It should not be capitalized. Several firms manufacture magnums. Semiautomatic, however. doesn't fit too well in 1 col headline. Call it a pistol. Call it a gun if the shot isn't looking.
Machine gun is two words.
Like deadly mother. Sten gun and Bren gun are crossword puzzle words. The Russian AK47 and its updated sibling the AKM, are both fully automatic weapons in their military versions. The civilian versions, semiautomatic only, an be converted illegally to full automatic, but it takes a knowledgeable gunsmith to do it. The AK-47 shoots a 7.62, called a Warsaw Pact caliber, which is a shorter cartridge than the standard NATO 7.62 round.