If you intend on using sniper rifle in combat, invest heavily in interrupt reduction.
I don't know if it's supposed to use the endurance as soon as it is activated, but you're definately meant to activate [Sniper Rifle] and wait patiently until it actually fires. But you're the first person to mention it; so I'm guessing that it's a non-issue.
All the snipes are like this. Except stalker patron snipes where the patiently waiting portion is quite longer.
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I went throught the exact experience and thought process you did with this power. I ended up assigning it to one of the extra buttons on my mouse so that I can still mash away at the other powers while being careful to click sniper rifle just once.
I haven't used sniper rifle in a long time (long since respec'ed out of it), but if I remember correctly, it *DOES* drain endurance even if the power doesn't fire and you're interrupted before triggering it. Working as intended. And sniper rifle will cost you 14+ points of endurance so a few tries at activating it (and failing it) will drain your end bar quickly.
1. Don't use sniper rifle in the heat of combat unless it is a) moderately slotted for interrupt redx or b) you have good defense
2. Consider spec'ing out of sniper rifle altogether. You have far better options and in terms of overall contribution to dps, it's one of the worst, if not the worst, powers you can use.
All the interruptible powers work that way.
Try using Aid Other/Aid Self in the middle of combat or while running around. You won't have an endurance bar very long.
With proper Interrupt Reduction slotting, the interruptible powers are quite usable in mid-combat situations, but they do require just a bit more attention than the non-interruptibles.
Sniper shots in general are rather lack-luster while on teams. They do great while solo or for pulling purposes though.
Heck, my AR Sniper Shot (slotted with extra range and perma-boost range power) can hit mobs from further away than my computer's draw distance....(I can't even see the mob, just a tiny little targetting reticle, but I can hit 'em and take a big chunk of hit points away).
6000+ levels gained and 8 level 50's
Hello, my name is Soulwind and I have Alt-Itis.
I'm quite aware of the End cost of interrupting a Sniper attack with movement or because you change targets. My advice is if you are in a pitched combat, don't use Sniper Rifle unless you are sure the attack can go off before the target is killed, and pause for about half a second before beginning the attack. Do not move or do anything else until the shot goes off.
Honestly, I find this in concept with a Sniper Rifle. It's a sniper shot, it's not supposed to be "shot from the hip". You need time to settle your aim, choose your target, and take your shot. I know a Sniper attack can be used in combat, I do it all the time, but it's not really its intended purpose. It's primarily an opening attack, or an extremely long range shot you can use to pick off targets prior to engaging.
If you compare the End cost to the amount of time it takes to cast the attack, it actually is one of the more efficient attacks. The End per point of damage is about the same as all others. But because it takes so long to cast it, you have more time to regenerate End while using it. (Basically, its DPS is low because it takes so long, so its EPS is low, too)
It's definately a problem that if you're in a team, though, your allies can usually kill a target before you get a chance to get off a shot. I would use it only to attack Bosses or AVs, or foes that are clearly outside of the combat and causing trouble from the sidelines. (Like mezzers) You really shouldn't be using it as often as it's up. It's more a situational power.
Another good rule of thumb, in keeping with don't use it as often as it's up. If you have Aim or Build Up, you should never use a Sniper attack unless you build up first. This isn't an option with AR/Dev, but you do have Targetting Drone, which while it's not a big bonus, you should obviously never fire your Sniper Rifle if the Targetting Drone happens to be down. If you got detoggled, put it up first. And I'll add that the only reason I really have Sniper Rifle on my AR/Dev is that I don't like the idea of wasting that bonus. If the devs put it on anything else, I'd drop it immediately. (Originally he didn't have Sniper Rifle, he had Slug instead, but I essentially use Sniper Shot in combat like I said, so at least I get some benefit from a mostly useless buff 
)
Well thank you all very kindly for the replies! Even though I may not be replying directly to the comments, information and advice all of you left, directly and individually, I did read them, and know better how I should use the Sniper Rifle in City of Heroes. Mainly I use(d) the Sniper Rifle as a sort of higher-power Slug shot, in doing a bit more damage to bosses (slotted with endurance reduction and reduced interrupt time), but I suppose later on I'll be able to pick and choose more effective powers for that sort of purpose--especially when I get my first respec'. I usually save those until I'm around level 40, then I go and select powers in the way I'll most need them; I'll probably get Sniper Rifle after getting the more combat-centralized powers now.
So thanks again, and I hope other Assault Rifle blasters, and other heroes and villains alike, will drop by out of curiosity and maybe learn a little bit more about the Sniper Rifle and how best to use it themselves! Thanks!
Hullo! Exposition first, you'll find the question below the following exposition...
So I was blasting away with my (guess it!) blaster, when I eventually tried blasting with the Sniper Rifle power. Within the narrow confines of a sewer, and the devastation that my full team was causing, I grew quite ignorant to the damage my attacks were causing. Lo! that suddenly I run out of endurance. This surprised me, as it seemed that before I had been nearly at full endurance. As I gobbled up a Catch-A-Breath, I thought of what it might have been, when my suspicions fell upon the Sniper Rifle power. My technique when in the middle of combat is to repeatedly press my attack keys until I'm certain that that attack had been activated and used. I perform this technique even more so on Sniper Rifle, which has a tendency to be interrupted quite frequently by moving slightly, or being hit, etcetera--as it normally does.
Understanding this however, I took to being somewhat more sparing of the ability--until quite suddenly, I ran out of endurance again. This led to my being completely baffled! My sparing use of the Sniper Rifle meant it was certainly not hogging my endurance so much as say, Grenade Launcher or Flamethrower. I therefore watched my endurance bar very carefully the next time I used it, along with my other powers.
Well. It turns out that the endurance Sniper Rifle uses upon being activated takes place immediately. This means that if you switch targets and activate Sniper Rifle again, it will drain an additional endurance cost from your bar. If, as I was at the time, you're involved in a fierce melee in which most enemies you target are defeated within a mere few seconds of being aimed at, and you use a power before and after the current target is defeated, then when the next target is auto-selected, it'll activate that power a second time and in the case of the Sniper Rifle, drain you of endurance a second time; whether the shot was completed or not!
Now I tried a brief search on "Sniper Rifle Endurance" and didn't come up with anything relevant to my question, which is this:
Is the [Sniper Rifle] ability supposed to drain endurance if used in this repetitive manner? The method is as follows: you use [Sniper Rifle]. The endurance cost is automatically deducted from your endurance bar. You repeatedly activate [Sniper Rifle], and select a different target whilst still activating the power. This will cause an additional endurance drain. If you continue in this manner, selecting a different target each time you activate [Sniper Rifle], then even if you never complete a Sniper Shot, your endurance will still be drained.
My continue-mashing-super power-button-until-cast strategy may not exactly be how super powers were intended to be used, but is that endurance drain caused by repeated use of the [Sniper Rifle] power really meant to be? For comparison, I harken to [Aid Other]. That too is a power that can be interrupted, and that I also mash viciously when activating, to make sure that my target is healed. But I have never had it drain endurance in such a way as [Sniper Rifle]. I think this is because endurance is only drained from [Aid Other] after it has healed the other player however.
But what are the opinions of you other super-folks out there? Is this a minor bug? Is this intended? Is this already a topic that I should have found instead of making this novel of a topic myself? Or what are your own opinions, maybe even on my super-hero'ing strategy of 'mash and bash', as I think I'll call it.
Thank-you for reading, I hope it wasn't too long a trudge to get here!