I have shot my Handi Rifles with the forend off to determine if the forend is causing the problem. Sometimes this has really improved shot to shot consistency. Then you could take the action needed with an O ring or bedding the forend.
Using a torque setting screwdriver to tighten the forend screw is another suggestion. I use 35 inch pounds on all my single shot forend screws. This takes one possible problem out of the equation.
