Is a gun which produces no sounds within the human range of hearing possible? I know of a couple issues:
- Most bullets move quick enough to make a whistling sound, but I may be incorrect in that regard.
- The bullet may grind against the barrel of the gun, creating a grinding sound one could hear.
- The firing mechanism would most likely produce sound. Even something that used magnets to propel the bullet would have something keeping the bullet back, and that may grind against other metal in the gun.
Any sound produced must not be within the range of human hearing (20Hz to 20kHz); sound produced outside of that range is fine. The measuring device we'll use is a human hearing the firing of the gun.
This should be a reliable weapon which can reliably kill a human at 200 meters, regardless of whatever is shot or projected. It should be able to survive more than one shot, at a minimum firing rate of three times per minute.
It should be silent within Earth's atmosphere, regardless of the weather.
The technology of the time is similar to the present time, but there are advances in magnetic technology (they have far more powerful magnets as they have super compressed magnetic material and have aligned it in the same direction. All magnets used are functionally 5 times stronger, a strong dose of handwavium included).