I just recently purchased an 870 HD 5077. Got to play (read fondle) with it for the first time today. I loaded some shells into the mag and could only get 5 in and the fifth won't go all the way in and jammed against the carrier.
My question is, is there a plug in the magazine and if so, why the hell would they put a plug in an extended mag? If there isn't a plug in there, then I guess it has other issues.
I looked in the manual and it says nothing what so ever about the model with the ext mag. So my other question is, do I just loosen the mag/barrel bracket and then loosen the part of the ext mag where it attaches to the mag tube and pull the two apart so I can take out the spring and hopefully a plug?
Dumb question I know, but it's the only thing I can think of.

That is strange.  take off the screw holding the barrel bracket on with a flathead and slide the bracket off, then unscrew the extension from the magazine tube and see whats wrong. The extension will probably be on VERY TIGHT, to get it off you may need a wrench and some leather.  Use the leather to protect the extension tube collar.  If you dont have the tools, improvise, just dont scratch that collar.  One thing I can think of is that for some reason, the magazine tube has dimples on the inside, normally found on the new 870s that dont come with the factory extensions.  To fix this you need to use something like a dremel tool to file them off.  

I have heard about the dimples on the mag tube and read  the different ways to deal with them, but you would think that since this is a factory installed extension these would not be there. At least you would think so. If they are there, I'm going to be pissed! Besides, from what I've read, I didn't even think the extension could be installed without first removing these.
I'll take a closer look at it later today.
From the posts I've seen more than a few people have purchsed this model recently. Has anyone else run into this problem?

OK, took some time to play with it and this is what I found out:

The first four shells slide in with no problem, if I slide the fifth one in with some authority, it goes in ok and the sixth slides in with no problem.

If I slide in the fifth one in slowly it hangs up and will not slide in any farther. I then have to chamber a shell and again slide in the fifth with authority and can get a sixth in. Once I get the fifth one all the way in the sixth always goes in easily.

I still can't get a seventh one in, it only goes half way before it stops. These are 2 3/4" shells and I'm pretty sure that 7 should fit in the tube.

I'm just wondering if the follower is hanging up where it slides past where the extension is connected.
But that wouldn't explain not being able to get the seventh shell to go in.

The tube should only hold six rounds, you hear that is holds 7 rounds, they mean 6 in the tube, 1 in the chamber.

Quoted: From Remington's web site: MODEL 870™ EXPRESS® SYNTHETIC (18" BARREL) Gauge Chamber Mag.Capacity Barrel Length Choke Overall Length Avg. Wt. (lbs.) Order No. MSRP* 12 3" 4 18" Fixed Cylinder 38 1/2" 7 25549 $319 12 3" 7 18" FixedCylinder 38 1/2" 7 1/4 25077 $345 As you can see it is 7 in the mag. I would e-mail Remington. MIKE.

Nope, mine and everyone elses only hold six in the tube.  They have two shot extensions.

Quoted: Nope, mine and everyone elses only hold six in the tube.  They have two shot extensions.

+1 Floppy.  Mine takes Six in the bottom hole and one in the top.

Everything I've read states 7+1, 7 in the tube and one in chamber. If it is only suppose to fit six thats ok.
As far as the fifth round hanging up, again if I slide it in easy it will jam and no matter how hard I push it it will not budge. The only way I can even get it out of there is to chamber it. If I slide it in there hard, it will go in ok.
I do have an email into Remington so I'll just wait till I hear from them.
This just seem strange and I was curious if anyone had similar problems.

It's likely the extension was mis-assembled at the factory.

The Remington extension is unique in that the tube and the collar are two separate pieces.

To properly assemble the unit, the collar is screwed on hand tight, THEN the extension tube is screwed in until it just contacts the gun's magazine.  
If the extension isn't contacting the magazine tube, OR the extension is over torqued against the gun's tube, the two tubes can get mis-aligned and cause hang-ups.

Another possibility is a defective extension or magazine tube being out of round.

