Well since I seem to be the one here with the longest interest and study of linear accelerators (Gauss style) I'll interject my opinion.
Well for one, you need a large power source, what's yours ?
I know you said 12v, but can you give further details, are you using a transformer, or just the 12v through the coils.
Because standard gauss experimentation uses around 400v DC. (AC doesn't work as well, don't know why)
Also you need a way to time your coils, using either photosensors, or magnets, improper timing will drop a coils distance by several yards.
Also consider dropping the size of your round, since there isn't a powerful enough source to create a decent man portable weapon you need a small round, I'd say get a box of copperhead .21 cal copper bb's.
This alone should increase your distance.
Then there's the matter of your wire size, and construction, if you run your wires backward, or your coils aren't tight enough, or you have a bad patch of wire, or your wire is too thick, you'll get a slower round with less distance.
Also your barrel will affect the round, if your using metal switch to plastic, the round is your core for the electromagnet, not the barrel, and make sure it's as thin as possible, and also as tight on the round without restricting movement, it should be about 1mm larger in diameter than the round, and be no more than 1/8th inch thick.
Also consider experimenting with a transformer style barrel.
Instead of using one coil, use two.
Here's how you build.
Get 32 or less gauge wire and wrap it around your inner barrel, then when it is complete, measure your barrel, and get another barrel just large enough to fit over the first, then use a slightly larger gauge wire (no more than 2 sizes larger)
Send the current down both, this will create a much larger magnetic force since the two seem to act on each other swapping current and magnetism, I haven't figured out why yet since it doesn't work like a permenant magnet does when it passes inside a coil, because if it did, it would create an infinite loop of electricity that would defy physics and eventually explode. (make sure you have a good enough resistor before it goes back to the power supply as this type is unstable and can easily spike enough to blow your power, I'd also suggest 2 fuses in series)
Also consider a heatsink for your power supply, and if your barrel can get small enough, get a few circular heat sinks to cool off your coils.