I need advice from you seasoned duck hunters.

My son has been hunting ducks a LOT lately. This is his first year of duck hunting and he's on fire for it. He's been using a fairly newish (15 years old, my newest shotgun for sure) Remington 870 with wood stock.  This gun had never seen any wet weather use until now.

Today he told me the action was pretty rough after cleaning it. I checked it out and it appears the wooden forend may be binding against the barrel when fully cocked, making it difficult to push forward and close the action. The wood doesn't LOOK any different, but I suspect hunting in the rain caused the wood to swell. The action operates just fine without the barrel installed. It's just friction between the wood and barrel when fully open.

He's not hunting tomorrow so we're just letting the wood dry for now and will reassemble in the morning. I'm considering sanding the wooden channel in the forend to allow a bit more clearance, following up with something to seal the freshly sanded wood.  Is this the appropriate fix?  Is there any other reason you guys can think for an 870 to start binding like this?

Check the big nut on the end of the slide that holds the forehand grip on. I had one loosen an me with the same result.

Thanks for the suggestion. The nut is tight, the wood looks like it's being held captive correctly by the nut.

Is it an express or wingmaster?
Is the forearm leaving polish marks on the front of the reciver?
It's been awhile for me dealing with 870s, but if there is no polish marks it might be debris in the back of the reviver.
Drop the trigger group, and pull the bolt.
My mossbergs get sluggish when this happens, funny how much duck weed and catail chaff can get inside.

Express. No marks on the receiver, but I'm pretty sure I'm seeing marks on the underside of the barrel. Nothing permanent as this is a new problem, but I think I see 2 lines dragged onto the taper of the barrel where the wood touches.

I taught him how to disassemble and clean the shotgun, the thing is clean! I'm sure there's no debris I'm there.

I'm sure you kmow about the crap wood and finish they use on the express, so I hope that is all that`s wrong with it.
My buddy had one that was shot with 3" loads all the time, with the same symptoms.
Turn out the reciver stretched, making it a parts gun.
Another sign of stretching is the fired hulls will look twisted or chewed up at the crimp.