An action for a firearm uses a rotating barrel on which are mounted lugs and a cam follower. The barrel is positioned coaxially within a tube fixed to the frame of the firearm. A cam, engaged by the cam follower, is also fixed to the frame. A gas space is defined between the tube and the barrel by a thrust surface on the barrel and a reaction surface on the tube. A gas port connects the gas space with the barrel's bore. A slide overlies the tube and defines longitudinally oriented grooves which receive the lugs when the barrel is rotated by interaction between the cam and the cam follower to align the lugs and the grooves. Motion of the barrel is arrested by interaction between the cam and cam follower, and motion of the slide relative to the barrel is permitted when the lugs align and engage the grooves.