Shielltec Inc. makes an interesting product. It is like a giant pez dispenser for gun magazines. You could call it a magazine feeding device or a Magazine of Magazines.
Here is their loadmaster. It holds five pistol magazines. As you retrieve one magazine from the dispenser, a new one is advanced and ready to be retrieved for your next reload.
The concept reminds me of the Moonclip Server that I often see at USPSA matches.The moonclip server does the same thing. You have a series of moonclips. As you take one to load your revolver, the server advances the follower to push the next moonclip.
Shielltec also make a larger backpack version that dispenses AR magazines.
And then there is the Cress. A backpack system that allows semi-rapid deployment of an AR with an integrated magazine dispenser. The AR looks to be using a Law Tactical folding adapter to fit it in the back pack. It looks somewhat clumsy to deploy. And the backpack does not look very discrete.