Had this same issue on both of mine hang on a sec.
Edit: You need a FBM (fireball mountain) snatch grip like the one on the left of my black and blue ion. Its a stylish talon snatch grip that doubles as a spacer that makes the needed room. You can get one from this guy
It's relatively cheap to get one but the downside is it will take about 3 months to hit ur door step so u have been warned
Your second option is to dremel out 1\4 inch of aluminum out the rear portion of ur stock frame as well as some material off the the inside where the mac33 will rest if I get a moment il take mine apart for pics
Where's my prize