Slide not locking

[Synchronizor]

It could be that the action bar is bent inward, either on its own, or because of the fore-end like Mercman suggested earlier. I'd pull your fore-end assembly off the gun and check to see if the action bars are straight and parallel. While you're at it, I'd take a good look at the end of the left bar to make sure it's not worn down at all.

Also, if the bit of the action bar lock that engages the action bar is bent slightly outward, it could make it easier for the end of the action bar to slip around it. Hopefully you can get an idea of how my action bar lock lines up with the receiver and action bar in the image I posted earlier, and see how yours compares.

[clevolver]

This evening I disassembled the 870 and had a look at the fore end and the action bars.

They appear to be straight to me, not bent in any way. I took a straight ruler and lined it up and it doesn't appear to be warped.

It's harder for me to judge the ends of the action bar and the action lock to know whether they're worn or not, but to my novice eye they don't appear abnormal. There's a bit of finish wear from where the parts rub together, but it doesn't look to me like there's anything obviously damaged about the surfaces.

It appears the only problem then is that the action bars are flexing when torque is applied to the foregrip.

I put the original foregrip back on, and it's definitely much harder to cause the action bar to slip past the action bar lock, but it can still be done. In practice I don't think it would ever happen accidentally with the original foregrip on, but with the added leverage provided by the vertical foregrip, it can cause the action to slip and come out of battery.

I could try replacing the parts, but as they don't appear to be damaged I don't know that it would do any good. Advice?

[Synchronizor]

It's harder for me to judge the ends of the action bar and the action lock to know whether they're worn or not, but to my novice eye they don't appear abnormal. There's a bit of finish wear from where the parts rub together, but it doesn't look to me like there's anything obviously damaged about the surfaces.

Are you able to take and upload close-up pictures of your parts? It's a little hard to describe what to look for, but here's an attempt:

Are the bottom and end faces of the action bar all perpendicular to the side faces? And do they meet the sides with clean corners, or have those edges become rounded off? How about the faces and corners of the action bar lock? Without applying any force to fore-end to make the action bars flex, is there any lateral offset between the inside-facing faces of the action bar and the action bar lock?

[clevolver]

I tried, but they come out very poorly. I'll try again tomorrow with better light and maybe read up on how to use the camera for macro shots. If they come out I'll post them.

[clevolver]

OK I finally got some halfway decent photos of the action bar and action bar lock. It's still not very clear, it's extremely hard to get light and a lens in there together. Hope you can see well enough to help diagnose it.

[Synchronizor]

Those pictures help. It definitely looks like something's off, your action bar lock shouldn't be riding on the outside of the action bar that way. The corner of the action bar lock should be riding closer to the inside face of the action bar, where it will have a more positive engagement. Here's my gun again, for comparison:


I'm still not sure your action bar isn't contributing to some extent, but in those pictures, it looks like your action bar lock is bent outward, and the tip might be worn as well. If I had to guess, I'd say the action bar lock is the primary cause of your problem. If you open your action, push up the carrier, and shine a flashlight in there, how much of a gap is there between the dimple on the end of the action bar lock and the inside of the receiver?

[mart6992]

Yes, this is an old thread but I found it because I was having the same issue and found out my problem and wanted to share.
It was working fine until I took the trigger assembly out to inspect the parts. I pulled the trigger while the assembly was out of the gun and the action bar lock ended up on the wrong side of the left connector. I didn't realize it had happened. This caused my action to fail to lock. After finding this thread I was able to correct the problem and save myself a trip to the local gunsmith.