The knockback a target takes depends on the larger of its ST and Bulk. For every full multiple of this number of crushing, cutting, or impaling damage the target takes before subtracting DR, the target is knocked back one meter.
If you want to get really picky, piercing damage can cause 1/10 knockback, rounded down. Only damage that does not blow through is counted. This will almost never be important, and this is realistic - man portable firearms do not actually impart enough momentum to knock a person over. However, when you have a man getting shot by a 120mm main gun from a tank, and his ultra-tech armor stops all of it, you might want to figure knockback.