Are you loading with a closed slide?
When you load with a closed slide, you have to compress the ammo another 1/2 cartridge to fully lock the magazine. If it is not fully locked, the magazine falls out when you start shooting, which is very funny to everyone but the shooter.

Try loading from an open slide and see if the problem goes away.

By the way, magazines should be seated with a firm push of the heel of your palm, NOT by drawing back and hitting it.

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Science plus Art