You got good advise from rantingredneck, just be careful not to crush the nut with excessive force! I only suggest that because I have had a "stupid" moment and done so. Otherwise it will work
fine.
There is a specific tool called a
fore-end nut wrench available, I got one in my armorers kit and it is like a waterpump wrench with curved plastic jaws that match the radius of the nut. However I am sure these days a tool like that would be very expensive, mine is 25 years old.
The design of this part has been questioned for many years and was key in my mind when helping Scattergun Tech design their mag extension. The wide over built nut on their extension excepts a 1" strap wrench, I gar-en-tee
it!
. Looking at other aftermarket mag extensions I think they liked the idea and have done some "coping".
Good Luck & Be Safe, Oh, by the way give your 870 a hug for me next time you get her out to play.