I recently purchased a Remington 870 Express Synthetic 12-guage pump-action shotgun with an 18" barrel. The gun will primarily be used for home defense and target shooting. I purchased a 24-inch replacement barrel through Cabela's, which is manufactured by Carlson's. I wanted the longer barrel for those occassions when I want to hunt waterfowl with my father-in-law (mostly geese). However, I am having a problem mounting the new barrel.

Here is my issue. My shotgun has a 6-round magazine (not just a magazine extension), so the barrel guide ring on the original 18" barrel is mounted about 17 inches from the back edge of the barrel. On the new 24" replacement barrel, the guide ring is mounted just over 12 inches from the back edge of the barrel. This means the guide ring on the replacement barrel is much farther back on the magazine (just ahead of the fore-end). As a result, there is nothing to hold the guide ring in place and keep it from sliding forward on the magazine. Should I return this replacement barrel and look for one with a guide ring mounted farther forward, or can I use a barrel clamp to hold the guide ring in place and keep it from sliding forward? If I use the clamp, should I put a second clamp at the end of the magazine to give it extra support? Which clamp would you recommend?

One other issue I have is that I need a plug to limit the number of rounds the gun will hold while hunting. I purchased a Remington 12-gauge shotgun plug, but it does not seem to be long enough as I can still load four rounds in the magazine with the plug installed. Is there a longer plug available?

Since you got the 7 round gun instead of the 5 round gun you need to return the barrel you bought and get your money back.
Problem is, I do not believe a barrel for your gun exist as your gun was never intended to be hunted with.
My advise is to take the money you get back for the barrel return and purchase a regular 870 express with a standard magazine and barrel with screw in chokes for hunting.
Do not use the clamps, this is dangerous and you will damage your gun, yourself, or both!
You can cut a wooded dowel rod to the length you need for a magazine plug....takes a little trial and error to get it just right, but you can do it!
Your guns bore is cylinder bore, this means that it has no choke constrictions and this is perfect for close up work in self defense and room to room combat, it sucks for hunting!! you have no way to change this.
You bought a great home defense gun and that is what you have. Now go buy you a great hunting gun as well.

Thanks for the advice, TuJays. That is kind of what I was expecting. The gun was actually given to me as a gift from my father-in-law. While I like having a good home defense gun, I tried doing some trap and skeet shooting with it and really liked it. Funny thing is I was hitting most of the targets even with the 18-inch barrel (unless it got past 15-20 yards out), so I imagine I could do even better with a 24 or even 30-inch barrel. I have never been hunting, but thought it would be nice to try some time since my son is now taking an interest. So if I do get another gun, I will look for something that will be good for both hunting and trap/skeet shooting. Any suggestions?

For high volume shooting such as clay targets with lighter loads and waterfowl hunting with heavy 3" steel loads I would get a used 1187 semi auto if your funding is tight.
If funding is not a issue I would pick up the Versa Max Sportsman.