They are fairly easy to get in the UKicon. Loose Mk2 gas cylinders were tightened up by slightly over-reaming the taper pin hole a tad more and replacing the taper pin with a new one but driving it in harder. Don't confuse a loose gas cylinder with a loose gas blast plate. (that's the blast plate immediately to the rear of the cylinder shown on the bottom left cylinder in Vince's photo). As for removing a worn one, as opposed to a loose one, then they can be a real pig. Accept that it is scrap and weld a xxxxxxg great nut to it, clamp the body, get it red hot and unscrew the cylinder.