Those pistol crossbows use
very light darts. I'f I were to make my own darts for it, I'd use a 3/16 dowel and cut them down to 4in with a gradual taper towards the point and very minimal DT fletching.
For a crossbow to be effective they have to ge very strong, 30lbs is quite low, 500lbs and even more is not unheard of in armor piercing crossbows.
Most hunting recurve crossbows are around 200lbs with a 16in power stroke.
The pistol crossbow I belive you are reffering too is mostly a toy. Treat it like a weapon, but it won't have that much KE.
If I were you, I'd save your money and buy a real entry level xbow. If you have your mind set on a pistol design, this wouldn't be too bad:
http://www.thecrossbowstore.com/150-lbs-Hunting-Crossbow-Real-Wooden-Cross-Bows-...