Brad, this is what I would do if I were you. I would take some liquid paper and coat all around where the mag fits into the mag well. It sounds from your description that it is in the side to side clearance where you are having the problem. After the liquid paper dries, carefully insert and lock the mag, and remove the mag. Look for the areas that where the liquid paper rubbed off, if any.
The respective areas of the Mag well are the areas that need to be relieved. I would use a sharp file and a steady hand before I would use a dremel, as it sounds as if you don't have much material to remove. I would also feel more comfortable if you had tried a wider selection of Magazines.
I have the opposite problem, a Mag well on a SAI cluster F*ck I am trying to finish is too loose and the mag wobbles from side to side. Too tight is better than too loose. Please hold off on taking a file to your rifle until a more skilled memeber can confirm my approach.