A gun comprising: a generally tubular barrel defining a barrel axis and having an external barrel surface about the perimeter of the tubular barrel a cradle having a cradle bore for accommodating a section of the barrel wherein the barrel is slidably mounted in the cradle bore such that the barrel can move relative to the cradle along the barrel axis across a range of barrel positions and wherein an interface defined by the slidable contact between external barrel surface and the cradle bore—i) prevents rotation of the barrel relative to the cradle about the barrel axis; and ii) is maintained at a single section of the cradle over the entire range of barrel positions.