Tha cam track on the Lanc was actually mentioned, briefly, in post No. 6. Depending on production block, this didn't actually prevent the guns from firing - it was designed to lift the barrels over the areas of the fins, the props and cockpit area, simply by the cams hitting the track, preventing the barrels from being depressed into line with these areas. Later Marks, and particularly those fitted with the Martin upper turet, had cut-outs, as did the Halifax mid-upper on the MkIII.