Slide not locking

[clevolver]

I have an 870P Police Magnum that I bought used for $350. I haven't shot it yet. I swapped the foregrip out for a vertical grip that I like for better control of the fore end. I had it out of the safe to look at it tonight, and discovered that the slide does not lock up solid when I rack it. Unless I'm mistaken, it should only unlock by pressing the slide release by the trigger guard, or by pulling the trigger. But it will come open with just a tug on the vertical grip. I tried pushing it forward hard to get a solid lock, and it still will come loose about 3/4 of the time when I try pulling the slide without pulling the trigger or engaging the slide release. Once in a while it seems to lock in place as I believe it should.

That's not right, is it? What could be causing the problem? Could I have reassembled incorrectly when I installed the new foregrip?

[Synchronizor]

Was the gun doing this before you bought it? And did you do anything else besides change the fore-end?

[clevolver]

I didn't notice it ever do it before swapping out the fore grip... for what it's worth. That's the only modification that I've made to it since I took ownership so far.

[Synchronizor]

It's possible the new fore-end is not allowing the slide to come all the way forward. When the action is open, the part of the action bar lock that you push on should be retracted, and won't stick out much from the trigger guard. When you close the action but it doesn't lock, does that "button" pop further out from the trigger guard, or stay retracted?

[mercman]

I had this happen when I attempted to install a ribbed polymer forend on an Express HD. The forend pushed the action bars inward and wouldn't allow them to lock in. I pulled the forend and installed it on a Wingmaster and there was no problem. Installed it back on the Express and had the same issue. I measured the action rail channels and the polymer forend was tighter than the original Express foregrip channels. I just set it aside at this point but could have relieved it with a little judicious Dremel work.

[clevolver]

I don't think the slide has any trouble moving all the way forward. I can try swapping the original fore grip back on and see if it locks completely.

When it fails to lock, the slide release button DOES stick out, past the trigger guard, as it should.

I've played with it a bit more, and noticed something more. When I'm gripping the foregrip by the vertical bar, my left hand tends to push the vertical grip to the right just slightly. The slide rails seem to flex a bit, and this is what causes the slide to unlock. If I pull straight back, it seems to stay locked pretty good. But the vertical grip gives me a little extra leverage, which seems to put some tortion on the slide rails, allowing it to come unlocked without the slide release button being pressed.

Moreover, if I pull the slide back more while the slide release button is still not pressed, the slide jams about halfway down, and will not go further. Most of the time it also seems to get stuck here, and will not slide back forward without significant force. When I first noticed the problem, I didn't try to rack the slide fully, and just tried to push it back forward to achieve a positive lock.

Still not sure what the problem is, but my guess is that with the original grip back on, it'd be hard to cause the problem, due to the lack of leverage from not having a vertical grip. However, I'd like to figure this out so I can run it with the vertical grip.

[Synchronizor]

It sounds like your action bar lock or action bar is worn or damaged enough for the lockup to fail when you twist the fore-end. If you push your shell carrier up, you can see the point where those two parts interact. Try instigating the problem while looking at that spot. Hopefully you can get a better idea of what's going wrong.

Here are some images of the action bar and action bar lock on my 870. I removed the shell carrier for the pictures, but you can see them well enough by pushing the carrier up.

Top to bottom: action locked, action unlocked, action open

870 slide lock.jpg (68.49 KiB) Viewed 13481 times

[clevolver]

Thank you, that picture helps a great deal.

I am able to look in with the shell carrier lifted and see the parts engage with each other. If there's wear or damage, my eyes aren't yet trained enough to see it.

If I press the slide release, the action bar lock lifts up, clears the action bar, and allows the action to slide open, as normal.

If I twist just a little bit on the vertical foregrip, I can see the action bar flex to where it is just barely engaging the action bar lock. If I then jiggle the slide just a bit, the two pieces disengage. The action bar lock does not lift, but the action bar slides around it. Thus, when the slide is about halfway down, the action bar jams against the action bar lock, preventing the slide from opening further.

Now that I can see what is happening, what can I do to fix it?

[Synchronizor]

It sounds like something's not quite right if there's enough play between those components for the action bar to slip around the action bar lock. When this happens, does the action bar slide around on the inside or the outside of the action bar lock?

[clevolver]

The action bar slips over the action bar lock, and then slides past it on inside of the action. Yeah, something definitely doesn't seem right if it can do that... I'm just not sure what that something is yet.