This is the Pentagon Magazine Coupler. Designed to work with the 10-round Ultimag, this device not only facilitates quicker mag reloads and offers a total capacity of 50 rounds, but also can function as an impromptu bipod and/or rifle stand.
At the very least, it’s a novel idea.
Tags: FAB Defense, The Mako Group
Or I can use a single Surefire 60 round mag. These guys seem to excel at finding solutions to non-existent problems, but I’m sure they’ll sell a ton of ’em anyway.