Fastest security mag pouch i have ever worn. - Gould & Goodrich Phoenix Open-Top Double Magazine Case

If you are required (or just concerned) to have a manual security device on your magazines this is the best option. It looks open, but the tab actually holds the magazine in place and mechanically prevents it from falling out. Once the mag is loaded (the retention does not work with empty mags) the spring tension in the mag locks it in place. Just bumping the lever in the exact natural way you would index the magazine and it slides out just like an true open top.

The stagger allows you to sweep right over the empty and hit the next.