A South Boston man was arraigned Wednesday on charges of illegally possessing ammunition and a semiautomatic rifle, the Suffolk district attorney’s office said. Alexis A. Laboy, 24, was arraigned in South Boston District Court. Prosecutors said he had a Calico M-100 and 48 .22-caliber rounds. A 911 call placed at about 8:20 p.m. alerted police to a man with a gun exiting his car and entering another vehicle near Father Songin Way and Kemp Street in South Boston, said a statement from the district attorney’s office. Police arrested Laboy when he returned, finding the gun and ammunition on his person, the statement said. He will return to court Jan. 29.