A basic tutorial for modding the gun Vincent (Final Fantasy VII) carries around during Advent Children. I don't claim to be an expert (in fact I want to remake this prop now that I know what not to do), but here's the basic guide I wrote based off what worked and didn't work for me.
Step 1: What You'll Need
A base gun (a plastic toy pistol will do nicely)
3 feet of PVC pipe the width of the barrels (I believe I used 3/4 inch)
6 inches PVC pipe slightly wider than your barrels
Three similar looking pieces to use for the hammers (I used the triggers from three other toy guns)
Bondo (it's for filling in dents on cars, can be bought at most hardware stores and auto part stores)
Sandpaper (having both a rough grit and a fine grit is optimal)
Matte black spray paint
Metallic silver paint
Metallic gold paint
Something to cut plastic with (you can use a jig saw, a dremel or a hand saw if it has small enough teeth)
Hot glue (for the detailing; I assume you could also use something like 3D paint or glitter glue)
Clay (for the tiny dogs on the barrels. polymer clay works great because you can bake it in the oven)
A toothpick or similar shaped object
The pendent (I skipped this because I didn't have enough time to make one on my gun, so I can't help you get one of these, sorry)
A long thin circular rod the length of your barrel (you can use just about anything, a wooden dowel rod would probably work great, I used a stick)
A piece of metal/plastic/wood about 4 inches long and 1 1/4 inches wide (this is for the detailing on the bottom of the gun, which I also mostly skipped)
The first step here is to take off the parts of the base gun you don't need. Basically, that's the barrel and most of the top parts. If any of the pieces look like something you can use for a different part of the gun, keep them. While you're at it, cut the wider PVC pipe into three equal pieces. Place one of these pieces at the end of your barrel piece and measure out how long you want your barrels to be. Mine ended up being a little under a foot, I believe. Measure and cut these pieces as well. Be careful to keep the ends as even as possible.
Step 3: Building the Body
This next part might take some time: you're going to make the grip look like it didn't come from a toy gun. That's where the bondo comes in. If you've never used bondo before, here's a few tips:
you don't really want this stuff on your skin,
this stuff smells nasty, don't do it inside,
mix small amounts at a time because it dries really fast,
and apply it in thin coats, it'll chip if you don't.
If your base gun was hollow, you're going to need to fill in the gaping hole where you cut off the barrel. I layered a thick craft foam in the hole until it was nearly level with the edges, but you could use cardboard or something similar. Mix the bondo according to the directions on the package and apply a thin coat to wherever needs to be raised. You can apply it with your hands (with gloves), or with one of those cheap foam brushes. It's also a good idea to cover the hole you just filled and make a nice flat surface on the top of your gun. If there's no place to clip the pendant, now is a good time to add one by placing a loop of wire or something similar and coving the ends with bondo. Since this stuff takes 20 minutes to dry and you're going to need several coats, there's a couple things you can do while you're waiting.
Step 4: Sanding
First you're going to lightly sand the PVC pipe to make the paint and glue stick better. You don't have to sand for a long time, just until it's not shiny anymore. Now is also a good time to smooth out the ends you cut.
Now you just basically build all the other parts you'll need, like the chambers and hammers.
Step 5: Chambers
Overlay the three wider pieces of PVC with craft foam. Use a weaker glue to attach the foam to the PVC. Once the glue is dry, take your utility knife and carve the detailing on the chambers (picture 2). Since you used weaker glue, you should be able to remove the pieces you cut. If the foam is coming off at this point, reglue it with stronger glue.
Step 6: Hammers
This part is a little tricky. I used one of the pieces I removed from the base gun, cut holes in it and glued my hammers (in this case triggers) in the back of it. This sounds like a good idea, but it was nearly impossible to cut holes in the plastic piece I used. What I would recommend doing is taking a smaller piece, gluing the hammers on and then building up the sides and top with bondo.
Step 7: Little Clay Dogs
Like I said, I used polymer clay for these. It should be available in most craft stores, I recommend using black because it will be easier to paint over later. It's handy because you don't need a kiln to fire it. You can do it in your oven, or, if you don't have an oven, a toaster oven will work too. You can use fancy tools if you have them, but a toothpick will work just as well. Please note that polymer clay is mildly toxic. Don't eat it. Don't put it on anything you're planning on eating with/on. Wash hands after using. Don't bake on a baking sheet you're planning on using for anything else (I use a little baking sheet I made out of aluminum foil with wax paper over it). Knead the clay until it's soft and workable. Roll the amount you need for each dog into a ball. For the sides, flatten the ball slightly and draw out the back into the dog's neck. Use a pen or a similar shaped object to create the mouth by moulding the clay around it. Pull a little bit of clay upwards to form the ear, then draw the detail on with your toothpick. Remember to make the two sides face opposite ways! For the top you're going to do relatively the same thing, but you'll be doing it on a slightly more three dimensional object. This time don't flatten it out, just pull the clay down for the neck and up where you want the ears. It should have baking instructions on the package.
Step 8: Mid Piece
I am referring to the piece between the barrel and the chamber. If it has an actual name, I don't know it. It should either be the width of the chamber or the width of the barrel, depending on what picture you go by. I made mine the width of the chambers. The easiest way to do this is to take the three chambers, hold them together and trace the shape of the ends onto a piece of paper. Adjust it however you see fit, add the rear sight, then use it as a pattern to cut out the actual piece. For this piece, I layered pieces of craft foam, but again, you could use cardboard. Cut another strip of craft foam the width of the piece and glue it around the edge to make it smooth.
Step 9: Back to the Grip
At this point, you should be done applying the bondo on the grip. Glue the barrels together. If the barrels aren't perfectly even, it's not the end of the world, just make them even on one end. You'll also glue the chambers at this point, but make sure the seams from your craft foam are facing inward. While that glue is drying, it's time to sand the bondo down! This will make a huge mess and probably shouldn't be done indoors. Or in clothes you like. Start with the rougher sandpaper (if you have it) and sand down any rough edges or bumps. The goal here it to make it look like smooth metal. Any other shaping you need to do on the grip should be done now. Sand it down briefly with the fine sandpaper to make it extremely smooth. If you want to be fancy, you can add wood grain to the part of the grip that's supposed to be wood.
Step 10: Assembly
Your parts should be all finished now, sans color. Gluing them together is the next step. Line up all the pieces before you start and mark where each piece starts and ends. Glue piece by piece, starting with the hammers and working toward the barrels. Make sure the barrel is straight while the glue is drying. Add the rod on top of the barrels and the detailing on the bottom of the gun. Once the glue is fully set, use the bondo to fill in any spaces, gaps or general shaping that still needs to be done. Like usual, sand this layer when it's dry. Make sure it looks exactly how you want it to- this is your last chance to change it. Glue the dogs onto the barrels. Draw out the detailing around the dogs, then fill it in with hot glue (or whatever you happen to be using).
Step 11: Paint
The final step is painting it. The base color you're going to be using is the matte black spray paint. Make sure to spray the inside of the barrels briefly so that white PVC doesn't show on the inside. Add the detail with a paintbrush according to the first picture. Finally, attach the pendent (if you have one).
Step 12: Congratulations!
You now have a non-working, non-metal replica of Creberus!