I think for us .40 cal guys it's an issue of heavier tougher parts (springs, metal guide rods, THE SLIDE, etc.) compared to lighter parts in the 9 mm. The 9mm's seem to be the best you can buy straight from the factory, whereas the .40s need some beefing up and fine tuning, probably just because of the heavier round. One has to keep in mind that the P99 was designed with 9mm in mind and they just crammed a .40 (which is just a scaled down 10mm) in the 9mm frame, and tossed a longer heavies slide on it, so it makes sense that the .40 cal P99s might need some 'beefing up' on the internals.