You need to take the forearm off completely. Then, you'll need a T-tool to take the retainer nut out of the front end of the forearm.
If you look inside the forearm (from the front), you will notice a round piece of metal with 2 slots directly opposing each other. That is the retaining nut, and this is what you must remove for the forearm to come off.
You could also try a pair of needlenose pliers, but be very careful when using pliers- they can muck up the inside of the forearm material in short order. Take your time and it should come out relatively easy.
There is a specific tool for 870s'
mossburgs are are slightly different sized.
If ya got some sheetmetal and a dremel tool you can Make one then just keep it in your tool box.
I made a wrench for these out of a piece of 1 1/4" tubeing and a 6"bolt for a handle. Use a hacksaw to cut away 1/4 of the tube leaving two opposing projections the same size as the slots in the retaining nut. When you reassemble it use some blue loc-tite or fingernail polish as a thread locker. Otherwise the nut will keep vibrating loose, and if loose the forend can crack.
