The Remington 870 shotgun is a marvel of manufacturing. They're made as inexpensively as possible, yet they are virtually bulletproof. Long a standard of police work, they're also well known in the duck blinds and bird fields all over the world. I don't know how many Remington has made since 1950, but the population has to be in the millions.
Tonight my daughter brought me her boyfriend's 870. It's a garden variety 870 Express Magnum with a 28 inch barrel. It wouldn't lock up and he asked that I look at it. It took a while, but I found a problem that I have never seen in the 870 line.
I took the barrel off and looked first at the locking notch in the barrel extension. Sometimes a piece of shot gets wedged up in there and won't let the locking lug engage the notch. It was clean. Then I took the trigger group out and inspected it for breakage or damage, or more loose shot. Sometimes a piece of shot gets in there and locks the trigger group. It worked fine took.
Scratching my head, I took the bolt out and inspected the locking lug. It was clean and serviceable. Sometimes trash gets in there and keeps the bolt from sliding completely forward. Nope, that bolt was just fine, although over lubed. We cleaned it and I started inspecting the forearm when I noticed it was loose.
The spanner bolt that holds the forearm to the action bars was loose. Really loose. I tightened it and reassembled the shotgun. It locked up fine. Evidently, that spanner collar had worked loose over the years until the bolt wouldn't slide forward enough to lock the action. If the forearm won't pull the bolt completely into battery, the shotgun won't lock up. Basic common sense, yet it was the last thing I looked at, and frankly, I was out of ideas.
Thirty minutes into the job, we were done. Boyfriend's shotgun is working fine. He owes me one, and that is a fine position to be in.