toddx,
I have a similar problem also with a Handi in .243. I get vertical stringing of about 10" at 100 yds while the entire string is 1.5" wide. I also had tried several different loads, but with that much stringing, it isn't an ammo problem. I read the FAQ's and got some good information, but nothing that solved the problem. I tried shimming the forearm with the equivalent of the rubber washer, but no improvement. I have not shot the rifle without the forearm, which I may still try.
I believe my problem is that the barrel is not fit the frame properly. This gun is new. When I hold the rifle up to the light with the chamber closed, I can see light between the top of the barrel and top of the frame. With the rifle locked up, I put a feeler gauge in the slot and a .003 slips right. I am going to send the rifle back to H&R and have them re-fit the barrel and order some more barrels at the same time. With this much of a gap, I suspect that the barrel to receiver fit is flexing quite a bit during each shot.
H&R told me that they will refund my shipping if there is a problem with the rifle.
John