Well, Im not a beta tester and dont see topics explain how it works.
First. The Schawarzer Reiter use handguns has main weapon, so, all skills learned when you was an acher are not able to use with handguns ?
Second. On ToS the classes equiped with bow use strenght as main attribute to increase the damage. Thats the same for handguns ?
Look leaving the logic, on bows if you tension more a shooting string more cinetic power you have. But in a handgun strenght increse damage… Is kinda wierd.
1: If the skills say that it has a required stance ie: Two handed bow or One handed bow, you can’t use it with pistols. It’s weird to have arrows coming out of a gun anyways. But this limit is alleviated with weapon swapping, an attribute you can get from any 2nd rank class master IIRC.
2: Yeah STR is the main damage for all physical attacking skills. And for guns… let’s just put it as magic or being able to pull the trigger harder… 
Maybe holding the gun for less recoil because ME STRONK? I dunno 