Athune

Athune, your method works, but seems overly complicated...homemade pumps are really simple things.

The "pump" you posted a link to is really just a piston. Pumps are simple, but they always require a check valve. Since check valves can be pretty expensive, and I couldn't even find one small enough to fit into a plunger, this was the simplest alternative.

I looked into this myself but abandoned it after I found an old bike pump I had forgotten I had. Before I gave it up however, I managed to figure out the basic mechanism.
First, the basics. As I'm sure you know, the simplest type of pump is basically a plunger in a tube with a one-way valve at the tip of the plunger so that air can flow into the tube when the plunger is pulled, but is then compressed when the plunger is pushed back in. When I dissected my Buzz Bee Big Blast, I discovered a very simple one-way valve that could be reproduced out of PVC with a dremel like this:
1. Carve a channel for an o-ring in an end-cap that would be about the width of 2 o-rings to allow the o-ring to slide back and forth a bit. If you can, make the channel a little shallower toward the end where the shaft of the plunger would be.
2. Carve two channels perpendicular to and deeper than the first one that lead from the tip of the end-cap to the first channel plus the width of one o-ring.
This would cause a good seal when the plunger was pushed, and allow air into the chamber when it was pulled. If you need pics, let me know and I could email them to you; I don't have the time to post them right now.

A bullpup design (like the SNAP 4) has the same advantages and is much simpler, but hey, build it and see how well it works.

Nah, if you want to skip the air stuff, then you want a good, old-fashioned railgun.

I agree. If you're going to make a electromagnetic nerf gun, rail guns are much simpler than coil guns (which use solenoids). Wikipedia has pretty good descriptions of both.

Couldn't you drill holes in the stationary piece of hardboard to allow for rear loading of the barrels? That would cut down significantly on reload time. Of course, if you think that the darts would simply fall out, you could make the holes offset so that they would only line up with the barrels half-way between barrels lining up with the gun... If this isn't clear, let me know and I'll post a diagram.