Re: I heard something about an impervious fortress...
Well, I know Hand Cannons (and really most firearms before the invention of... those lines inside the barrel, the ones that make the bullet spin) were grossly innacurate, thanks also to poor quality of gunpowder at the time.
But I am not a proponent of weapons that have an innate attack roll penalty on them even for proficient users. I dunno, it just doesn't seem right to me. Specially since, given the weight of those weapons, plus their ammo, most would-be users would have to sacrifice some dexterity (considering point buy and on character creation) for additional strength.
I would say that most "common" users (those who aren't PCs or wouldn't go into the PrC) just wouldn't take the feat, sacrificing accuracy for the greater damage, and maybe compensating with ranks in Profession.