A firearm suppressor has a tube and a baffle stack carried within the tube. A first stack portion has a first inner volume defined by a first inner wall having a first diameter. A second stack portion has a second inner volume defined by a second inner wall having a second diameter unequal to the first diameter, the second stack portion being located forward of the first tube portion. A first outer volume has a first radial thickness formed between the first inner wall and the tube, and a second outer volume has a second radial thickness formed between the second inner wall and the tube. Vents in at least one of the stack portions allow gases and particulates to flow between the inner and outer volumes.