There's been some really good write ups on RWS primers and there performance, check them out.

Size & Shape

All large rifle primers are 0.210-inches in diameter and, although checking the dimensions of such small component isn’t easy without specialist equipment, it was possible to establish that the RWS primers are very consistent in both shape and size. RWS primers seem to have better sensitivity for more positive firing in all rifles. RWS Primers are carefully controlled with shape, size and weights of primer mixtures with precise measurements and tolerances for anvil heights and stability in extremes of temperatures and humidity.