Take away the screws, and you get the crappy internals - the plunger is tiny, and largely useless for any real water blaster fight. Ditch the tubing and slice off the front of the PT.
The PT is actually the perfect size for putting PETG barrel tubing into. Take a 1" long piece, and super glue it into the end (but only a little bit). I wrapped the gap in E-tape for extra security.
Note that I've removed the o-ring from the plunger. Since we're going to directly push the dart, a seal no longer matters. Speed is the goal. By the way, go ahead and remove the front ridge that formerly retained the o-ring. Otherwise, your plunger will attempt to jam itself into your PETG dart holder.
Right now, after removing some of the plastic at the front of the blaster, I'm getting 15-20ft. ptg out of this blaster. Still not that impressive, but it's better than before.