Finally built one. Now this is for a special R&D project that isn't going so well. The rotating breech made out of a drain leaked a fair amount of air. Thought this would solve my velocity issues but it didn't. The gate valve is air tight though. I used the 3" gate valve for two reasons.
1. All the 2" gate valves were an little undersized. You could Squish a Nerf into it without too much trouble but I couldn't find one that did have clearance. Guys have posted they found one that did but I have been in a dozen RV parts places between Texas, Oklahoma, Arkansas, and MO and haven't found one. So have a nerf with you when you shop for one.
2. My special project required an air chamber all the way around to feed air into the center 2" barrel tube. You don't need that.
You will notice I trimmed the female fittings of both the gate valve and PVC T fitting so the inserted Nerf bulb will be in FRONT of air inlet of the T fitting. Glued but also pinned with some small wood screws just to make sure I don't get a facefull of PVC. With a 2" T fitting, it is quite a bit easier to get the bulb in front of the air inlet.
Open Breech view with a Nerf loaded.
You can see the barrel section extends all the way back to the breech and for my special project FLUSH with the closed gate door. It was relatively easy to ream / trim out the ridge in the 3" x 2" reducer fitting in the front of the T fitting to allow the barrel section to slide through. Used a pocket knife and a band aid.
You can see the air inlet holes into the 2" barrel section. All behind the bulb of the Nerf.
Muzzle view with Nerf Loaded.
You might note the 2" breech barrel section is Schedule 20 Thin Wall PVC. That is part of the special project so use schedule 40 if you want.
This 3" thing is big, bulky, and heavy for a handheld. Below is the mockup using a 2" PVC T fitting on that shower drain. You can easily imagine how much smaller and lighter using a 2" T fitting would be. Use the 2" gate valve and it takes up the same space. If you want the clearance, put the 2" x 3" reducer fitting between the gate valve and the 2" T fitting.
Just so you know, I like detachable, screw on and off barrels. That is why it is so short-some assembly required. Handy for Transport and R & D projects. The 3" Gate valve was $20.
FYI, if you buy those 2" slip x 2" Thread PVC fittings, Take a Nerf with you. Some will pass a Nerf with clearance but other brands will not clear a Nerf. Look for the schedule 20 drain fittings.
Have fun, be safe.