Sir Joseph Whitworths Fluid Pressed Steel Tubes, with 3 1/4inch Chambers. It also 'Sports' the Improved Mills system with Cashmores patented Improvement No 17,040 of 1895. When I test Fired this Choice Blend of Steel&Wood, I shot some 'Very Hot' 2 3/4" Live Pigeon Loads, 50 rds to be exact, over the space of aprox 1/2 hr. The Locking system is easy to Open and closed "Like a door on a Hooper Bodied 1933 20-25 Rolls Royce."