I've looked at that guy before and it's the closest I can find But I'm not a fan of that model.
also It looks more like a handgun than a pistol to me
also, I should mention I'm aware of this model:
but again, I'm not a fan. I think he looks to bland for an assassin adept
some more searching revealed the existence of a Black skaven with Pistol, http://www.solegends.com/citcat2000/c2000p365-01.htm
again the model doesn't look menacing enough to be my warbands leader and it's pretty much a regular pistol. the search will continue...