There have been barrels made to shoot 9mm from 40S&W guns, but the breechface is larger in the slide of a 40. Most external extractors will hold the 9mm cases and for range use it's probably OK but I wouldn't bet my life on one. 40 ammo has been running $2-3 box more than 9 in my area, so you would need to run a couple thousand rounds before you would come out ahead.