The layout is fairly similar, especially with regard to the valve and gas reservoir (which really are in the only positions you can put them in). I didn't like what I did in FANG 3, so I scraped the thing shortly after taking that photo. PVC seems to have avoided a number of issues I had by building the entire thing off a tube and using electronics. I considered a similar layout for FANG 4, but I'm going for something completely different at the moment. Watch FANG 5 be a copy of this.
PVC, if you ever have any questions about math, engineering, or physics, I'd be happy to help. I might not have the free time to help often as I'm practically in grad. school right now and I'll be officially starting grad. school next semester, but I'd still be happy to help you. (And yes, I am studying fluid dynamics. Interior ballistics is basically a subset of that field, so I'd be even more knowledgeable about Nerf ballistics during and after grad. school.)
I appreciate that; I'm sure I'll encounter many difficulties over the next five years. I'm taking the five year masters route which combines your senior with your first year of grad... with some overloading of course.
I just finished learning about the joys of sequence and series in Calc 2. Had I not refused my AP credit I would be in DiffEq right now but I think it was good decision. My calc foundation is pretty solid.
I owe the layout of this gun to good fortune. I was sitting around with a list of pipe diameters and a calculator trying to find a combination that would make a good hub+barrel assembly. 2" PVC and .625" tube came out pretty close to giving a 15-barrel assembly. Sure enough, I pieced together some actual tubes and they fit just fine.
From there I knew I would base the gun on a 15-barrel turret. The 2" PVC worked out nicely too because I could house most of the guts inside it. I picked sch80 specifcially because 1.5" PVC fits inside it well. The motor and regulator are housed in 1.5" PVC sleeves which then slide into the 2". This let me safely secure both of those with screws. Another benefit of the sch80 is the wall thickness- this thing is solid, those screws aren't coming out.
Like you said, there is really only one place for the valve to go. Originally though, the reservoir was a narrow line snaked through the top handle parallel to the supply line. But the volume was inadequate so I flipped it around to thread on that makeshift reservoir. I'll be switching it to copper when I get time.
The seal between the valve outlet and the barrels is made with some adhesive backed foam. It's all that's needed considering the low mass of the projectiles and the low/brief backpressure.
The grip is made from Fimo clay like others i've made before. It's pretty comfortable even though I made it poorly.
The Arduino circuitry is pretty simple. Three digital pins read the state of three switches (trigger, selector, limit). Two more digital pins send signals to two logic level N-Channel MOSFETs. These send power from an external battery to the solenoid and the motor. There are flyback diodes on both to prevent voltage spike(though I don't think the motor produces much because it stops dead in its tracks due to friction).
Hopefully this answers some questions until I do a writeup.