Drop-in fixed magazines have a tubular body defining an elongated passage and having a lower end and an upper end, the body having an exterior surface that closely fits the profile of the magazine well, and the body including protrusions of the upper end extending laterally beyond the profile. The protrusions may engage a surface of the rifle to limit downward movement of the body within the magazine well. The rifle may have to be operationally disabled to enable the magazine to be installed into or removed from the magazine well. The rifle may include an upper receiver and a lower receiver, the lower receiver may have a top, and the upper receiver may have to be removed from lower receiver, thereby exposing the top of the lower receiver, in order to enable the magazine to be installed into or removed from the magazine well.