Got my new 870HD today. I noticed on the mag extension clamp it has a hole in the bottom for what I would assume would be a sling swivel.
Where can I get a proper sling swivel? Is there a better way of doing it?? Then end of the mag extension is not threaded to accept a swivel.

Maybe something with a  bayonet lug??

You can get a swivel and base direct from Remington.

As I recall, there are two types of swivels made for the extension.  One is a swivel and screw that fits the side of the clamp, replacing the screw you have.
The other is a swivel base that fits into the hole in the bottom of the clamp.

For details on what's available, contact Remington.

I'm pretty sure you can buy a plain swivel that fits the hole in the screw that's already in the clamp you have.
For this, take a look at Uncle Mike's:
www.unclemikes.com/adtemplate.asp?invky=8781243&catky=1279575&subcatky1=4548842&subcatky2=2000190

They do sell a complete clamp and swivel set for your gun that includes a new clamp, but I'd stick with the factory clamp if possible.

I would get a Wilson Scattergun sling mount.  www.wilsoncombat.com/sa_slings_mounts.asp

The factory one that swivels on the front will let the sling twist up.  

Quoted: You can get a swivel and base direct from Remington. As I recall, there are two types of swivels made for the extension.  One is a swivel and screw that fits the side of the clamp, replacing the screw you have. The other is a swivel base that fits into the hole in the bottom of the clamp. For details on what's available, contact Remington. I'm pretty sure you can buy a plain swivel that fits the hole in the screw that's already in the clamp you have. For this, take a look at Uncle Mike's: www.unclemikes.com/adtemplate.asp?invky=8781243&catky=1279575&subcatky1=4548842&subcatky2=2000190 They do sell a complete clamp and swivel set for your gun that includes a new clamp, but I'd stick with the factory clamp if possible.

So I called Remington this morning to buy the mount for the hole in the bottom of the barrel clamp. I was told that I could not buy just the mount, but rather had to purchase the entire clamp and the mount for $34. She said they do not sell the mount separately. She clamed the barrel clamps were attatched at the factory minus the mounts because most police departments wanted them that way anyhow, so they stopped including them. I asked if there was any other way of attatching the sling and she said no they had no other means to do so. She then told me I should call Brownell's and promptly gave me their phone number.

I am not sure what to make of the phone call. It left a bad taste in my mouth.

I think I might just buy a second barrel clamp from Midway for $8. Keep the factory clamp and add a second.

Take heart, I think I've found your solution.

FIRST:
I would keep the factory clamp at ALL costs.

The Midway clamp is a Tac-Star, and it's a direct copy of the original Choate design.

While good, these are NOT as sturdy as the factory clamp, and these have a bad habit of moving forward under recoil, marring the finish on your barrel.
Try to tighten them down tight enough to stay put, and the screw strips.

Choate recommends putting black electrician's tape on their's to provide enough friction to hold it in place, and to prevent damaging the barrel's finish.
First time the gun barrel gets hot, the tape melts.

OPTION #ONE:
I just checked my Police gun, and the hole in the bottom of the clamp, is a perfect fit for an Uncle Mike's Push-Button sling swivel.
www.unclemikes.com/adtemplate.asp?invky=9004408&catky=1279575&subcatky1=4548842&subcatky2=2000188

On my gun, the actual sling swivel actually locks into the hole in the bottom of the clamp, I don't even need the lock base.
You can buy one of these sets from Brownell's or direct from Uncle Mikes.  
I'd bet that Mike's would sell you just the swivel and the base "just in case".
If for some reason the swivel didn't fit the hole, you could attach the base with a short machine screw and lock nut, and that would work just as well.

OPTION # TWO.
Another option is to buy one of the between-the-extension sling mount plates.
These fit between the extension collar and the barrel support ring.
These are available with the sling mounting on the bottom, left, or right side.
Here's a picture of one from Brownell's.
www.brownells.com/aspx/NS/store/ProductDetail.aspx?p=12519&title=REMINGTON+SLING+MOUNT&s=29821#29821

This particular one is the same model that is used on the US Marshall's Witness Protection shotguns.
Here's some more numbers to look at on Brownell's:
800-100-005
100-000-525


My advice, go with the Uncle Mike's Push Button swivel.  

