The 990 Aftershock is manufactured from the start with a unique “Aftershock birdshead grip” and maintains an overall length greater than 26 inches. This is why it’s considered a “firearm” and not an NFA-regulated short-barrel shotgun (SBS), despite its 14.75-inch cylinder bore barrel. In other words, the Mossberg 990 has no stock and serves as a pistol-grip only shotgun.
athlonoutdoors.com
After the author went through the ATF’s NFA registration process and pay a $200 tax stamp, he added a Mossberg 590 Speedfeed stock.

SBS'ing Mossberg's 990 AfterShock: Short On Barrel, Not Performance
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After the author went through the ATF’s NFA registration process and pay a $200 tax stamp, he added a Mossberg 590 Speedfeed stock.