You found out about a CPVC coupler fitting in PVC conjoiners didn't you?
Which part of this are you talking about, the finished rifle, or the unfinished, tiny at2k? If you're talking about the barrel of the airsoft dealie, then no, I didn't use couplers (nor did I use any of the whole gun). It's a series of smaller and smaller tubing/pipes. If you really want to know what they are, I can take some pics and make some measurements. I also used a long (probably around 3') strip of bicycle intertube to act as a spacer/seal. You twist it around the smaller pipe/tube like a candy cane as you twist it into the larger pipe. Seals perfectly everytime. I can post more details about that as well. Actually, if you want more details about the airsoft dealie, maybe you should stick to pms; I don't want to violate the CoC by talking too much about airsoft in a nerf forum.
I didn't use a bic pen or anything, because like Suave said, the pin is right up against the trigger. If I were to make this into a real nerf gun, I would deninitely put the airtank as far back as possible in order the make the gun as compact as possible.
Thanks for the complements guys!