The ramp at the lower rear part of the adapter will need to be removed to permit the modified magazine to fully seat into the grip. This ramp does not serve a useful function in routine use. Its purpose is to make the insertion of the magazine a little easier. This may be an important function with a hand gun, but it's not a significant aid in the case of a long gun.
Trim the adapter to remove the ramp, as shown in the photographs below. It's difficult to make this mistake, but MAKE SURE YOU TRIM OFF THE CORRECT END OF THE ADAPTER!
The adapter is trimmed so that the cut is just short of the bars across the adapter body, as shown in the third photograph. The trimmed length of the adapter, measured from the top end of the long extension, should be 3.35". To ensure the best mag fit, leave the cut surface a little long initially and then trim it carefully with a sharp file until the mag just pushes into place with a nice, solid click. This is a matter of trial and error.