I'd check for proper assembly, and if that doesn't fix it, send it back to Remington.

To disassemble the extension, REMOVE the bracket screw, then remove the bracket.
Unscrew the magazine extension CAREFULLY.  Once the extension tube is unscrewed the magazine spring will attempt to force it's self out of the gun, and it's a STRONG spring.
With the extension tube off, unscrew the extension collar, and remove the barrel.

As for magazine capacity, most shotguns and magazine extensions are listed as the TOTAL number of shells the gun will hold.
The 870 with a 18" barrel will hold SIX shells in the magazine and ONE in the chamber for a total of SEVEN.

A 20" barrel holds seven in the mag and one in the chamber for a total of eight.

Well, looking at the Remington 870 synthetic HD page, it reads "Mag capacity: 7"


 I'm gonna have to get mine out and load the mag to see.  This is bugging me.

Quoted: Well, looking at the Remington 870 synthetic HD page, it reads "Mag capacity: 7"  I'm gonna have to get mine out and load the mag to see.  This is bugging me.

This what I'm saying, I've read numerous places where it says 7+1. But, if it only holds 6 that's fine.

faris -
Appreciate the guidance, I know I will most likely need to take it apart, but you say "unscrew the extension collar and remove the barrel". Is it really necessary to remove the barrel?

I will probably wait to take it apart because I am going to order an after-market follower and only want to take it apart once.
I hope I don't have to send it back to Remington, I hate playing around with that crap.
Thanks guys!

Quoted: Quoted: Well, looking at the Remington 870 synthetic HD page, it reads "Mag capacity: 7" faris - Appreciate the guidance, I know I will most likely need to take it apart, but you say "unscrew the extension collar and remove the barrel". Is it really necessary to remove the barrel?

First of all, just remove the collar with the extension tube still attached, easier that way.  

Secondly, once the collar is removed, the barrel slides right off, its not complicated at all.

Talked to Remington today. The lady in the warranty said 7 in the mag. She said just shoot it to get it to break in.
Wish I could have talked to a regular repair guy. Think I'll fill the mag with as many shells as I can get in there and leave it sit for a time.
As far as the fifth shell jammin in the mag, again she just said shoot it, it needs time to break in. Probably their standard answer.
I know I'm going to wind up taking the mag ext apart because something is just not right. I can see the mag spring needing to relax a little to get the seventh shell, but she did say 7 in the mag.

I think someone at Remington gave you bad info.
Assuming you do have an 18" barreled gun, there just isn't room in the magazine for 7 rounds of ammo.

Try this:  Find the rear of the magazine were it enters the receiver. Lay the gun down on a table and line up seven shells along the side of the tube.
Taking into account the room needed for the spring to compress, judge if the seventh round would fit.

Again, Remington advertises these as "seven shot" guns, but typically don't stress that the seventh round is in the chamber.

I suspect an office worker with no real knowledge of guns just read the total capacity off a fact sheet.

True, the strong spring can make getting the last shell or two in the tube hard, but when the tube is full, there's no doubt.  
The follower makes a "hard" stop and no amount of force can get another round in.

Quoted: Again, Remington advertises these as "seven shot" guns, but typically don't stress that the seventh round is in the chamber.

This is common advertsing practice in the firearms industry.  

"Total capacity" = both holes (top and bottom)

My 870P w/ Wilson Combat +2 ext holds 6 in the mag, and 1 in the pipe.

I have no doubt that she was just reading info off her computer screen or some Co. literature.
I really didn't intend on making a big deal out of the mag capacity. I was more concerned with the problem of the fifth shell jamming up. I have six shells in the tube now and will leave it that way for awhile. If the spring relaxes somewhat, maybe that will take care of the jamming issue.
If it doesn't, I will be contacting Remington again and demand to talk to someone other than a customer rep.
Thanks again for all the help and suggestions.

JUST SHOOT IT, DAMN IT  !!!!