I wanted it for my Extend-a-blast, but before I sawed the thing apart for a clip system that might or might not work, I built one on a barrel outside my EaB -- a barrel I could attatch to any of my guns. The result? A breech-free 8-dart deodorant clip. It cost $1.97 for the deodorant can, and ~30 minutes to cut and glue.
First, I hotglued it to the end of my EaB's current barrel, to see if it would work with a spring gun -- a gun that draws air back first, then blasts the dart forwards.
See the video - it did.
Then I stuck the barrel on what will be its permanent home (relatively; all my barrels are detachable) -- my titan, to see if it would work with pump guns, that only blast air forward.
See the video - it did.
Here's a picture of the clip system integrated into my EaB. Click the picture to watch it in action:

Ranges:
Note: These were measured by lining up tennis balls at 10' intervals in my backyard, then approximating from there. Margin of error is probably +/- 3 feet.
Titan:
-Unclipped 2' SCH80 PVC Barrel, flat, 5 pumps: Avg 35'
-Clipped 2' SCH80 PVC Barrel, flat, 5 pumps: Avg 32'
Extend-A-Blast
-Unclipped 12' SCH80 PVC Barrel, flat: Avg 40'
-Clipped 12' SCH80 PVC Barrel, flat: Avg 40'*
Darts: 2" micro stefans, Hot glue only; no weights.
*I lined the clipped barrel with a sheet of printer paper to guide the darts and provide a tighter fit. It averaged 30' flat with atrocious accuracy without the paper.
How to make your own:
Step 1: Make sure you want one
This clip is not ideal for pump guns. When you use it in a pump gun, one of three things can happen:
1. Work correctly
2. Jam
3. Fire multiple darts, one good, the rest pooey.
It all depends on how fast your pump gun empties its air pressure. If it dumps its air pressure quickly, the clip will work. If it's slow about it, it will catch multiple darts, but only one will really fly, and even then less than if it worked correctly. Or, if your pump gun is somewhere in the middle, it will shoot the first dart, pull the second one partway into the barrel, then jam when you fire again.
If you gun does #2 or 3, you can fix this by pulling and releasing the trigger very, very quickly. I had to do this with my titan, and it works... but if you ever forget and squeeze and hold the trigger, your next shot will jam or be a dud, depending on your gun. My titan just fires one dart and spits another; you may not be so lucky.
#3 could probably be fixed by successively layering hot glue in the barrel right in front of the clip until the gun doesn't provide enough power to cause the second dart to leave the breech... but the specific amount would be unique to each gun and the amount of pumps you intended to use it at, so you can work that out on your own.
This clip works much better with spring guns.
However, there's still one thing you should be aware of... this clip is dependent on a barrel that leaves the darts with a fair amount of play. If your foam doesn't slide back and forth from one end to the other in your barrel, find different foam or a different barrel, or this won't work.
To be on the safe side, I reccomend building a barrel with this clip that can easily be detached (any gun you've put a coupler on for removable/interchangeable barrels is a good one to build it for), so that if it doesn't work, you haven't chopped up a good gun for nothing.
Step 2: The "Secret"
This clip consists of an SCH80 PVC barrel that leaves the darts with room to freely slide back and forth when tilted,

a "Personal expressions" (generic "old spice") deodorant case,

a sheet of paper, and buttloads of hot glue. You could use a clear deodorant case if you wanted, but I couldn't find a tall enough clear one. A lot of this write-up is specific to this brand of deodorant; if you use something else, you'll have to figure it out on your own.
This clip is designed for 2" stefans, but can be adapted for shorter.
The theory is this: The cap is attached to the SCH80 barrel, like so:

The deodorant can is then filled with darts, and stuck on top.

Gravity pulls the darts down, and one dart will sit in the barrel. Because the barrel is not wide enough for TWO darts, when the first dart leaves, even though it does pull on the dart above it, that dart doesn't go anywhere until the first dart is out of the clip's breech area. A dab of hot glue is placed behind the clip's breech area to prevent spring guns from drawing the first dart back far enough for a second dart to drop in. This can be omitted for a double-shot (or more, if your spring draws darts back far enough) effect.
Step 3: Creating the clip
Take your deodorant stick and turn the screw on the bottom until the stick is all the way out, then throw it away. Keep turning the wheel until this thing pops out:

See the goo around the edges and in the hole in the middle? Scrape it out with a toothpick, and wash in hot water to get the residue (and smell) off.
Now take your favorite cutting instrument, and cut off the peg on the bottom.

Now find two small, round, metal things. I used a quarter and a dime. Put the quarter underneath the hole in the middle of that plastic thing, and fill the hole with hot glue.

Fill it over the top just a little, then press the dime down on top. This will cause the hot glue to cool and fill the hole nice and flush, and because they're metal, you'll be able to easily remove the coins. Set that thing aside for now to cool.
Now grab your deodorant case. Clean out any deodorant that is stuck to the sides, then take a look at the bottom. See that wheel we turned earlier?

