A gun dealer in South Carolina who is suing Mayor Bloomberg was indicted on federal gun charges on Wednesday.

The dealer, Larry Mickalis, is charged with selling a firearm to a felon. If the charges are proved, the development could sink the man’s defamation case against the mayor. Mr. Mickalis has claimed the mayor slandered him by describing him in the press as a rogue gun dealer whose firearms were regularly found at crime scenes in New York. The case appears to be headed toward trial in South Carolina, and could result in a multi-million dollar verdict against New York City.

The charges against Mr. Mickalis were filed in U.S. District Court in Charleston, S.C. He is accused of selling a .223 caliber rifle and ammunition to a felon, Jesse O’Conner, in January, 2006.

A message left Mr. Mickalis at his gun shop, Mickalis Pawn Shop in Summerville, S.C., was not returned. A lawyer for Mr. Mickalis, Justin Kahn, declined to comment.

Mr. Mickalis’s suit against Mr. Bloomberg came after the city investigated whether certain out-of-state gun dealers were responsible for a disproportionate number of crime guns recovered in New York City. The city claimed that 77 guns from Mr. Mickalis’ shop were used in crimes in New York over an eight-year period.

A gun dealer in Georgia has also filed a defamation case against the mayor.