In order to make this project, it is necessary to work with high voltages, so don't even think about making a project like this unless you are familiar with electricity, especially capacitors.(ie. Don't blame me if you hurt yourself)
If you ask a question I'll try and answer it.
I made this for my science fair project in order to see how efficient I could make a railgun (not very). See this here:http://www.instructables.com/id/1kJ-Railgun/
If you want to see video of this its on youtube: https://www.youtube.com/watch?v=HNhfc-MJg2M
Step 1: Making the actual railgun
I used 6x20mm aluminium, and 6mm perspex.
This top down photo shows the "fake" perspex rails on the right, and the aluminium rails on the left. If you don't get how this works, i'll explain it at the end.
The transition from the perspex rails to the aluminium rails needs to be smooth so sand it lightly.
There really needs to be a close tolerance of the projectile fitting inside the barrel. I had a bore of 6mm, which made it easy to use the 6mm aluminium as a spacer to make sure the gap was 6mm all the way down.