These high quality o-rings are the perfect size for fitting round the inner barrel of virtually all AEGs to hold the hop chamber snug against the gearbox shell. 

Made from Nitrile Butadiene Rubber, they are chemical resistant and ideally flexible. 
They make ideal replacements for the common tensioning springs as they apply a more constant, reliable pressure. 
This can help reduce chamber vibration especially in high RoF builds ie. DSGs