Thread from the WWn wiki - originally developed by iSoaker, Martianshark, and Andrew
A piston conversion is when you take a piston gun, add a pressure chamber, and give it a firing valve. This modification turns a gun that shoots with every pump into a gun that you pressurize and then shoot. It works best on piston/elastic hybrids such as the Super Soaker Shot Blast, Quick Blast, and Water Warriors Steady Stream. This guide shows martianshark's way of doing it on a Shot Blast.
This is a rather complicated modification. It's not for beginners, and it's not really worth buying a Shot Blast to do it. It might be worth it if you used a cheaper piston/elastic hybrid such as a Steady Stream or a Quick Blast.
You will need:
-some 1" PVC pipe
-a pressure chamber (a rubber bladder is best)
-PVC parts to connect the 1" pipe to the pressure chamber
-some sort of trigger
First, open up the Shot Blast. This is difficult to do. You have to unscrew it, snap off the outer shell, and then take off the inner shell. I forgot to take pictures of the stock internals, so here's a pic, courtesy of Nerf Mods & Reviews.
Shot Blast Stock Internals
Unscrew the back of the pressure chamber (the dinky little tank that looks kind of like an Airtech tank). You will find a plunger and spring inside. Take this out. Because of the seal, you may have to open the pull valve to get it out.
Shotblast Plunger and Spring
By the way, don't worry about messing up the trigger system or something. We're going to get rid of that anyway.
Epoxy a piece of 1" PVC pipe into the PC tank. I used epoxy putty. Be sure to stuff the putty in good so that it doesn't leak.
Shot Blast PC and 1" Pipe
Attach a pressure chamber to the piece of PVC. I used a rubber bladder from a HydroBlitz. You can use anything you want as a pressure chamber - even the stock one, although this would give it worse performance than it was stock. It might be an easier mod to attatch an air-pressure chamber, using a bottle or large-diameter PVC pipe. Here's my PC:
New HydroBlitz Bladder PC
Now take out the water reservoir and put a good sized hole in the front of it. This is going to be the PC casing for my rubber bladder. Also, get the reservoir cap and tether out.
Stock Reservoir with Hole
If you're making an air pressure chamber, skip this step - you don't need to touch the reservoir, but you'll probably have to put a hole in the top of your gun for your PC to stick out.
Now stick the bladder assembly into the hole in the reservoir. (if you're making it air pressure, skip this step too of course)
Bladder PC Inserted into Reservoir
You need to make a trigger for your Shot Blast. I made my trigger by twisting a piece of wire onto the pull valve pin. Then I tied the other end onto the slide of a Nerf Strikefire. The Strikefire slide connects to tactical rails, so I can connect it onto the Shot Blast's rail. Then I can pull it back to open the firing valve. You need to cut a hole in the top of the gun for the wire to go through. This is just my method; there are many other ways to make a trigger system.
Now close the gun. Closing it is a lot harder than it was to open it You have to get all the parts in place, and you need to tear out a lot of the stuff in the way to get the new internals to fit. You will also need to cut a hole for the reservoir intake tube to stick out of.
Re-assemble Internal Components
...then snap on the inner shell...
Snap on Inner Shell
...then snap on the outer shell...
Snap on Outer Shell
...and then screw it all together, and put the pump grip back on. Also, clip the Strikefire slide onto the tactical rail if you're using my trigger system method.
You're done! Now hopefully it works... The first time I did this, there was a leak, and I had to open it back up and apply more epoxy putty. The second time, it worked for a while, and then another leak appeared - apparently the epoxy wasn't completely dry. Now I applied more epoxy, and I'm going to wait several days to be sure it's dry. So be sure you stuff the epoxy in really good, and wait several days before using it. The top-mounted trigger system works great.
Unless you found a method that doesn't use the reservoir as the PC, you'll need a homemade water backpack.
 External Links
Ben's Piston Conversion - http://www.sscentral.org/mods/piston_conversion.html