Building a cold sniper

[King of Nowhere]

I'm preparing a campaign world with an industrial tech level, they have some decent guns, since i didn't like the pathfinder gun rules i made my own (tldr, i swapped out the touch attack thing with an "armor penetration" factor that in most cases can be considered as a flat to-hit bonus, and i removed misfires). in particular i have made the sniper rifle with a special ability that can have interesting applications in a build

Sniper rifle special rules: sniper rifle are big, heavy and very hard to point at close targets. Their recoil also make it near impossible to achieve a quick rate of fire, even with magic-assisted reloading. Their scopes can be used as spyglasses.
Using a sniper rifle at less than 30 meters of range against a moving target (one that is not flat-footed or immobilized somehow) entails a -4 penalty to hit. Using it in melee entails a further, cumulative -4. The recoil from using a sniper rifle entails a -4 penalty to all eventual subsequent attacks made in the same round. Using a sniper rifle to its full range incremement requires taking a full-round action to make an attack; otherwise, use the range increment of a regular rifle of the same size.
If a character who is proficient with the sniper rifle takes a full round action to make a single attack, she can apply any source of precision-based damage (including, but not limited to, sneak attack and death attack) regardless of distance, if the other conditions for applying that effect are met. In addition to that, if the the result of the hit roll beats the target AC by more than 20, the hit is authomatically a critical.[and it has critical x4]

I have ideas for at least one npc sniper that may be either an enemy or an ally depending on the players choices, and i think a sniper that actually works can be an exciting and interesting encounter.
The party is walking down the road as the wizard is suddenly shot through the head. quickly the cleric revivifies him, while the warrior drags them both behind a wall as another bullet sprays a fountain of dirt just where the cleric had been a moment before. The rogue pulls out a spyglass, but only has a few seconds to look around before the spyglass is destroied by another bullet. For now the cover holds, but the attacker is surely moving around to get a clear shot. where is the mysterious shooter? how can the party locate it before being picked off one by one?
it's the kind of encounter that would make the players go all creative. something rare at high levels, when players have the magic and stats to go through most problems by brute force.

anyway, i want to build a high level npc sniper taking full advantage of the sniper rifle special rules.
now, the only thing that comes to my mind is to take 10 levels in assassin and maximize death attack DC. not only it's a good source of save-or-die, but some assassin spells (nondetection, greater invisibility, dimension door to shift position) would also help staying hidden.
on the other hand, looks too much like a one-trick pony; and average BAB means less chances of getting that juicy free 4x critical for overshooting the AC by 20.

so, i'm turning to this forum's creativity and greater knowledge of obscure sources. how would you build a sniper to take advantage of the sniper rifle special qualities?

[Venger]

Very hard to improve on perfection: Tippy's bird of prey (https://forums.giantitp.com/showthread.php?286818-Bird-of-Prey-(Tippy-s-Awesome-Flying-Sniper)) an apex predator. He'd be even better with a more powerful gun.

[SangoProduction]

Take note: Snipers are one of the most obnoxious of possible enemies, in any genre, because typically, the counter play comes from knowing their position, and how you are supposed to approach...which, obviously, you don't know the first time through.
It's less easy to "learn" the way to approach things, when death is anything but ephemeral. And becomes nearly insurmountable if the sniper is properly optimized (and worse, optimized against the party), and given omniscience (either via scrying or ....being a DM's npc), and gets to act intelligently.

With all that said, let's get into it.

There are a couple parts of the sniper kit you need:
1) Sneak attack (because the weapon you want to build around)
2) Mobility (Shoot and scoot is a thing for a reason)
3) Visibility (Preferably by a spotter, so that no resources are spent by the sniper)
4) Ability to hit (which doesn't come naturally to most sneak attack classes)
5) Range. (because duh. Also helps stealth.)
6) Stealth (also duh)

1) Obviously, rogue-types get the sneak attack that you are basing your build around, so let's just take it as a given that you will fill out the build with sneak attacks.

2) One level of barbarian (preferably with an ACF / archetype that changes the rage strength to dexterity). This is because movement speed. Then take any other dips that grant movespeed.
Then take the Run feat. Also take the Endurance feat. These two feats, which players never take, ensure that almost no character can catch up in an extended chase. After he gets distance, just plink away, and run again.
Back it up with a Boots of Springing and Striding.
And a wand of expeditious retreat, in case they have the slightest chance of catching up to you.

You could also instead just...teleport by some means, but does soak resources (either build or gold). It also doesn't tend to get as far as running, unless you have terrain obstruction or it's a high level spell.

Having flight could allow you to ignore terrain obstruction

3) Hire a scryer. With telepathy. Train for years to coordinate with each other. Sure, the scryer could probably also just scry and die on his own, but that's not the point.
Alternatively, you need a way to reduce penalties from distance to perception. The scope's spyglass function works wonders, as it doubles the effective range of your perception check. Since the sniper is presumably starting the fight, he can effectively just take 20, and from that alone, he can see a DC 0 at 400 feet before bonuses, ignoring obstruction.
At 400 feet, you can get off 2 full round shots before anyone with 30 ft speed can get anywhere nearby, and 3 shots if you want to give them a chance to see you.

If you can get the 138 perception that comes up in the thread I link below, then that becomes 2760 ft...at which point, I'm pretty sure, just because of D&D ranges, unless the weapon has a significantly greater range increment, will simply never be your bottleneck.

4) Weapon focus feat. Able Sniper feat (in 3.5, Races of the Wild). Magic enhancement modifiers. Dexterity improvements.
True Strike wand.
Something to somehow make you target touch AC with attacks.
Ability to reduce miss chance / cover / concealment. (True Strike wand or a number of classes / feats / or magic enhancements)

5) Far Shot feat.
A couple of the classes that focus on range also work (such as Deepwood Sniper in 3e), though it takes resources from sneak attack classes.

6) I have an entire build linked in my signature related to getting as much stealth as possible. Sure, it uses SoP, but you can replace most of the SoP spells with Greater Invisibility and get basically the same result.
https://forums.giantitp.com/showthread.php?553522-Stealthiest-Stealth-that-ever-Stealthed
Came out to a max of 165 on a natural 20. And I think I reduced the sniping penalty to -10, rather than the normal -20. But that's kinda pocket change at this point.
Definitely different if no PF material is available.
With enough stealth, you can just keep sniping, and there's bugger all they can do about it other than mass fireball, or its equivalent.



Also, special shout out to Master Sniper feat, which, while referencing a completely different "sniping" action (the one in PF, and thus why I think this is a 3.5 question), it does specifically say that normally you get 1 shot of with a full round snipe, but now you get 2.

[zlefin]

The combination of True Strike, plus something to bypass armor/target touch ac, like Brilliant Energy waepon, should yield a near-guaranteed hit w crit.