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An emergency reload is needed when you have spent all the rounds from your magazine and your slide is locked back. This happens after the slide has locked back on an empty magazine. .. Be sure not to lose your old magazine, and bring your new magazine into place below the magazine well. (they should essentially pass each other during the drill). Place the rear of the magazine against the rear of the magazine well of the gun, align the two, and with some force (though there should be little resistance)... Give it a light tap, you should feel it click into place. Be sure not to put your fingers in the slide opening, or anywhere else they might get caught. This will seat the next round in the chamber, preparing it for discharge.
Identify the need. Grab a fresh magazine (likely from a magazine pouch). Move the fresh magazine toward the gun, as you do this. Eject the empty magazine. Insert the magazine. Seat the magazine using the heel of your palm. Roll your hand over the top of the slide and pull it back toward your chest. Release the slide, so it can go forward with full force.