Took my 870 express out to the range last week, started experiencing a problem.  The action now opens up after most shots.  I made sure that the action was fully closed before each shot; wasn't able to open it up using arm strength, but after I shoot, the action is left open as if the recoil/force is popping it open.  I'm shooting Winchester white box bird shot, nothing fancy.  Thoughts?

Don't know if this is related, but I've already sent this shotgun in for extraction issues.  It was repaired, functioned fine for about a box and a half of shells (same Winchester) until this started happening.  Sorry, I'm not all too familiar with pump-action shotguns.

If you are pulling back on the pump while shooting, it will release the action after ejection. My wife was having a similar issue with mine. It doesn't happen with me since I push forward on the forearm while shooting.

Could this be your issue?

I don't think so, I tried a few times to shoot while intentionally pushing forward on the foregrip, still happened.  At what point does the action release to be cycled after firing?  Connected with the trigger pull, I'm guessing?

It would seem so. While trying one-handed skeet shots (challenge between friends) my action was open partially after every shot.

The action unlocks when the hammer falls. The inertia of the gun moving under recoil causes the fore end, bolt, etc, to keep moving backwards when your should stops the gun from recoiling. As long as the action stays locked in battery before the gun fires (ie the hammer falls) I'd say the gun in functioning correctly.

Really?  So the inertia is really all that is making the action open up after firing?  Seems unsafe to me, because after I've shot, I've now pumped the action halfway and will have to chamber the next round when I close the action whether I want to or not.  Also, is there some kind of break-in period that passed?  Because this didn't happen before.

Every pump shotgun that I can recall shooting unlocked the slide when the trigger was pulled.  It should lock up and not come open without pressing the release lever before the trigger is pulled.  Once you press the trigger, the guns are designed to allow a faster follow up shot by eliminating the need to press the release.  Both Mossbergs and Remingtons do this, but I have more experience with Remington.  I probably shot a different 870 every year for almost 20 years qualifying at work, and they all have worked that way.

I do not think it is inertia making this happen, but engineering of the internal components.  I have yet to see one pop open on its own regardless of being dropped/banged/beat on.  If you think something is not right with it, take it to a Remington smith and have them check it out.  A few bucks may be worth the peace of mind.

I once had a Winchester Mod 1200 Whitetails Unlimited with what I think was a 20" rifled barrel.  With slugs, the recoil was so violent, it damn near acted like a semi-auto... I sold it rather quickly... I'm not into that kind of abuse...

At times, my 870 opens slightly...

It definitely unlocks after each shot (i.e. don't have to press the button), but I distinctly remember not having the action open up on me automatically due to recoil, only when I actively "pumped" the shotgun to load the next round.  I haven't changed anything in the way I hold or shoot the shotgun.  I rarely shoot the 870 though, I think I've only put through less than 75 shells, all the same Winchester white box.  

Sounds like it broke in a bit. What you are seeing is normal for a pump shotgun.

Winchester used to have a spring in the bolt that would unlock and open the chamber after the shell fired called the "Speedpump".  You have to consider that since the shotgun is reaching its full rearward recoil is when the bolt actually opens.  The shot has already left the barrel and pressure has dropped before your shoulder stops the gun.  Perfectly safe as long as hand pressure on the pump doesn't open it BEFORE the hammer falls.

I've fired old Wingmasters that did the same thing.  Its a sign that the shotgun is smooth and broken in.

How did you think it opens up after you fire the gun?