This writeup was done post-assembly, so there aren't pictures for each step. I called this Mega seeing as it uses 2" PVC for the plungertube, instead of the usual 1.25" plungertube.
First, the catch
I chose to use a PVC ring catch, based off SGNerf's, so I'm not posting steps to make this. It's made with a 3/4" T and sanded out 3/4" PVC.
Next, the ends of the plungertube. These are made with 2, 2" couplers and 2" to 3/4" reducer bushing(for the back/plungerrod) and 2" to 1/2" threads reducer bushing(for the barrel) assembled like so
For the plungerrod, I used 1/2" PVC and a washer sandwich attached to an endcap
The catch goes into the back bushing assembly like so
And then attaches to the plungertube
The spring I chose was 2' of my K34. A piece is shown here
Here it is all put together
It got ranges of less than 70' with streamlines the few times I was able to shoot it before the catch gave up.
The threaded reducer bushing on the front allows you to put any fittings with 1/2" threads on.