I had this same problem with an M2. Was shooting obscenely high and to the right. I put on the C shim (didn't come with a D) that wasn't enough, then used a dremel to stack 2 shims, then 3 shims, all trying to get more drop...still couldn't get the pattern off the top of the paper. Even put a taller front bead on.
It's likely your magazine tube is ever-so-slightly cocked up and is pushing up on your barrel. I'm talking thousands of an inch, imperceptible to the naked eye. Once I fixed mine, it was instantly better.
Here's what I did to finally fix it. Take off the barrel, and wrap the receiver in an old t-shirt. Clamp the receiver in a bench vise, and slide a close-fitting pipe over the magazine tube. Put a SMALL amount of pressure on it in the opposite direction from your POI (for me this was low and left). Don't crank on it - your making incredibly slight bends in it (or rather, relieving bends) Go shoot it, and test it out. Make corrections as needed.