Cut some holes in some of the pipes.
You can see the hole cut in the 1" PVC Pipe connected to the hose nozzle -- that's for that bolt over there to pull the barrel (the smaller PVC pipe) forward and back to reveal a hole for loading Nerf darts. That ring on the smaller pipe is for sealing the barrel mechanism as well as possible.
That hole there is for loading Nerf darts.
View of the barrel itself. I actually came across a problem, where the air would leak around the pipe. So I'm going to line the hole with electrical tape until it seals.