Quad stack, aka “coffin” mags have been around for a long time. While prototypes existed earlier, the Finnish Suomi KP-31 sub gun of Winter War fame ran one.
Later, the SITES Spectre of the 1980s followed up on the practice at the same time the Russians tried to field a series of AK74 quads (later revisited by U.S. Palm a few years ago). Since then, Magpul thought about it while Surefire actually put it into production for the AR.
However, the metal-body Surefire 60-round quad runs upwards of $130 and doesn’t have stellar reviews.
Speaking of reviews, I’ve been watching the Schmeisser company of Germany (what did you expect, Fiji?) for a few years and have recently been working with some of their latest Gen 2 S60W mags, which offer a quad-stack polymer 60 rounder for about half the cost of the Surefire.
More in my column at Guns.com.