It's got to go. Grab a pair of pliers and rip it out. It's okay if some stuff breaks off.

Now find yourself a piece of thin cardboard -- we're going to use that to make the interior shape of the deodorant stick a little more rectangular, so the stefans don't jiggle around inside and jam. The clip on the right is finished; that's what we're going to do to the one on the left.

I used two 1" wide strips and one skinnier one. My skinny one measured 5/16", but what matters is that your longest stefan has just a little wiggle room between one end of the deodorant clip and the cardboard.
Before we can glue them in, though, grab that funky plastic piece we hot-glued earlier, and stuff it all the way down the clip, sawed-off side down.

See how there's a little recessed oval running around the outside of that plastic piece? We can use that to line the bottom of the cardboard strips up. I used hot glue to attach them, but you can use your favorite adhesive.

Then cut them down to the length of the clip:

The skinny strip of cardboard is the back of the clip.
Finally, to make the seal between the clip and the cap tighter, take some scotch tape and tape around the lip of the clip. You will actually have to trim the width of the tape down a bit for the *best* fit, but it is not necessary.

Now the clip is done! Grab some stefans, and load them weighted-end up as shown:

Holds eight.

Step 4: Making the barrel
Grab yourself some SCH80 PVC (or any barrel, really, that allows darts to slide freely through it) to use for a barrel. Here's my Extend-A-Blast barrel.

Cut a breech-hole in the barrel that is longer than your longest stefan, yet shorter than the deodorant cap. For my 2" stefans, I cut a 2.5" hole. It should extend slightly more than halfway down from the top of the barrel to the bottom.
Before:

After:

Now cut off the closed end of the cap.

I messed up horribly with the hacksaw, but that's okay -- hotglue will fill the gaps.
Now cut the cap so it fits into the breech you made.


Once you do that and stick it in, you'll notice that the rear of the cap is obstructing the barrel; darts cannot slide backwards. Cut here on both sides:

So that it fits in like so:

Note that you are cutting it so that the deodorant cap does not obstruct the barrel, not so that it looks like it fits nicely from the side.
Now smother that mother in hot glue, to seal up all the cracks, so that the only openings to your barrel are where the clip goes and the two ends:

Finally, you'll want to put a small dollop of hot glue in the barrel, just behind the breech-hole:

From there, I had a lot of sawing and gluing to do to assemble it inside my extend-a-blast, but once I got it in and tested, I realized I was getting a significant range loss and my accuracy was shot. (Heh...). This is of course because the barrel we chose was a loose fit on the darts; air could rush around them, instead of pushing, decreasing range, and unless the dart was perfectly straight, the air would spin it, decreasing accuracy.
The solution is to get a tighter fit... but we can't do this with a smaller barrel, because then gravity won't have enough force to load the darts for us... The solution is paper. I grabbed a sheet of printer paper, cut a strip about 1" wide, rolled it up, and slid it down my barrel.
This is the inside of my extend-a-blast, cut away, with the rolled up paper.

As you can see, the paper flares nicely under the clip, to better channel a dart, and narrows nicely as the dart enters the main barrel. In fact, the darts will no longer slip forward out of the barrel with the paper there; it makes it that tight, yet, because it flares outward under the clip, gravity still feeds the darts in with no problem.
You'll want the paper to be a bit longer than your barrel; that way you can secure it in the back with another dab of hot glue (because if you don't the paper flies up into the clip with each shot).
To secure the front, cut the stuff sticking out the front of the barrel from the tip of the paper sticking out, to the base of the barrel. Fold the strips back, and tape.

At long last, you're done. Try your gun.
If darts are getting jammed in the clip, it's probably because they're getting caught on the PVC where the dedorant cap overlaps the uncut part.

See how there's only a little bit of a gray lip? If your clip leaves a significant bit of the barrel visible at either end, adjust the location of the clip until it's not showing... if you still have trouble, post pictures + a description of what happend (a picture or video of you detatching the jammed clip is good), and I can try to help. There is a lot of trial-and-error and hot-glue involved in aligning that $#!#$!@#%$#@$ deodorant cap correctly. Once you've got it working, slather some epoxy on top of the hot-glue and let it dry for an airtight (hot-glue), strong (epoxy) clipped barrel.
Step 5: Tips to avoid a jam
1. Cock/pump the gun pointing upwards.
2. If you hold your gun significantly behind the clip, try to avoid waving is; centrifugal forces could slide another dart into the barrel.
I'm pretty sure these aren't needed with the paper in the barrel, but I haven't tested.
Because replying in rapid succession has caused the board to add my three posts with an acceptable number of images together into one post, I can no longer edit the large post, so here's what you'll need:
Step 2.5: Getting your Toolz
You will need these:

That's a
Hacksaw
Screwdriver
Needlenose Pliers
Quarter
Dime
Deodorant case
Scotch Tape
Scissors
Ruler
Hot Glue Gun
Sharpie
Exacto knife