40 breech face is approx. .6mm (.024") wider than 9mm.

Since most extraction pressure comes from blowback (when firing), the extractor doesn't need to work that hard.

Whether or not the 40 extractor would have sufficient purchase on the 9mm case rim in a marginal (and possibly desperate) situation like a round sticking in a dirty chamber during a shoot out is a difficult question to answer and would NOT be a situation in which I would choose to be.

In other words, Range Use Only!

John