The classic synthetic rubber 'Magpul' loop which gave Magpul their name, fits most 5.56mm magazines.
Allows for easy handling and fast changing of magazines at speed and in the wet, reduces rattling of magazines in the same pouch.
Also protect your magazines when resting the mag on the floor or dumping them into hard floors during mag changes.
Fitting Tip - to make fitting easier, try putting them in a bowl of hot (but not boiling water) for 5 minutes to make them more pliable, then remove and work onto your magazine immediately. They will shrink on as they cool to give a snug fit.