Can you fire a 9mm bullet in a 40 cal Glock?

No, you cannot safely fire a 9mm bullet in a .40 caliber Glock. The .40 caliber Glock is designed to handle specific .40 S&W ammunition, and using a 9mm round in this firearm could result in catastrophic failure, including damage to the gun and injury to the shooter. The dimensions and pressure specifications of the two cartridges are different, making them incompatible. Always use the ammunition specified by the manufacturer for your firearm.