Equipment: .40 Dummy in a 9mm?

In a recent Tap-Rack class I took, Bill Blowers used a piece of equipment that surprised me: a 40 caliber dummy round in a 9mm handgun. We had just done type 1 malfunction clearances (tap, rack, reassess) and were moving on to type 2 (rip, rack, reload) malfunctions. When we did this, Bill handed out 40 caliber dummy rounds. He explained that the rounds would fit with out 9mm live rounds, and would cause a failure to feed that needed to be cleared. While not all dummy rounds worked in all guns. He had a few styles to create the proper issue.

The best part, is that a proper 9mm dummy can create one type of malfunction, and the 40 dummy could create another. Best of all, you can set them up with varying numbers of rounds in the magazine and train yourself to fix the type of issue you have in an appropriate manner, building your conscious and then unconsious competence with your handgun

DISCLAIMER: Bill Blowers even state that he has not found a combination that works in any other caliber. A 45 in a 40 doesnt work according to Bill. Though I suspect that a 9mm in a 380 would work, provided you could get it to fit the magazine.