I did not expect this to become so popular a topic among Snipers on the TOR forums. The suggested build is usually some variation of 2/18/21 (with more points in Lethality, to take Cull).
The real problem is that this build does look so attractive on paper.
The people building it aren't totally off-base about "good abilities" to use, either. Cull, Interrogation Probe, Corrosive Grenade, Corrosive Dart, and Explosive Probe (with Cluster Bombs) are all strong abilities. They just happen to be the most expensive abilities in both trees (none of them costs less than 20 energy and Cull costs a whopping 30). All of them also have front-loaded energy costs. As much as I'd like a hybrid build to be viable, there's just no way to balance this kind of rotation in order to have enough energy to do good, consistent damage without being constantly out of energy (aka: it's non-viable).
This build could potentially work with EMP Discharge (though, at that point, you're probably better off being a pure Engineering spec). The real kiss of death is trying to use Lethal Purpose as a primary energy regeneration talent (you only have two abilities that count as poison damage, even as Lethality). It's not worth much even if you're being optimistic about your DoT crit rate.
Question: Is it possible to put enough damage and crit on a mid-level talent to make it better than a 31-point talent? Wouldn't being able to do this make a hybrid build viable? Fen, why do you hate hybrids?
Answer: I don't hate hybrids. Is it possible to make a mid-level talent "mathematically" better than a 31-point talent? Maybe. In a "game mechanics" sense, I would argue that "what's better" is situational (isn't it always?). The fact that it may be possible should not, however, lead you to conclude that a hybrid build that relies on a mid-level talent will be viable. 31-point talents (and the five points in the tier directly below them) are designed to help your specific spec balance resource management with damage output. Those talents are also where you begin to see specific playstyles take shape. I wish hybrid builds were viable, but they just aren't.
Hybrids are more viable for Operatives. They are bad for Snipers.
Let me emphasize that. Operatives have ways of regenerating energy with Tactical Advantage that Snipers do not. Stim Boost is why Operatives do not run out of energy even with an energy-intensive build (though they still may sacrifice damage to keep their resources up). Sniper has no equivalent tool and must rely on pure pacing and/or Adrenaline Probe.
Long Version and Hybrid Analysis from the TOR Forums
I felt the detailed explanation was worth reposting and saving here (so that I could link people instead of repeating myself).
I'll give you some background.
1. Gunslinger/Sniper combat mechanics and balance have been my primary testing focus for the last ten months. Three months ago or so, Series of Shots was still Marksman-only and Followthrough didn't even exist. Even longer ago, Entrench was also usable only by Marksman. Gunslinger/Sniper was a promising class without much mechanic synergy for about 5-6 months out of the ten I tested. Hybrid Gunslinger/Sniper specs have never been viable in any build I've tested -- and that's not for lack of trying! I've done some weird 12/17/12 builds that were "almost" viable last April but still clunky to use.
2. Lethality was made a tree "unique" to Sniper/Gunslinger only within the last three months as well, during the "renaissance" in AC development when Followthrough was added (but before the recent streamlining changes to talents and certain cooldowns, for example the reduction of Cull's CD -- it used to be 30 seconds instead of 9, big difference!). If you didn't play the class before the major revisions, I'm pretty sure it's impossible to appreciate how much the class changed in that short timeframe.
3. Engineering/Lethality is the only possible hybrid that could work right now, and even then the specs I've seen floating around on the forums are all sacrificing damage for survivability and the wrong energy regen talent (Lethal Purpose is your worst option, don't count on it ... when you need it, it will refuse to work). This doesn't make sense to me, as a Pure DPS-type player, because my opinion has always been that the best defense is a good offense. Spend too much time putting points into survivability talents and you'll escape just fine ... you'll just never kill anyone. Killing things fast, by the way, is the point of being a Sniper/Gunslinger in the first place. The thing about 31-point talents is that they complement the abilities the rest of your spec is using. I'll start by analyzing the most frequently proposed hybrid build.
NB: I'm of the firm opinion that anyone considering a hybrid spec should play a pure spec first to get an idea of the "baseline" performance of their chosen tree. It's hard to know if you're doing a bad job, or if the pure tree is more or less comfortable, unless you've experienced the difference. If and when I believe hybrid builds are viable I'll be the first to let you know. Right now Engineering is too energy-intensive a spec to tack onto Lethality (and the energy regenerated from poison crits is not enough, imho, to feed even the pure version of that spec that natively uses less energy than any other build!).
1. Lethal Purpose is the most inconsistent energy regneration talent in any tree. Lethality has low energy costs and I still frequently ran out of energy as a pure spec designed to use it.
