Ammunition magazine and hold-open bolt carrier

An ammunition magazine configured to bottom-feed a plurality of ammunition rounds into an upper receiver of a firearm, the ammunition magazine comprising:

• a magazine housing
• including:
• an ammunition reservoir,
• a proximal guide channel disposed proximal to the ammunition reservoir,
• a storage compartment disposed distal to the ammunition reservoir and accessible through an opening in a top surface of the magazine housing;
• and
• lips disposed on the top surface of the magazine housing, the lips configured to align the ammunition reservoir with a feed ramp of the upper receiver;
• spring within the magazine housing;
• a floor plate retainer disposed at a bottom end of the spring;
• a floor plate disposed at a bottom end of the magazine housing and reversibly mated with the floor plate retainer;
• and
• a magazine follower disposed at a top end of the magazine housing, the magazine follower including:
• a follower protrusion disposed on a superior portion of the magazine follower, and
• a bolt catch piston disposed at a proximal end of a body portion of the magazine follower and configured to slide along the proximal guide channel of the magazine housing, wherein a region of the body portion defines a recess adapted to removably receive a tool.