I am answering my own question...and the answer is NO, this rail will not work with a Mossberg 590a1. I have the 590a1 with the 18in heavy-walled 18in. barrel and the front of the Warhammer rail will NOT fit over the clamps...barrel is just too big. Just received the Warhammer from Optics Planet - great place to buy from !! - and tested it out and it would not work
I am answering my own question...I purchased this rail and found that it will not work with a Mossberg 590a1. I have the 590a1 12ga with the 18in. heavy-walled barrel and the front end of the Warhammer will NOT fit over the clamps.