BudsGunShop.com - Fit a Mossberg 500 12 gauge?

Yes it will Gary. 18inch is just the shortest length you can legally convert with this kit. [This conversion kit only fits 2 ¾ and 3 inch 12 gauge Mossberg 500 shotgun with plain barrel with bead sight. It will not work with a 3 1/2 inch receiver, ribbed barrel, 18.5 inch barrel with rifle sights. Mossberg’s with bayonet lugs or with a barrel brake that is non-removable. We have found that on some Mossberg’s the metal forearm tube is shorter than others, in this case approximately 1-3/16 inch must be cut off the front of our forearm tube so the tube nut can be reinstalled.
Maverick’s/Mossberg’s with the one piece forearm will not work]. FAQ's here : rocksolidind.com/product/mossberg-model-500-bullpup-conversion/

Yes it will Gary. 18inch is just the shortest length you can legally convert with this kit. [This conversion kit only fits 2 ¾ and 3 inch 12 gauge Mossberg 500 shotgun with plain barrel with bead sight. It will not work with a 3 1/2 inch receiver, ribbed barrel, 18.5 inch barrel with rifle sights. Mossberg’s with bayonet lugs or with a barrel brake that is non-removable. We have found that on some Mossberg’s the metal forearm tube is shorter than others, in this case approximately 1-3/16 inch must be cut off the front of our forearm tube so the tube nut can be reinstalled. Maverick’s/Mossberg’s with the one piece forearm will not work]. FAQ's here : rocksolidind.com/product/mossberg-model-500-bullpup-conversion/