Dona Ana Sheriff's Department investigator Bo Nevarez says both are in critical condition but their wounds aren't believed to be life-threatening.
He says the bullet struck the boy in his stomach and continued through to the grandmother and hit her in the abdomen. Nevarez declined to identify the man.
Investigators regard it as an accident but are sending the case to prosecutors to determine whether negligence played a role.
Information from: Las Cruces Sun-News, http://www.lcsun-news.com