As Kurt suggested ... or possibly you knocked the shell latch loose when you removed the trigger group to change the safety.  If the shell latch moves back and forth with the trigger pins out, you need to restake the shell latch.

Juggernaut,

This is first time I've seen someone post about the same shotgun and the same problem I'm having.
I put on the SideSaddle; Choate forend and buttstock and/or pistolgrip; Choate extened mag and spring.
Removed the extended mag and spring and replaced the factory parts, too.
Nothing would help.
It continued to have the next round in the magazine tube stop before loading.
If I reached in and pressed on the latch, it would load.
Next round would hang up again.

Sounds like the same problem.

Can one of you explain how to 'restake' the shell latch, please?

I'm guessing that the new side saddle you added is too 'long' and interfering with the full rearward movement of the forearm.

I believe you either chop the forearm, or cut the side saddle (removing one shell placing) so to restore full pump range to the action.  Probably much better to do the latter! :-)

BZ

Jaggernaught:

You really should be able to puzzle out the exact problem yourself, though weather or not you can fix it is a different issue.

First, figure out which shell stop isn’t working.  Let’s assume it’s the one opposite the ejection port that actually releases a shell into the receiver.

There’s only two parts interacting:  the one shell stop opposite the ejection port, and the one action bar opposite the ejection port.

The shell stop has a hard to see tab at it’s front, top.  This tab extends into the action bar’s slot in the receiver.

The action bar has a 1/16 inch step along its bottom edge about 5 ½ inches from its rearmost end.  This step goes up and down and is probably hidden by the rear of the forend.  The step has a left to right taper.

Normally the front of the shell stop extends in front of the magazine and holds the shell in the mag.  When the forend is cycled back, the step on the action bar hits the shell stop tab and cams the shell stop away, releasing the shell.

Look carefully from the underneath of the shotgun while cycling a round and you should be able to see what’s happening.  You’ll probably need a flashlight.  

If the other shell stop (the one on the ejection port side) was causing the problem, the drill is almost the same.  However, this stop moves INTO the magazine when the forend is cycled to the rear to catch the second round in the mag and prevent it from following the first one into the receiver.  

Then, when the forend is pushed forward, this stop should release the round and let it catch on the other stop.  The step on the action bar for this shell stop is 3 ½ inches from the action bar’s rearmost end.

This should reveal what the problem is.

Do not remove the triggerguard assembly when doing this.  Otherwise, one or both of the shell stops might pop out and need to be restaked.  

My experience has been that 870’s function quite well even when one or both of the shell stops have become unstaked.  However, this does make removing and replacing the triggerguard assembly a real bear.  

You just about have to have a restaking tool (such as sold by Brownells) to restake the stops.  I’ve tried it with punches with no real success since things are so hard to get at.

Hope this makes sense!

Some 870's do interfere between the forend and the sidesaddle. I noticed that newer ones have been notched to accomodate the sidesaddle.

Also, I think that the sidesaddle can be over tightened and it will affect the action if it is pinching the action down.

Try removing the side saddle and see if the action works properly. If it does, you know what to do from there...

Ray

I agree with the suggestions that you remove the sidesaddle.

Bud

Ok, I recently purchased a 6 shot tacsar side saddle, and the speedfeed 3 tactical stock for my Remington Express 870 Super Mag.  I had the exact same problem, BUT I fixed it.  The problem is that the forend is not coming back far enough.  Either your side saddle is blocking it(not the case in a super mag) or your forend is hitting the receiver before it should.  Remove the side saddle and test, if you still have the problem, check to see if the forend is hitting the receiver when you cycle the action.  If it is, then bingo, you've found your problem.  I fixed this with a dremel, I removed about 1/4"-1/2" worth of material from the inside of the forend where it hits the receiver.  Now it cycles all the way(the action stops without the forend touching the receiver).  Now it feeds properly.  Since the material is removed from the inside of the forearm, you can't even see or notice the change(except the forearm does surround the receiver for about 1/4"-1/2" when the forend is in the rear position).

Hopefully this will get you guys up and running, I know it aggrevated me until I figured it out and fixed the problem.