Raheem Sterling (Getty Images)
Manchester City striker Sterling, 23, posted a picture of himself training with his England team-mates with a tattoo of the rifle on the winger's right calf clearly visible.
Lucy Cope, who founded Mothers Against Guns after her son Damian was shot dead outside a club in July 2012, said the depiction was "totally unacceptable".
Sterling said the unfinished tattoo served as a reminder of his vow never to touch guns after his father was shot to death in his native Jamaica when he was just two years old.
The England boss claims Sterling justified his decision to protect the troubled England star despite his booking for diving against Nigeria.
Southgate kept faith with Sterling after a troubled week for the Manchester City winger and he responded with a mixed performance in Saturday's 2-1 win in the World Cup warm-up at Wembley.
Sterling had been forced to apologise to his England team-mates after turning up to their training camp a day late following a flight mix-up on his way back from a holiday.