Very simple!
You need to clear the clips regularly, (I do it every turn) Then when the rim gets close watch CLOSELY as you do NOT want to cut the rim recess any deeper.
Many fined accuracy INCREASE after reaming!
As for head position, I am speaking of shooting a Handi, other platforms are different and the AR is very different.
Yes, to some degree different body builds have an effect on a rifle site/mount. But the design of the stock on a conventional rifle (like the H&R) allows for this. On a AR the butt is straight behind and round.... On a conventional rifle the stock has drop or raise and may be cut to allow formed cheek weld. in short, the rifle fits you, you do not fit the rifle. If your craning your neck up or sideways its not good. Your head should only tilt a bit to get scope alignment. If you need more the butt is not placed properly to the shoulder. Tough to get this across via a key board. But if you have a gun club you belong to see if you can get involved with some hi power shooters. Find someone with some experience and just watch him/her shoot. Be open minded and let what you see sink in. If available see if one of them can coach you.
This is something we have done for many years. My primary discipline is pistol, but I also shoot rifle and air pistol. Your shooting will improve I can guarantee it!
CW