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Bill, Mossberg does not make this readily available so I would ask Mossberg Customer Service directly at Tel: (203) 230-5300 [Call: (203) 230-5300] | 8:00am-4:30pm Eastern

I suggest asking Mossberg at : (203) 230-5300 [Call: (203) 230-5300] | 8:00am-4:30pm Eastern

The O.D. of the mag tube on the Mossberg 500 20 gauge is .875 ". i just don't know if the 590 is the same, but I would imagine it is. If you need precise measurements, then yep, time to call Mossberg. Good luck.