Abstract: Apparatus, systems, and methods for an adjustable magazine lockup. A magazine body includes a slot in a first side with feed lips connected to the top side. The feed lips have a top edge positioned away from the top side of the magazine body. A first insert with a first lockup notch may be positioned within the slot so that there is a first distance from the top edge of the feed lips to a centerline of the first lockup notch. The insert may be replaced to change to the top edge of the feed lips. A second insert having a second lockup notch may be inserted into the slot in the first side of the magazine body to increase the distance and a third insert having a third lockup notch may be inserted into the slot in the first side of the magazine body to decrease the distance.