To me, a jag is still a short brass finger on the end of a cleaning rod that is wrapped in four-by-two before being shoved up a rifle barrel with a cry of "up yours!", sar-major.
To anyone in Scotland, a jag is a needle injection, usually preceded by a verbal warning "just a tiny prick". When the person offering just a tiny prick is male, polite patients suppress a smile. Fully-vaccinated children are referred to as having "had all their jags".