I purchased a storm-lake barrel for my M&P 40 it has two ports at the end making my barrel 4.9 in. i have a problem with it when i shoot it the mag. falls out the bottom every shot but when i use the factory barrel i have no issues can some one help please I purchased the firearm in 2006

Does your magazine seat fully when using that barrel? I don't see how the barrel really could affect the magazine seat. Perhaps lock the slide back, insert a full magazine, and see if any part of the barrel prevents it from seating correctly. Other than that, the obvious question would be, "when using that barrel, does it affect your grip to where you may be pressing the magazine release button inadvertently?"

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M&P40c/15-22/SD9VE/Mossy500

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.