Hello in Mesa, I am in Phoenix. Extended mags (in the 20 count range) are often poor quality and cause jams. If you are talking about self defense that is bad, if you are talking about fun in the desert, its ok and can help teach you clearance drills.
A lot of Auto guys carry a spare mag. Its not so much as a percieved need for more ammo as a way to keep in the fight. Most jams and stoppages are Mag related. In this circumstance a standard 12 round mag should serve you well.
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