Arquebus + modal (+20 acc) is nice for alpha strikes like Assassinate + Backstab. Also with Assassination it doesn't hurt that much that guns have blunted criticals - because the +50% on crit from Assassinate and the +30% from overpenetration still work well enough.
Thankfully "+50% crit damage" actually seems to mean +50% total damage on crit, rather than +50% of the extra crit damage (which would be +12.5% total damage without blunted criticals, and 0 with). Hopefully that's intentional.
With Assassinate and Sharpshooter you get so much Accuracy and Penetration that it may be redundant in almost all encounters. (Except maybe to deal with Wizard's Double or Escape or other deflection boosting spells/abilities. Firearms should bypass the +50 deflection from Arcane Veil too.)
Firearms have reload rather than recovery, which means you can escape/stealth immediately after using them. Huge boost to survivability, especially since you have to get close to backstab with firearms. Arquebus is better than blunderbuss or dual-wielding pistols because backstab and assassinate only apply to the first attack roll (so for blunderbuss, just the first projectile).
Even if you don't one-shot, wounding shot will let you do good DoT. That will also give your pet a damage bonus against them, and the pet iirc has a special attack that can interrupt spells/abilities.
Edited by SaruNi, 20 April 2018 - 11:33 AM.