You would be surprised over on a glock forum how many people accidently (some on purpose) try to shoot 9mm thought a 40 barrel.

I DONT RECOMMEND THIS NEXT PART IT COULD BE EXTREMELY DANGEROUS

One of the guys even went as far as buying a extra 40 barrel and shooting 500 rounds of 9mm thought it. It worked but to me that's extremely dangerous. He just wanted to prove the point in a emergency the 9 would work in a 40.