I’ve been theorycrafting around this for a while and haven’t come up with much good.
First off, there’s the itemization issue: No legendary pistols deal acid, poison, or frostburn. Arcanum Sigillis only deals a small amount of cold as part of its elemental portfolio. No legendary pistols have +all nightshade skills or bonuses to any individual nightshade skill, other than the PBlades bonus on Demonslayer’s Life-Ender. (There are also no relevant MI pistols, and and only one relevant faction pistol.)
That’s not necessarily a problem. We can farm vendors for a Venomfang of the Defiler or a Frostborn/Glacial/Rimefrost of Shattering- or just use an Elite Death’s Revolver, or the few relevant blues. (Chillborer has no cold conversion making it questionable, and the blues situation for acid is pretty bad.) But the problems go deeper than itemization.
Why use ABB in the first place? What make it different from other spammable weapon attacks? ABB itself isn’t that great an ability- it’s got a high power cost on a short cooldown, which means it’s not really sustainable at ultimate levels, and tops out at 200% weapon damage anyway: it’s not going to light up the single-target DPS chart. Most people stop at 6/16 in order to get 100% weapon damage (for full activation of weapon effects like Viper) with a sustainable power cost. The real beauty of ABB is Lethal Assault.
Lethal Assault adds a bunch of flat and percentage damage to acid and cold, plus 100 OA (max 190 at 22/12), to all attacks for four seconds. In this sense it works very much like Cadence’s modifier Deadly Momentum. What makes ABB+LA interestingly different from Cadence+DM is that Cadence only continues to activate if you keep spamming Cadence, while ABB can be reactivated every 1.8-4 seconds while you use other attacks in between. So, the two main target audiences for ABB are players who have lots of WPS skills replacing their basic attacks, or, players who are weaving other (non-basic) attacks in between ABB activations. If you’re just going to hold down ABB LMB and you don’t have a bunch of cold or acid from passives or WPS, you’re probably better off with Cadence.
So, if we’re using pistols, we can’t use the Nightshade melee WPS line. (And soldier WPS have no cold or acid, so no synergy there.) So the point of pistol ABB is, or ought to be, to weave in other attacks in between activating ABB, in order to take advantage of Lethal Assault’s flat damage and OA bonuses. So, what should we weave?
There are basically no weapon-based attacks anywhere in the class arsenal that deal cold or acid damage. Arcanist has nothing weapon based except (kinda) CT. Occultist has nothing weapon-based at all. Shaman has no cold or acid. Demo has no cold or acid. Soldier has… transmuted Cadence, with a max of 17% conversion to cold. If we look further afield to item and component skills, we find either unsustainable energy costs, cooldowns, or low or zero weapon damage factors- nothing we can use which will be spammable and also leverage LA’s bonuses.
To me right now, it’s looking like a Blademaster with ABB on RMB and Cadence on LMB, is probably the best option for using ABB’s new dual pistol capabilities. Stack all the cold conversion you can find: Elite Death’s Revolver, Empowered Marauder’s Justice, or a Frostborn/Glacial/Rimefrost rare; Ellena’s Necklace or Night’s Embrace; and Deathchill bolts will get you to 35% conversion, on top of the transmuter’s 50%/3. Cadence’s weakness is lack of AOE while ABB’s is lack of single target, so using both together actually solves some problems neatly.