If the Storm-Lake barrel feed ramp ISN'T interferring with the magazine seating, check both the magazine release/catch blade and the magazine catch slot for wear, and that the magazine release spring has plenty of force when engaged.

It's quite possible that the difference in recoil force and direction with the ported barrel could release a "marginally" engaged magazine, where the recoil from the standard barrel doesn't. Magazines and other gun parts do wear out, and if this weapon has been fired regularly since 2006, it may even be the likely cause.