Quoted: Take heart, I think I've found your solution. FIRST: I would keep the factory clamp at ALL costs. The Midway clamp is a Tac-Star, and it's a direct copy of the original Choate design. While good, these are NOT as sturdy as the factory clamp, and these have a bad habit of moving forward under recoil, marring the finish on your barrel. Try to tighten them down tight enough to stay put, and the screw strips. Choate recommends putting black electrician's tape on their's to provide enough friction to hold it in place, and to prevent damaging the barrel's finish. First time the gun barrel gets hot, the tape melts. OPTION #ONE: I just checked my Police gun, and the hole in the bottom of the clamp, is a perfect fit for an Uncle Mike's Push-Button sling swivel. www.unclemikes.com/adtemplate.asp?invky=9004408&catky=1279575&subcatky1=4548842&subcatky2=2000188 On my gun, the actual sling swivel actually locks into the hole in the bottom of the clamp, I don't even need the lock base. You can buy one of these sets from Brownell's or direct from Uncle Mikes.   I'd bet that Mike's would sell you just the swivel and the base "just in case". If for some reason the swivel didn't fit the hole, you could attach the base with a short machine screw and lock nut, and that would work just as well. OPTION # TWO. Another option is to buy one of the between-the-extension sling mount plates. These fit between the extension collar and the barrel support ring. These are available with the sling mounting on the bottom, left, or right side. Here's a picture of one from Brownell's. www.brownells.com/aspx/NS/store/ProductDetail.aspx?p=12519&title=REMINGTON+SLING+MOUNT&s=29821#29821 This particular one is the same model that is used on the US Marshall's Witness Protection shotguns. Here's some more numbers to look at on Brownell's: 800-100-005 100-000-525 My advice, go with the Uncle Mike's Push Button swivel.

I appreciate all of the leg work and advice. I hate to impose further, but you wouldn't happen to have a picture of the push button swivel actually in  place on your gun? Do you?
I ask because the space between the clamp hole and the magazine seems to be very shallow. The detent stud would in turn have to be somewhat short.

Ok, I took some pictures, BUT, I don't know how to resize them yet, so these may be huge:

First picture is my magazine clamp showing the hole and a ruler for scale.  There's 5/16" clearance between the extension and the inside of the clamp.
The hole in the bottom of the clamp is 3/8".


Next, the sling swivel and the base.  The base is intended to be installed in a hole drilled in a wood stock, with it flush.


A view of how the screw is installed in the base.


The push button swivel installed in the clamp.


Clearance inside the clamp.


How it would look if you attached the base on the clamp.


How the full assembly would look.


My advice, buy a push button clamp as long as your factory clamp is still set up like this one.

Second option, is to buy a quick detachable stud that has the machine screw shank with the nut.
You could attach the stud to the clamp, then just lock a QD swivel on the stud.

DFW, wow thanks for the great pics. I really like the push button swivel. It seems to fit yours very well. It does look like my clamp has slightly less clearance from the inside of the clamp to the extension. It is only 3/16". I don't think I would be able to make it fit, The hole is 3/8" diam. and 1/2" wide at the inside. Maybe it would work with the stud mount as you have in your pictures.

I really appreciate the help here. Thank You.


BTW How do you get such clear, bright, close and in focus pics? I can never seem to get pictures as nice as yours.

"How do you get such clear, bright, close and in focus pics?"

Try absolute blind LUCK.
I've got a Minolta Dimage Z1 I bought last year, but I've never really sat down and figured it all out.

I simply turned on a florescent Magi-Light I use for a bench light, switched the camera to the Macro setting, and made sure to focus it closely.

Wanna see some NICE pictures?  Go over to The Firing Line or The High Road and check out the photos in the revolver forums by DHart.
He's a pro photographer, and his look like something in American Handgunner magazine.