A magazine for a firearm having a magazine well has a body with the chamber for receiving ammunition. The body has an upper opening to the chamber, and has a contact portion with adjustable screws that contact a surface in the magazine well. Adjustment of the screws is used to establish a precise selected position of the magazine with respect to the receiver, permitting adjustment of the feed height, feed angle and ejector position. Four screws are provided, one at each corner of the upper surface of the magazine.