Because I have a spare At3k turret lying around, I used a Dremel to sand down one barrel and put in a piece of 1/2" CPVC. While I was able to do it, I wouldn't recommend it because (1) If you're doing a barrel support mod like Cxwq's, your barrel support will have to be sanded down until it is very, very thin, and (2) Without a Dremel, you will have to suffer a lot of pain to get all six barrels in there. Trying to get anything thicker than 1/2" CPVC will be next to impossible, in my opinion.
Yes, the Dremel moto tools I see at Home Depot are usually selling for 60+ dollars.
If you have a dremel, then go with Bag's advice...that will make anything fit, and it won't cost you any more.
I also went around 5/16" under the line, and my 9/16" brass fit in just fine, but that will cost you more money (~$6 for two feet of brass at my local Ace Hardware), and you should be able to get PETG to work.
If you want to just stick the stock dart in the muzzle and fire, 9/16 OD K&S brass should be fine (supposing that ALL stock darts are EXACTLY the same). All stock micros I've seen work fine in it (I've tried ones from the AT3K, NiteFinder, and Maverick). I've heard, however, that PETG will do even better, if you can get ahold of some.
If you want to use a ramrod/breech/bolt/nested barrel, I'd take poo's suggestion on the the 1/2", Sch. 40 PVC.
EDIT: 1/2" Sch. 40 PVC has an inner diameter of 5/8"...I don't know what the outer diameter is, but it's not as important.
I have no idea why you would want to do this...NF lights are completely and utterly useless in a war.
If you take a look at the NF innards, you'll see that pulling the trigger lightly brings two metal pieces together, completing the circuit to lite the NF light. Now you have to figure out a way to install these metal pieces in your other gun (which I'm guessing is not easy, if it's even possible). Then you'll have to find a way to cut the battery case out of the NiteFinder grip...and then unscrew the laser light, snip the wires, and then rewire it up again correctly in the new gun.
If you have your heart set on one of these things, your best bet is to go to Wal Mart and buy a 'Daisy Electronic Point Sight' or the like. They are about $8 ea, and then you can either make your own dovetail mount or just take some e-tape and tape it to the top of your gun (or the side, if you prefer).
Could you say a little bit more about how you made the PVC pump you mentioned? Also, supposing that a pump on an existing nerf gun (say an AirTech 3000) broke, would it be possible to use a PVC pump like the one you made as a replacement?
As a newbie looking at your ranges, that looks like a VERY powerful weapon. How did you test the ranges? Was that a level shot, and was there any dart skip? Does it hurt to get shot from a closer range (say 30')?
Also, do you use all the pressure up in one shot, or does it only let out a little bit of pressure at a time, like in Osiris' homemade?
I don't think I've seen sizes of foam backer rod other than 3/8", 1/2", and 5/8". For what foam backer rod is meant to do, precise sizes aren't needed. If you would like to make your foam a little bit thinner (say, for airguns), you could stretch your foam like tucker suggested. Take a look at this thread.