[Parabellus]

The .40 cal. P99 has a heavier slide than 9mm. That's one important difference. You can see the 9mm slide is scalloped near the front to reduce mass. There are probably other internal dimensions that would differ as well. Springs are another consideration.

Russell

 

[Parabellus]

I thought the recoil spring are the same in both the 9mm and 40?

You must be right.