Australia also doesn't have a quite porous border, while it's well known that illicit materials - guns, narcotics, people - flow relatively freely across the US

Australia also doesn't have a quite porous border, while it's well known that illicit materials - guns, narcotics, people - flow relatively freely across the US-Mexico border. While I would wager that mass shootings would likely decrease to some degree, expecting to eliminate all gun violence is a goal not currently achievable.