My old BuckMark came with the 4.5" slab-side barrel. I ordered up one of the 5.5" barrels from the Camper when it was introduced. I got it direct from Browning in Utah for around $85, IIRC. After you remove the sight rib and field-strip the gun, you can unscrew the barrel in front of the trigger guard. You must use a properly fitted hollow-ground 'gunsmith' screwdriver! My particular gun required a great-deal of effort to get the screw started, and it's very easy to slip if you are not careful. They must use a torque wrench at the factory to assemble them. I would ask if there are any fit or compatibility issues with your particular gun, just to be safe.
