After reading other writeups as examples I decided to compose my own version of the obligitory backstory.
Here goes:
I went to target.
I bought a Doubleshot.
I opened it.
My goal was to eliminate the most annoying aspect of the Double Shot (the shell ejection) while retaining the sweet break-action breech and mix in the efficiency of rear-loading barrels.
Materials
Hacksaw / Dremel
Needle Nose Pliers
2-1/8" of 17/32 Brass Tubing
EDIT: I have come to the realization this is an uncommonly small diameter for barrels, I use ACE FBR which is on the small side.
Hotglue
1/2 Drillbit (actually 17/32 would probably work better)
Goop or your favorite epoxy
Instructions

Open that badboy up, and saw the barrel off at the end of the yellow faux forestock... I'm not sure why this is performed, other than for minimization, compactness, and the coolness factor of having a "Sawed Off" Shotgun. I just did it because everyone else was doing it.

Now to attack the shells.

Grab your hardcore battleworn needle nose pliers and rip the stabilizing sticks out of two of the four shells you are provided with.
Now here's where you have options:
Option 1: (The way I did It)
Grab your drill and 1/2" bit and drill from the top of the shell all the way through the butt of the shell. Cut a 2-1/16" length of 17/32" diameter brass and put your blood sweat and tears into jamming said brass into the the 2" newly drilled shell so that the 1/16" excess of the brass sticks out of the rear of the shell.
Option 2: (The way I should have done it)
Grab your drill and 17/32" bit and drill from the top of the shell all the way through the butt of the shell. Cut a 2-1/16" length of 17/32" diameter brass and firmly slide your brass into the the 2" newly drilled shell so that the 1/16" excess of the brass sticks out of the rear of the shell.
It should look similar to this

Next Hotglue the gap between the brass and the plastic butt of the shell. Make sure no hotglue goes higher than the rim of the 1/16" excess.
Once this is completed thinly epoxy/goop the thin part of the barrel to the plastic springed ring in the barrel of the Doubleshot. MAKE SURE YOU GLUE IT IN SUCH A WAY THAT IT DOES NOT RESTRICT THE MOVEMENT OF THE SHELL SLIDING IN AND OUT OF THE BARREL.
Finished Product:

Another View

Gets about 15' better than stock, shoots stefans, and loads from the rear of both shells. The 1/16" excess helps to succeed where the other "No Shell" mods have problems, the seal.
Here's a Youtube of how it works.
Watch it.
Edited by Radio, 16 November 2008 - 04:46 PM.