Environment for it's upbringing
After WW2 and the inevitable stalemate between the USNA and the Third Reich, the world renowned gusnmith Caxton Storm (the genius behind the Sword Double Action Design) managed to sneak out of Nazi-Controlled Belgium to the Free Irish Territories, where he was then evacuated to the mainland North America. The tense three-way socio-cultural conflict between Capitalist Republo-Democracism, Aryan National Socialism and Russian-karl-Marxist Communism may have been a stalemate, but the military conflict was balanced decidedly in favour of the Nazi Third Reich, with their advanced AG49 assault rifle. The shattered remnants of the Soviet Union were relying primarily on cheap, mass-producable submachineguns and the United States of North America and other Allied territories were relying on full-sized semi-automatic battle rifles.
Caxton Storm managed to give the Allies a lethal edge over nations equipped with Nazi-produced assault rifles, by developing a lightweight, versatile, fully-automatic, compact and cost effective personal weapons platform. The M15 was developed specifically for a revolutionary new 5.56x45mm round. It was built to have both reliable and accurate iron sights, and a primitive multi-purpose optics rail attachment.
The M15 was produced with lightweight aluminium-steel alloy and lightweight polymer furniture.
The first magazines produced were 20 round aluminium box-magazines, with weak springs. These were subsequently only loaded to 18 rounds.
The second generation magazines were 30 round aluminium box-magazines, but again, the springs were weak, leading to only 28 rounds being loaded.