Allegedly the angle of the feed tower and its length.
From Magpul's websiteUse of this D-50 GL9 in pistol caliber carbines (PCC) is not recommended, because the increased length of the feed tower will be partially unsupported in most pistol caliber magazine wells. This unsupported condition may cause premature wear on the feed tower magazine catch, and (in firearms which deviate from factory magazine well specifications) may result in improper round presentation and failures to feed.