I don't have a 320, but am not surprised the .357/.40 slide assy. is heavier. SIG and Glock .357 and .40 slides I am familiar with, have less steel machined away during manufacture than slides intended for 9mm. The FN/Browning Hi Powers were another that used a heavier slide with the .40s, etc. The heavier .357/.40 slides compensate for the heavier recoil impulse of these calibers...