Some guns are not really worth using most of the time, but I don't think Firearms is underpowered. You just need to use the right gun for the job, and you also need a katana to fall back on.
The Deagle and S-AUG are OP with a high Ranged ability, and the Jamie Sue lets you snipe 2 bosses from relative safety. The S-AUG is the best weapon in the game, it's deadly at all ranges, but it runs out of ammo fast if you use it exclusively - with 10 Ranged ability, the crosshairs do not drift while zoomed and the cone of fire while running is fairly tight. Most mini-bosses can be killed with 1x 30-round mag of 9mm, and some vampires can be killed with half a mag. Revolvers are great against zombies; pop their craniums and watch them experience critical existence failure. For ranges > point blank and < long, the Deagle will drop any 2x Medium-size opponents without reloading, including vampires.
The shotgun takes a long time to reload, so don't reload it. Fire 6 shots, then pull out something else. Reload between fights.