For one thing, I notice that for some reason the Blackhawk has a longer average range than the Silversix. For another thing, it does worse damage than the Blackhawk, even against enemies it's supposed to be good at.
A Silversix does less damage to a Construct than a Blackhawk would to a Jelly, for example.
And aside from the undead, there's very few enemies in the game that are weak to Elemental and/or Piercing and not resistant to one or both. Not even the speed of the weapon can make up for that.
Blackhawk does more damage (to the enemies it is good against) and has more range, and it's supposed to be the equal opposite of the Silversix.
I also feel that, even at their current weapon speeds, the weapons are somewhat underpowered. Both of them. A Silversix takes down a Construct slower than a 3* Alchemer.
The reason the blackhawk does more damage to jellies than the silversix does to constructs is that not all monster "weaknesses" are equal. Jellies take an inordinate amount of shadow damage. Try comparing the silver six's construct damage to the blackhawk's Gremlin damage and you should see the numbers are pretty much in line.
As for the range complaint, I think it's somewhat unfounded. The entire Antigua line has godly range compared to any of the other guns including the Magnus/Callahan, which confuses the heck out of me considering the Antigua guns are the "well rounded" ones and aren't really meant for people who use guns as their primary...