Considering installing a new breech, because current modified Unknown breech has the following shortcomings:
1. When the dart is inserted, it must be pushed inwards a little bit to create a seal with the barrel.
2. 1/2" PVC doesn't hug the dart too closely.
3. The seals on the breech are a tiny bit too tight. That makes for hard reloading.
4. The hole through which the darts are loaded is just terrible. :P No possibility for interfacing with a gravity-fed magazine as formerly planned.
Reason #1 had the most impact, as a breech was supposed to make loading easier. It's hard to do something that exact while running and dodging darts and tracking targets at the same time.
So back to the drawing board. Partially. Air tank and hose nozzle seem to be very effective, though. On retrospection, picking a small air tank to be refilled after every shot was a good choice, as the hose nozzle had only two settings: all sealed or all released. No way the airflow could be controlled consciously.
So new design: If 1/2" CPVC fits smoothly inside 3/4" PVC, then I will cut a semicircular cross-section off of the 3/4" PVC for a breech, and "cone out" the inside of the 1/2" CPVC. Because the hose nozzle requires a 1" PVC pipe to attach to, I'll need a 1" --> 3/4" PVC adapter as well. Back to Ace Hardware, hopefully for the last time.