I built a small gauss weapon for a physics project in high school.
I'll post a picture if I can find some.
I had eight rare earth magnet bars encircling a little barrel, which were all housed in pretty substantial plastic pipe. I had two of those 12.8 volt batteries hooked up to it but found I only needed one. It shot ball bearings.
The weapon kind of destroyed itself after a few days. The plastic pipe became very distorted form the magnets pushing against each other, and the whole thing was being held together by zip ties.
It worked pretty well, and got me an A, so I was happy with it.
Like I said, I'll post pictures if I still have them.