Grey Ghost Gear is modernizing an old school idea. They have finally released their magazine bandolier, which was designed to function in many ways like an ammunition-only Bail-Out Bag, or for "those situations when you need additional mags to supplement your current equipment or for those times when you aren’t able to grab your vest or armor carrier, but need additional ammo." It is manufactured from MilSpec webbing and reinforced elastic webbing and can be worn cross body or around the waist.
Note - the bandolier was not designed to carry USGI magazines. It is built specifically for PMAGs and the like (their construction are key to retention). That said, there are several GGG staffers and field testers checking 'caveman style' to see if it will effectively hold onto AK, G36 and some other magazines.
Stickman talked about them a few months ago over on Gear Scout, and they appear to largely function as he originally reported, though the price is lower ($24.99) and this carries six PMAGs, not four.
Follow Grey Ghost Gear on Facebook, as they apparently will be giving some of these away soon.