I had an H and R Handi rifle in 223 a while back. Must say it was the most difficult rifle to develop a load for that I have ever had. It had a gunworks suppressor. I too had the drifting zero problem and would also split groups. 2-3 in one area and another 2 of to the right and high.
Between your zeros, did you pull rifle apart? I found that if i applied different torque to foreend screw it was applying different pressure point on barrel. Fixed it by doing two things.
One, I used a torque driver to ensure i did the screw up the same every time, and two, I made a thin washer and put it in the screw recess in the forend, essentially floating front portion of barrel. After that I stared to get consistent tight groups..
Good lunk with yours they are a fun rifle to have.
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