A single-feed magazine adapter used with automatic or semi-automatic firearms and a twin-drum magazine, which converts the dual-feed arrangement of rounds created thereby into a single-feed arrangement that permits dual-feed twin-drum magazine use with automatic and semi-automatic rifles and pistols of differing caliber requiring a single-feed system. The easily-installed adapter can be separately manufactured for attachment to an existing dual-feed twin-drum magazine, or attached to a new dual-feed twin-drum magazine during manufacture. In an empty adapter prior to magazine reload, three specialty rounds are located adjacent to the feed opening, with numerous standard filler/dummy rounds behind them. A flexible linked-connection between two of the three specialty rounds creates a pendulum motion in rounds within the adapter's top inwardly-tapered portion that enables successful conversion of rounds close to the feed opening from a dual-feed arrangement into a single-feed arrangement during their forward advancement toward the firing position.