First, do you have a Wingmaster, a Police or an Express?
If you have an Express you have to modify your gun's magazine tube.
In the Express magazine tube are two projections, or "dimples" that interlock with the magazine spring retainer.
These dimples can be drilled out, or pressed out in several different manners.
Once the dimples are out, the magazine extension screws on the gun.
Once the gun is modified, it's difficult to change it back to the standard length magazine, since the retention assembly for the magazine spring is no longer usable.
There are ways around this.
Of the two Choate magazine extensions you linked, one has a satin blue-black finish, the other has a military type dark gray parkerized finish.
Which you buy depends on the finish your gun has.
If you have a Wingmaster or Express, the blued version is the best match.
How long an extension you buy depends on how long your shotgun's barrel is.
An 18" barrel takes an extension that adds 2 shots, for a total of 7.
A 20" barrel takes a 3 shot extension for a total of 8.
This gets complicated since some sellers advertise extensions by the number ADDED, and some by the TOTAL number of shells the gun will hold.
In the case of Midway, you'll have to actually contact them and tell them how long your barrel is, so you don't order too long an extension, and have it sticking out beyond the end of the barrel.
For black stocks, I recommend looking at Brownell's Gunsmith Supply web site.
They sell at least three brands of magazine extensions, and a number of black synthetic stocks and forearms.
http://www.brownells.com
In order to change out the forearm, you have to unscrew the forearm nut at the front inside of the forearm assembly.
This requires buying, borrowing, or making a "spanner" tool to allow you to unscrew the nut.
Often, a local gun store will have wrench they will either loan or rent you, or they will do it for you.
So adding an extension is fairly easy even if you have an Express and have to modify the magazine tube.
This is a 30 minute job for most people.
Although there are ways to return the gun to the original length tube, this requires some effort.
The Choate magazine extensions are about the best buy from a price/quality stand point, and come as a complete kit, with the extension, longer magazine spring, heavy-duty follower, sling swivel, and the all-important barrel clamp.
USE THE CLAMP.
The clamp supports the extension and prevents knocking it out of line or even off the gun if you bump it.
If you fail to use the clamp, and bump it, the gun's magazine tube threads are damaged, not the extension, and this is an expensive trip back to Remington for a new tube.
A new stock is installed by removing the old one, and screwing the new one on.
The forearm requires the special wrench or a home-made version.
