Here you see the Cleanout (1), the 4" to 3" bell adapter (2), the short piece of 3" PVC (3), and the 3" to 2" Bell adapter.

One of the very first of a few modifications you need to make to the PVC is to cut out the ridge in the 2" to 3" bell adapter.  This ridge normally stops the 2" pipe received by the adapter from going all the way through it, but in the case of the potato gun, it (the barrel) will need to do just that.

Here you can see how I ground it down, first with a rotary rasp on the air die grinder, then to smooth it, with a half round stone.

Take the time to make sure you grind all of the ridge off, and err on the side of taking too much plastic off, than not enough.  This because you will be shoving the whole barrel through this fitting, and if there is interference, you may stress or even crack the adapter.  My 1 1/2" x 3" gun eventually cracked right here.  Do not mar up the near side of the bore where the glue will go.

Now in this case, I had to make up effectively a 4" to 2" adapter out of the 4" to 3", the piece of 3" pipe, and a 3" to 2".  Not sure if you can get a 4" to 2", but my Home Depot didn't have one.  If you do this, you can go ahead and glue it up as shown at this point.

Now here is another critical step, cutting the rear end of the barrel.  It needs to be perfectly square and smooth.  I use my miter saw to cut the PVC.  It cuts nice and square and clean, as long as you pull the blade down slowly.  If you have to hand saw your pieces, use a miter box for a square end.  If by chance your 2" barrel piece comes from the store with a nice square end, you can use it after a little sanding.  The "business end" of the barrel is less critical.

Once you have a good square end, sand the face of the cut with a block sander.  I used 220 grit paper, then 600.  This end of the barrel is a critical sealing surface with the diaphragm, take your time and get a good clean, smooth face.

Here is the completed compression chamber.  The 4" PVC section is 55", but I'm sure something considerably shorter would work fine also.  At this point the homemade adapter and the cleanout are firmly glued on.

Now here is another critical area, one that if not addressed will guarantee that your gun will not fire well, if at all.  This is the face of the cleanout plug.  You can see not only a mold seam running the circumference of the face, but also on either side, sharp injection "nipples".  These must be sanded down smooth or else the diaphragm won't make a good seal when firing.

To sand the cleanout plug, put a full sheet of sandpaper on a known perfectly flat, hard surface.  I used a 1" piece of MDF board, a thick plate of glass or mirror would work equally well.  Use 220 grit, then go to 600 grit.  The thin face of the cleanout plug must be perfectly smooth.

Ok, now the tough part.  Lay your compression chamber on the floor with the cleanout end  against a wall.  Start the barrel, with the square, smooth end you dressed up first, into the adapter.  With a block of wood and a rubber mallet, hammer the barrel down into the compression chamber.  On both my guns, this took a while and was quite an effort.  Do not try to lubricate the barrel with anything as it will interfere with the adhesion of the glue once you get it all the way in.  If it is really hard to get going, pull it out and inspect the smaller adapter, the one that you ground out the ridge on.  If you didn't grind it down enough,  the barrel may not go all the way through, or worse yet, you may crack the adapter.

Note though that once you start whacking it into the compression chamber, it is very difficult, if not impossible to get it back out.  The only way I know how is to get another long piece of 2" pipe and hammer it from inside the compression chamber.  Then you need to figure out how to hold the chamber while you hammer too.

This one was snug, but every square whack with a very large rubber mallet scooted the barrel in about and inch.

So the trick is to stop hammering when the barrel is about 3/4" short of where you want it to be.  So measure the barrel end, when it is 1 3/4" short of being flush with the end face of the cleanout housing, stop.  Then mark the barrel with a marker exactly 3/4" from the edge of the adapter as you can see here.  Then, slather lots off glue all around the barrel just at and to 3/4" out from the adapter.  Don't be stingy with the glue here, this is the highest stress area in the gun (unless you are a potato).  After you apply the glue, quick grab the hammer and block and drive the barrel the final 3/4" home (glue sets up very fast).

When you are done your marker mark (not seen here) should be right at the adapter edge.

About that plug;  you will need to make a small hole in the back of it if you are going to use blow gun to fire it.  One day I hope to figure out a more sophisticated valving system, right now I just use a blow gun on the compressor hose.  I drill a hole about 3/16" in the center of the plug and then use a countersink bit on the drill press to give it a bit of a relief for the tip of the blow gun.

So here is what you see when you look into the back end of the gun, plug and diaphragm removed.  You can see the barrel end (squared and smoothed) comes right about to the end of the 4" compression chamber PVC, 1" from the end of the cleanout adapter.  The 5 pieces of garden hose (I used only 4 in my 1 1/2" x 3" gun) hold the barrel in the center of the compression chamber.  Without them, the barrel end would be a bit floppy down here, especially with 55" of compression chamber (55" from there to the bell adapter, the only place where the barrel is actually glued to the rest of the gun)

Another pic of the end of the chamber, showing the relative position of the barrel end.

Now, the most critical part of the whole gun; the diaphragm.

This is the one that has worked well for me so far with this gun.  It is cut from the bottom of an office drawer, black plastic "organizer tray".  It is remarkably thin and strong.  You really have to get creative when finding diaphragm materials.  Plastics work best, they should be fairly flexible.  Here's a couple other that I tried.

Another pic of the "desk drawer" disk (thin one on the left) and another I cut out of some thicker, very strong plastic that was a divider in an old tool road case.

Here is the thin diaphragm in place in the gun.  Cutting it to fit is just trial and error.  You want it to bee loose in the bore, but it should not be any  smaller than the outside edge of the that surface you smoothed on the plug face.  Note that I still have virtually all of the threads available for the plug, the barrel is positioned just right.

Finally, the tip of the blow gun I use.  Nothing special here, just a brass nipple.  Just about any narrow tapered blow gun tip (metal or rubber) will work.