That is, the Charged Accelerator Gun. While not so great on it's own, it's a required component for other weapons like the High Velocity Pistol and the Rifled Cannon (Schreck's Pistol and Bronwyck's Gun were disappointing, to say the least). Everyone says the schematic shows up in random tech shops, but I've literally aged my party two game years advancing time to refresh the shop's inventory and teleporting around Arcanum to look for the damn thing. Is there a guaranteed location, or in other words, a chest or person's pockets that will most definitely have the schematic the first time I look in it, and not just a higher chance than most other places?

I don't think so. I'm pretty sure the only way to get it is to build it from a bought schematic. You really do just have to camp the tech vendors unfortunately (both the gunsmith and the normal technologists).

Though I have never found any of the guns particularly good other than the hand cannon, looking glass rifle (especially before the patch; ahh, those were the days), and droch's warbringer.

The best ranged weapon I have (and I have the Grenade Launcher, Blade Launcher and Tesla Gun) is the Aerial Decapitator I got from the Old Blind Master they ripped right out of "Master of the Flying Guillotine". Actually, the Tesla Gun's on par with it, since it bypasses Physical Damage Resistance, and the Aerial Decapitator only has a Range of 8 (though it seems to score more Critical Hits on Called Shots to the Head).