Pretty sure Bkreutz nailed it... Bubba'd feed ramp.

Brass would be "belled" above the base toward the case mouth if 9mm is fired from a .40 S&W chamber. I witnessed this a couple of times when shooters had matching Cougars and XDs in each caliber and inadvertently switched a .40 mag with a 9mm.

Nothing catastrophic happened, but the groups tended to be just a bit wild, shall we say?