As kaashif points out, railguns have recoil. Recoil isn't a special trait of chemical propellants, it's Newtonian. The railgun pushes the needle, so the needle pushes the railgun.
Add to that, the rails and the capacitor bank have mass, substantially more mass than the needles. The same force that sends a railgun projectile down the rails also tries to push the rails apart. The rails need to be strong enough to resist this force trying to separate them. You can't even let the rails flex, because if the rail loses electrical contact with the projectile then there's no current flow and therefore no force to push the projectile. For a railgun this small you can probably wrap the whole assembly in carbon fiber and call it a day, but there's no way any of this comes out weighing less than the needles themselves. Just one of the capacitors will weigh more than a bunch of needles.
An air canister weighs to much? A standard CO2 cartridge weighs about 50-100 grams and has ample capacity. A CO2 cartridge in an air pistol lasts for dozens of shots, that's plenty.
And silent and transparent? Now I think you might be trolling. Neither UAVs nor railguns are silent and only a few components in them could conceivably be made transparent.
Add to that, the rails and the capacitor bank have mass, substantially more mass than the needles. The same force that sends a railgun projectile down the rails also tries to push the rails apart. The rails need to be strong enough to resist this force trying to separate them. You can't even let the rails flex, because if the rail loses electrical contact with the projectile then there's no current flow and therefore no force to push the projectile. For a railgun this small you can probably wrap the whole assembly in carbon fiber and call it a day, but there's no way any of this comes out weighing less than the needles themselves. Just one of the capacitors will weigh more than a bunch of needles.
An air canister weighs to much? A standard CO2 cartridge weighs about 50-100 grams and has ample capacity. A CO2 cartridge in an air pistol lasts for dozens of shots, that's plenty.
And silent and transparent? Now I think you might be trolling. Neither UAVs nor railguns are silent and only a few components in them could conceivably be made transparent.