Start with 10k Firearms for that perfect Glass Cannon power, then add 6 piece Nomad to pour a protective layer of adhesive over the whole mess. You can't facetank a whole squad of enemies, but you can stand in the open at distance long enough to click a few heads while taking incoming fire. This build isn't up to soloing heroics - too many rushers and you melt no matter how many "get out of death free" cards you have - but anything under that and you should be able to breeze through... so long as you can land headshots.
If you can land headshots, this build excels at dropping tough enemies before they can really even set up. Named can drop in 2-3 shots, while non-tank Elites are usually one hit kills. Red bar enemies don't even need headshots to go down in one most of the time. Veteran enemies on the other hand almost always take two shots simply because there are only so many ways to add bonus Enemy Armor Damage.
Most times the stats on the gear is more than enough to let you one shot enemies, but if you need a little extra this build has several options that'll get you just a little bit more so you're hitting for 102% of the enemy's health instead of 98%. Booster Shot and Pulse are obvious, but Tactical Advance should not be overlooked: even a 5 meter cover-to-cover move adds another 10% damage to your shots. Most times you won't need it, but when you do need it the difference is pretty huge.
Oh, and if you do want to play a little recklessly the high Health on Kill means you can, so long as you know you'll be able to secure kills. This build is set up around sniping and staying at distance, but if you need to run down enemies up close and in person you can. Just don't expect to be able to take point as a regular matter of course.
Although this build is aimed at PvE, it isn't completely useless in PvP. If someone tries to jump you in the DZ you have a fair chance of coming out on top if you play smart, but you'll probably use up your Nomad Cheat Death proc and Recovery Link in the process. Switch to a different build if you intend to throw down past that.
Nothing complicated in terms of set pieces: you want 6 pieces of Nomad Classified all rolled for Firearms. What's important are the major and minor stats; in order, you want to prioritize:
Anything beyond that is not critically important, though the other stats I've picked will help noticeably in one way or the other.
The Gear Mods
Pretty straightforward: 4x Pulse Duration, 5x Firearms Damage to Elites. If you expect to be using this where Elites aren't as common you can sub in a few Firearms Skill Haste in place 1-3 of the Firearms DtE mods, especially if you can get some in the 265 FA range. At that point you're talking "fraction of a percent" differences though.
Also, if you can't get 4% DtE mods then you will probably be better off doing 3x FA DtE and 2x FA Skill Haste.
Primary: Classic M44 Carbine. This has the highest per-shot damage of any weapon in the game, and fortunately it's craftable. Getting the right talents on it though... well, all three talent slots are open, so make sacrifices as necessary to appease RNGeezus. You need Destructive as the free talent, and an M44 without Brutal is just not worth using for this build.
Do not try to roll an M700 as an "easy out" to getting the right combination of stats since it has a fixed free talent. For one, you're always going to have to reroll the free talent, so that doesn't help. For two, the lower damage doesn't seem like that much of a damage difference, but the M44's damage is just barely high enough to one-shot some of these enemies even with all of the associated damage bonuses. The M700's is just barely not high enough.
Secondary: Bullfrog. You could go with a Lightweight M4/Liberator if you're more comfortable with that, but the only way the LWM4 wins out is if you can land almost all headshots with it, and when you wind up switching to your secondary is often when things are a little too close and/or frantic to really be placing your shots that carefully. You will wind up using it at midrange from time to time though, particularly in the Underground, so the LWM4 is not a bad substitution. The House isn't a bad choice either, but personally I find the lack of inherent Enemy Armor Damage means enemies die a bit too slowly.
Sidearm: 93R. Three-round burst means you're more likely to land headshots and trigger Precision to pulse the enemy you just shot, but that's not vital. 9 times out of 10 I use my sidearm to destroy traps and such, so really just use whatever you like the most.
Booster Shot: A great way to bump your outgoing damage up just enough to secure a kill, and it has the advantage of giving you a little Damage Resistance as well. You'll wind up using this as much to take down heavier targets as you will to keep yourself from getting killed, and with as little health as the build has even the low heal power is enough to get you about a third of your health back. The Damage Resistance is a decent amount, so feel free to use it on teammates who are in a rough spot or are taking point regularly.
Pulse: I could compose a sonnet to how wonderful Pulse is, but I think we all know at this point. Tactical Scanner is my default for when I want to maximize the damage bonus and don't care about the lower crit rate, but I'll often run Recon Pack for the 20m of extra range, particularly in Underground.
Adrenaline: You could swap this out for Critical Save to ensure those panic Medkits keep you alive, but personally I find Status Effect Immunity more useful on average.
Tactical Advance: A potentially huge damage boost if used well, but one of the nice things about this build is that you typically don't need it. When you do though being able to reposition and get a damage boost as a side effect is one hell of a nice perk.
One is None: Utterly vital. Although the cooldown on the talent is a few seconds, if you're placing your shots carefully it'll often come off cooldown just in time for you to land the next headshot. With the low magazine size and long reload on the M44 this can really make the difference on your DPS.
Precision: A great way to ensure that if you're playing peek-a-boo with a particularly shy target behind cover they won't disappear on you. The bonus damage from them being pulsed also means that if your first shot just wasn't quite enough to take them down, the second one almost certainly will be.