[Alacrity]

As I read it a silencer does both, thereby reducing the pop caused by breaking mach and also muffles the explosion of gun powder.. As for speed:

·Light Spear (equivalent to a javelin): 2 pounds, 50 mph, 1" diameter
·Heavy Spear (equivalent to a Roman pilum): 5 pounds, 25 mph, 1 1/2" diameter
·Light Atlatl Dart: 3 ounces, 85 mph, 1/2" diameter
·Heavy Atlatl Dart: 6 ounces, 70 mph, 5/8" diameter
·Primitive Arrow: 500 grains, 165 fps, 3/8" diameter
·Modern Hunting Arrow: 540 grains, 220 fps, 11/32" diameter
·.357 Magnum handgun bullet: 125 grains, 1450 fps, .357" diameter
·.30-06 rifle bullet: 180 grains, 2600 fps, .30" diameter
. 152 mm tank shell Muzzle velocity 560-945 m/s or about 1600-2900 fps

Mach 1 is 1130 fps and a .357 is a fairly slow slug.