2. You're spec'ing Lethality and then taking Explosive Probe ... useable only in cover. Lethality is designed not to use cover AT ALL. Ever. This means you are either taking a cover-spec (Engineering) and trying to add mobility talents to it -- or worse, spec'ing higher into a mobility tree and then essentially weighing it down with cover-based abilities that, if you were a pure spec, you would never need to use.
3. Cluster Bombs can wear off before you have a chance to blow them up. They're only detonated by blaster fire, not DoT ticks (check the tooltip, this is the kind of detail it's easy to overlook unless you play the class). This is why Series of Shots + Electrified Railgun is so good for Engineering. Series of Shots will detonate two charges and ... guess what ... requires you to be in cover to use it! The burn is really nice when you have Cluster Bombs on a target, too.
4. Hybrid specs are going to run out of energy really, really fast. In one rotation or less than one rotation (approximately six seconds ... I'd think you'd want to be viable for more than six seconds out of every 1:30).
5. If your heart is absolutely set on being a hybrid, drop Cull and take EMP Discharge in Engineering. Having Cull does nothing for you when you have no energy to use it (this happens more often than you might think). If your energy regen gets too low, you get stuck down there. And if your Adrenaline Probe isn't available you're SOL and it takes forever to get out of that hole (at that point, you're better off just dying to reset your energy pool, it's faster).
6. Let me repeat that a pure 31-point Lethality spec already has energy problems. You don't need to compound that with a very energy-intensive tree like Engineering.
7. Time to break out the "Energy Cost" calculator! Let's do a theoretical rotation of what you have to do to engage a target.
Apply your first two DoTs. Corrosive Dart costs 20 Energy. Interrogation Probe costs 20 Energy. Corrosive Grenade costs 20 energy. These are all instant cast abilities and can be used in rapid succession. You have just used 60 energy in three GCDs (over half your bar of 110). And you haven't done any damage yet.
Let's just assume no one is smart enough to cleanse your DoTs and that your DoTs are crit'ing every other second (which means you get at least two energy back from them every second ... which is being generous).
You take cover and apply Explosive Probe (20 Energy). Still no damage other than the DoT ticks (approximately 800 damage, assuming one or more crits every second in the time you've been waiting).
Your bar now contains 45 energy and you've got 3/tick energy regen (moderate/slow, assuming your energy regen is working, you're getting 4/tick).
Now let's break that Explosive Probe and do some damage! Lucky for you, Cull is a blaster ability and will break it. It costs 30 energy to use.
You now have 20 energy (again, assuming your DoTs are being kind) ... and are regenerating energy at 2/second. You have enough energy to use Ambush. Once. No, you don't get to use Takedown, because this rotation hasn't done enough damage until your DoTs have ticked the full duration (15-18seconds).
In my experience, this rotation will do about 8000 damage and put someone at roughly 35-40% health (assuming they have bad gear or aren't level 50). This means you still have 5-10% health to wait before you can use Takedown. Did I mention you have no energy?
If you want, at this point you can use Adrenaline Probe to kill your target. And you won't have enough energy, even using Adrenaline Probe, to do a second rotation. As a Sniper, you can't kill anyone in one rotation (from full health) -- ever. It's always "almost" one rotation. It's just your misfortune that your rotation costs enormous amounts of energy in a very short timeframe without a consistent way to get your resources back.
NOTE: During this timeframe, a similarly-geared and leveled Sharpshooter on the opposing team will now have had 7GCDs of time in which to do damage. I'm going to be as generous with the Sharpshooter's crits as I have been with your DoT crits (by the way, his energy regen is constant and doesn't care whether or not his abilities crit ... and they cost less to use!).
Our Sharpshooter has done the Sniper equivalent of Explosive Probe (1500), Ambush (2800), Followthrough (2000), Snipe (1500), Snipe (1500), Followthrough (2000), Series of Shots (3000) in the same number of cooldowns. That represents, modestly, 14300 damage. That's enough to about kill most level 50s, excluding tanks, and anyone even remotely squishy.
These numbers are based on my experiences at level 50 with half-assed gear and average damage -- without using Shatter Shot (which, incidentally, doesn't help your DoTs)!
The reason the proposed Energy/Lethality hybrid is so weak is that it relies exclusively on the most expensive abilities in two talent trees to do damage (anything that costs 20+ is expensive, and Cull - at a cost of 30 energy - is one of the most expensive abilities to use in any spec).
This is one of those posts I have a feeling I'm going to need to quote and reprint for a while.