A magazine for a firearm is formed by a plurality of sidewalls that define a space for holding ammunition cartridges. An opening between two of the walls provides access to the space. Feed lips are positioned on opposite sides of the opening. The feed lips contact and retain the cartridges within the space. Feed ramp surfaces are positioned on opposite sides of the opening. The feed ramp surfaces are angularly oriented to tilt a cartridge so that it may be fed into the chamber of a barrel as it is pushed out of the opening along the feed ramp surfaces. Guide surfaces are positioned on opposite sides of the opening between the feed lips and the feed ramp surfaces. The guide surfaces guide the cartridge as it disengages from the feed lips and moves up along the feed ramp surfaces.