This is what I am thinking. Grab 1"-3/4 PVC reducer X2 (This may require some sanding to the inner portion of the gun to accommodate the larger outer diameter of the reducer).
Place one reducer right in front of the cylinder, and another towards the front of the blaster, secured in place with your choice of adhesive or other means.)
On the front reducer, drill a small hole in the side, Use a tap and appropriate size threaded screw. For your friction lock. Now you can remove and replace the barrel any length you want.