Here you can find Pouches for the Machineguns. They can fit almost all type of Machinegun mags, like M249 or PKM and similar one. All of them reinforced with tough fabric inside, to avoid rubbings and damage the pouch.
Here you can find pouches for Light Machine Guns. Usually it's RPK or M27 IAR. This pouches fill 40-45 rounds mag. Not only machinegunners use this pouches. Specops operators usually like to use hight capacity mags, to delay recharge. It's important in CQB battles.