Well, xbow build can be pretty damaging, i easily reached 1k+ damage on crit with it.
Sniper builds are pretty straightforward and easy. You will need 7 Int, 10 Per (at start, max it for more damage), 5 Str (to actually wield sniper rifles) and 7-10 Agi. Feats: Nimble (so there would be no penalty to stealth), Aimed Shot, Snipe, Gun Nut, Sharpshooter, Critical Power and possibly Blitz. You, of course, will need to max Guns and Stealth skills, stealth is needed because it increases damage from Snipe ability. Mechanics is necessary to build a good rifle. For all your rifles i recommend Anatomically-Aware Scope to increase your critical hit damage and Smart module to increase damage from special attacks. You will need to craft at least two rifles, one 12.7mm using Corsair or Harbringer frame, depending on which you will find with higher quality and one 7.62 Spearhead with smart module and rapid reloader so you can shoot twice in one turn.
To further increase your damage, wear Smart Goggles and stealthiest outfit you can find (Ninja Tabi, Leather armor with soft padding and black overcoat, Tactical west with black overcoat, etc.).
To deal with enemies with high mechanical resistance, buy/craft W2C ammo when you can. JPH is good for enemies with low mechanical resistance if you want to increase your damage even more (which isn't really an issue at this point).
I would recommend to have some sort of sidearm (AR, pistol or SMG) to use when enemies got to close. SMG is best in my opinion since you have free feats to spare and with Commando + Spec Ops, SMGs will have best damage per action point ratio. You can also craft silent 5mm SMG when you want to kill your enemies one by one. Remember that Burst is a special attack so Smart module will increase damage form it.
With this build you can pretty much one-shot anyone in the game with Aimed Shot or Snipe attacks (sometimes you will need W2C ammo for this though).
