take it apart. clean it. reassemble.
I bought one years ago and could hardly get it apart. the quality control on it was horrible. the barrel was wedged into the receiver. it took me hours to file down so it would fit and even then it was tough to reassemble. it was tight, but shot reliably. after a hundred rounds or so, it started to cycle more smoothly and i certainly trust it today.
the 870 is top notch in terms of reliability, but remington has been through some ups and downs with QA. if you're trusting it to defend your castle, it's worth seeing what the innards look like and making sure things are lubricated.
mine wears a Knoxx recoil reducing pistol grip stock and a Leupold prismatic optic on the top. one of my go to guns if the need arises. good luck.