You right about psionics needed for pistol build, it's a sideways solution. When I saw how Dead Eye would/could seem to work(a lot of shots in very small time), I thought it would be so cool to include Temporal Manipulation as they ""realistically"" per say, fit together(like seeing time stopped haha). I didn't mention it as to say pistols should become entirely psionic builds to be more effective, but rather spewed a fun idea I had that could be incorporated for flavor.
You're right about per/dex for pistols, but don't forget every weapon build's requirements first! For starters here, smg to an extent is per/dex too; heavier weapons require high str- Definitely don't forget that str requirement. Also Snipers are stealth(agility-eske) + movement + perception, Crossbows really benefit from Deadly Snares and are arguably definitely Per/Dex as well for the full-leaning, high value crossbow builds. Sledgehammers need high str and really benefit a lot from high con + heavy armor for full stat-synergy and tactical necessity(sitting ontop of enemies? better not die instantly man!).
Pistols being dual-stat required isn't directly a problem. It's that pistols aren't damage-to-ap efficient, comparatively to other attack strategies. But pistols(irl; peering at the accuracy, or objective look) are weak weapons, an assault rifle is used in military for standard combat use because of it's advantages. The pistol user must have some advantage from skill or it just doesn't fit right imo.
Pistols compared to Smgs, pistols have way more ap-cost than base-smg attacks but also do more damage(which honestly makes sense but makes pistols feel even weaker, even whilst smgs are also weak like pistols though- as they are hurt more by flat-reductions immensely, off-topically?).
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I still do think feats are the right direction for pistol builds to make them "a self-complete build" as otherwise they would be too essential back-up weapons for all gun builds(even though that would be thematically perfect and ok too?).
In my opinion, an alright start for pistol users is a feat that makes Pistols get increased critical chance based off of your dexterity just like melee weapons do, in addition to the already included light-melee and ranged weapon's ap reductions from dexterity.
Another feat could be added I give would be one that builds off of and requires Gunslinger could add some bonus(maybe just +15% flat universal) accuracy for firearm pistols, separating the build from Smgs which are usually close-range, burst-fire. They would then have very accurate, trusty but steady and slow damage. I know this is just a numerical upgrade but it doesn't feel out of place.
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I also feel that Steadfast Aim is very underwhelming. First off you need str 5 on a build that would be assumed to not take much benefit from being above the 3 minimum..(that's what pistols are like, no big requirement right? Not even Plasma Pistol nor Chemical Pistols needs a Strength score). A single extra ap-cost on a weapon is pretty large and significant in Underrail. This feat here then gives HALF a crit chance per ap above 10, which is basically nothing. At 20 ap(most expensive AR shot base is only 16 ap during single-fire...) you have +5% crit. This feat wouldn't be too strong even if it was just giving a flat +5% crit for firearm pistols (ignoring the alright position of chemical-pistols at least for damage potential, energy pistols can scale off of Int too). If this feat was for firearm pistols only, it wouldn't even be insanely broken to give +10% crit per ap above 10. Critting on a pistol shot for a .44 hammerer is nice, but it still isn't as good as just shooting 3 times(not even bursting) with an AR, if considering just damage in a vacuum. The fact that the AR can burst for 7 shots- or maybe 2 bursts for 14 just blows any pistol out of the water, but for higher requirements, of course.
A better course for Steadfast Aim imo, is to just make Aimed Shot available for firearm Pistols once every round, which then still benefits more heavily to the heavier pistols, just like the feat already does.