In response to splitlip's suggestion of buying a variety pack of resistors, you would need to make sure you get resistors rated for enough wattage or run a LOT of higher value resistors in parallel. The resistors you would get in the variety pack are either 1/4 or 1/2 watt, so if there's any real load on the motor they will quickly get hot and fail. It might work for a quick test, but I'd suggest buying a power resistor once you know how many ohms you need for the speed you want.
Sam I think it will. The "POP" you hear when you fire a spring gun is the sudden release of pressure from behind the dart. The silencer slows down the pressure wave from that air. Imagine the difference between popping a balloon and letting the air out slowly.
Why would you want to silence a NF? Anyway, if you drill holes in the barrel then obviously air will escape, which will keep the pressure from accelerating the dart. What you need to do is keep the barrel solid, and then have your silencer attach to the end and extend farther out. As for length, you only really need about a three inch long silencer for that small volume of air.