Good way to get a ring bulge is to leave oil in the barrel before shooting . A close examination of many guns will reveal a slight ring bulge behind the choke cone .Having said that I have measured a lot of Spanish guns [new] that appear to have again slight ring bulges behind the chokes as a result of the way the chokes are formed by swaging or from lapping into the cone . These will I'm sure increase in severity with prolonged use especially if using maximum loads.