Analysis addressed the changing impact of gang membership, drug sales, and peer gun ownership for protection on gun carrying at nine separate points over the early adolescent to young adult life course. In early adolescence, gang membership was a strong motivation for gun carrying. At somewhat older ages, drug dealing--particularly high drug sales and illegal peer gun ownership--replaced gang membership as the primary determinants of illegal gun carrying. Gun carrying is a response to the situational imperatives faced by youth. The constellation of situations in which they encounter other illegal gun carriers is so large that the probability of their carrying becomes high. The many vicious cycles of contagion come and go over the adolescent life course to produce the milieu of gun carrying that produces homicide among young males. Notes, tables, references