Strac Technologies has developed an interesting magazine carrier called the F.A.S.T. System model M4. They tout that this system allows operators continuous sub 2-second reloads.
It does look pretty nifty but it only holds 3 magazines at at time and is a little bulky for those 3 mags. Military operators would probably want to be able to haul more mags than these things offer and the almost $300 price tag is a bit steep. I can see it’s value as a specialized piece of gear for SpecOps or Civilian Tactical Units where 3-6 magazine load-outs would be sufficient.