Duxman, move your cel phone, or go bluetooth, you must always carry an extra mag, it must be in a place that makes a mag replacement quick!
The reason for the extra magazine is twofold, #1 malfunctions, the second reason is more bullets!!!! for down range delivery, in a hurry.
A Glock 19 has 16 rounds up, the spare is from a G17, full of 127g new talons, when you get into the extra mag, due to expending the 16 rounds in the pistol! you may have bitten off more than you can chew!!
Keep Safe. Mike H.