house. They have sniper weapons in that game, but they make it nearly impossible to hit things at close distance when you are not looking through the scope. The idea is to balance the weapons and keep the sniper weapon from being too powerful. Personally, i think they go a little overboard in GRAW with sniper difficulty at close distances, but the premise is a good one.