If you want to learn more about building off of the SNAP design, there's no need to re-invent the wheel through trial and error. Here are some very good write-ups on several variants of the SNAP design. Alternatively, you can also read through any/all of Carbon's homemades in the Homemade directory.

True, I just find a lot of these are a bit overkill for what I'm looking to do. I will certainly be taking from these once I "graduate." Though I find starting from the ground up lets me discover reasons for doing things certain ways that may be taken for granted by other people.

After learning about plungers and finding some compression springs, my second attempt at a SNAP is much more compact and powerful.
I built this with a 1" PVC body instead of 1 1/4" and used a bolt for the plunger rod instead of thick-walled PVC.

The plunger:
A 6" 1/4-20 bolt and two small compression springs from Ace (don't remember which product #) I then have a sliver of electrical-tape-wrapped PVC for my o-ring sandwiched between a couple washers. I use a third washer, spaced by a nut, for my catch. It has a hair over a 2" draw. At first my plunger head was made of 1/2" CPVC fittings, which are smaller in diameter than the washers. I was able to suction-feed darts with it. However because I couldn't make a catch with it, I switched over to the washers. They aren't perfectly straight, and I think them rubbing on the inside of the PVC is ruining my perfect seal, but I'm not sure.




I used electrical tape around the end of the bolt to get rid of the slack and prevent the plunger head from extending too far into the body.
It's important to note: I had to drill a bunch of holes in the end-cap because the seal was too good and air couldn't fill the plunger tube as fast as it was pushing it out. Without them, it wouldn't function.




The trigger:
I used the same trigger design as my first homemade: A clothespin and a shelf peg.






The barrel:
1/2" CPVC barrel wrapped in electrical tape to fit inside a 1"->1/2" reducer. Reinforced with hot glue.




The final product:






Comparison with my first homemade:




Summary:
Tools used: PVC cutter, drill, hot-glue
Relative difficulty: 0.0. A few holes were drilled, but other than that it was just cut-and-assemble.
Range: ~50 ft flat with snug darts.
Cost: Under 10 dollars.
What I learned: Having the washers with a diameter almost exactly the same as the PVC's inner diameter causes problems if they aren't perfectly straight. My PVC ring under the o-ring isn't straight, so the washers have a slight tilt and can rub. Either I'll try to use smaller washers, or find a silicon spacer to use instead of PVC so that I can guarantee it is straight.

Correction: 3/4" fittings go neatly inside 1 1/4" PVC.

Also, Extension springs aren't always a bad thing; Carbon has experimented with them, and largish blasters like the Ultimator or XXL Bazooka use them. It just requires more ingenuity.

Finally, at HD I know there's a 4-spring pack, two of which work wonders in reinforced NFs, or BBBs. Perhaps you should grab that for your next attempt at a homemade that's easy to get all the parts for assembly!

I'll have to check back to see if they got more springs in. Though I went to Ace and they had a wonderful selection of washers, o-rings, and springs. I got my parts for my second homemade there

I've always want to make some Nerf guns, and I finally decided it was time. This was also an awesome excuse to start buying my own tools. I may not have lurked enough, so let me know if I need to fix any terminology or vague areas.

Goal: Make the simplest SNAP possible with things I can get from Home Depot. I didn't want to make any overly-custom parts. I wanted to learn about making effective plungers and reproducible methods of construction.

The plunger:
I used a piece of 3/4" PVC for the plunger rod because 3/4" fittings fit near-perfectly inside 1 1/4". I first tried a neoprene washer for my plunger. I couldn't get one the right size and ended up cutting one down, but that just caused uneven edges. There was also too much friction to make it viable. (I need to go back and try this again now that I have white lithium grease, vaseline didn't work for beans). I ended up using the method of sandwiching an o-ring with pieces of pvc and using electrical tape to get the right diameter. This ended up working well. The draw is exactly 6".

The catch:
I simply used a another fitting in order to get the edge I needed for the catch. I don't need a ramp because my trigger pin is rounded (more later)



The trigger:
I used a clothespin trigger because it seems to be the simplest, fool-proof option. Instead of a nail, I used a shelf peg because it is rounded on one side, and flat on the other; Perfect for letting the plunger move smoothly one direction, and catch in the other. It was difficult getting such a large hole drilled in a clothespin, and the force of pressure-fitting the peg caused it to crack. I reinforced it with thread and hot glue, which has been holding. On my second attempt, I drilled the hole slightly too big, and put hot glue in the hole before inserting the peg. This seems to hold much much better (plus no splitting)






The Barrel:
The barrel is a reduction from 1 1/4" PVC down to 1/2" CPVC. The CPVC couple pressure fit nicely inside the 1/2" reducer.



The spring:
Unfortunately, the only viable option at my Home Depot was an extension spring. This is obviously not a good option considering the dead-space and constant impacts deforming the spring. It was also a struggle to get around an eye-bolt. I used an 18lb spring from this.




Final product:






Summary:
Tools used: Drill, Dremel, PVC cutter.
Relative difficulty: 0.5. The only reason this isn't a zero is because I had to use the dremel to make a big enough hole in the end cap for the 3/4" plunger rod to move through. I only had to glue 1 part because it had a loose fit. I didn't bother figuring out a handle or proper trigger because this is honestly a throw-away piece.
Range: ~35 ft flat. Nothing to write home about.
Cost: Obviously for certain parts, I had to buy extra. From nothing I'd say it was less than 20 dollars. (more expensive for me since I bought a drill and a dremel )
What I learned: O-rings were a simpler, easier method for getting a good seal on the plunger than a neoprene washer. (I probably did it wrong the first time, though). PVC plunger rods are heavy, and cause a lot of power loss. Shelf pegs worked awesome for the trigger, it's very smooth.

I've already made a smaller, better version which I'll do a write-up on next.