liliysdad,
The rear base on rifle sighted Remingtons is too far forward to allow a large diameter aperture (aka ghost ring) rear sight to perform properly. The ghost ring needs to be mounted on the receiver closer to the shooter's eye in order for the eye to utilize it as a ghost ring.
This means drilling and tapping your receiver for the (pretty much) permanent installation of the rear sight. If this is to be a dedicated defensive gun that's no problem, but if you want to switch barrels and use it as a fowling piece the rear sight would be a distraction to put it mildly.
Given that you have the factory bases already installed, changing out the sight inserts to a more useful style is likely your least expensive approach. The factory originals are awfully small to use quickly or in less than optimal light. I like the XO express sights myself (see
http://www.expresssights.com/catshotgun.html ). This style of sights has been used on dangerous game rifles for many years and is very fast. It will allow keeping the sights on the barrel, if you plan to use the gun for other purposes as well.
I've used replacement sights from IWI which fit the factory bases as well. These are Patridge style (square notch rear, square blade front) sights which are far more visible than the factory originals. Their web site is at
http://www.ptnightsights.com/ . Meprolight makes a similar set- see them at
http://www.meprolight.com/products.asp?id=2 . I like tritium replacement sights, by the way.
If you are going to istall ghost rings, I like the MMC version best for the rear sight, but that's only one option (
http://www.mmcsight.com/shotgun.htm ). You may have noticed the XO version at their site earlier, Wilson/Scattergun makes their own version, as do a couple of others to include Mossberg.
HTH,
lpl/nc