I seem to recall you mentioning (in a different thread) that this is for a school science fair. I'd like to suggest that for the science fair, you redesignate it as a "functioning model electromagnetic mass launcher".

That way, you don't have to worry about The Authorities fretting over a student building a "gun". And it's plausible -- an EM mass launcher is (theoretically) a remarkably cheap way to boost a satellite to an altitude where a standard upper stage can place it in a decent orbit (assuming you can convince someone to let you borrow their mountain).

Details! Details! Electromagnetic mayhem is one of my favorite topics!

If you're winding coils, then it sounds more like you're building a coilgun. A railgun is much simpler, basically two parallel bars of copper or aluminum, a whomping big capacitor bank and a very fast high-current breaker.

Not to kibitz, but I'm pretty sure relays aren't going to be able to open and close fast enough even to move the projectile down the length of the tube. You might want to learn about Silicon Controlled Rectifiers (SCR's) They're just the thing ya need, and they're not difficult to use.

Keep us posted!

Max.

Here's some links:

http://www.oz.net/~coilgun/home.htm

http://www.jamespaul.clara.net/coilgun/index.html

http://web2.airmail.net/jd4usa/

I wonder if they'll make a paintball gun like this...

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I are moose octopus, I having much buttons to push. So many buttons...

Hie thee hence to your local Radio Shack store and ask for the "Engineer's Notebooks." They're little booklets (they used to have one big one) written by a guy named Forrest M. Mims III. They're well written, explain things from the ground up, and they're chock-full of practical examples. The big book I started out with years ago has umpteen million notes and soldering-iron burns on it, and I *still* keep it by my workbench. I highly recommend them for electronic-geeks-in-training.

At a minimum, get the one on the 555 timer chip (one of the most useful chips ever, one of my faves!) and I think they have one on power control circuits and another on optoelectronics. You don't want to use hall effect sensors, that would be kinda like using an ultrasensitive parabolic microphone at a Metallica concert. By definition, an EM gun is putting out monster magnetic fields. What you need is an optical sensor to tell where the slug is in the gun barrel. You can get some off-the-shelf modules from Hewlett Packard Opto and others that have gaps of up to 1 inch or so.

Good luck!