Here is our prototype for shooting Frisbees. Our idea is a basic skeet shooter, still working everything out.
That looks like a fun idea. Have you looked into what would happen if your shooter was concave instead of straight? I think that geometry could get you a way to impart more spin (flight stability) and have it exit straighter.
It would be interesting if the arm had a curve, as mentioned above; think Jai Alai.
We are trying all types of arm. We have learned to much curve does not give a very good flight. Next we are going to add a little curve to it.
Have you tried using a larger sweep on your arm?