2. Cut down a piece of 1.5" thin-wall PVC (or anything else with an ID larger than your spring), and trim one end so that it can fit in the stock.
3. Shove a spring in there. I used a full AR-15 spring.
4. You're done!
With a really short Stefan, I got around 30-35' flat. Now all it needs is a better barrel. I'm also really liking the extra weight behind the handle.
The parts:

The notch:

The finished product:
