A retired commander of a Navy commando squad and an Arizona arms manufacturer were indicted by a federal grand jury in Alexandria yesterday for conspiracy in a scam in which the commander allegedly secured a Navy contract for the manufacturer in exchange for a $100,000 kickback. The secret Navy hostage-rescue squad, Seal Team Six, has been the subject of various corruption investigations since the unit was formed in 1980. Richard Marcinko, once the commander of the squad, allegedly prepared documents in 1984 that steered a Navy procurement contract for 4,300 grenades to Accuracy Systems Inc. in Phoenix. Charles M. Byers, owner of the munitions company, allegedly agreed to pay Marcinko any amount above the cost noted on a price list Byers had submitted to the Navy if Marcinko could ensure that Accuracy Systems won the contract. The $310,000 contract awarded to the arms manufacturer was marked up by approximately $100,000, according to the indictment. Shortly after Accuracy Systems secured the contract, it said, Byers sent a check in excess of $100,000 to Ramco International, an Arizona company Marcinko and others allegedly formed for the purpose of selling arms to military and civilian customers. Marcinko and Byers were charged with conspiracy and conflict of interest. Marcinko was also indicted for lying to the government. The two men face maximum sentences of up to 12 years in prison and fines of up to $30,000 each.