Above is a 3d render of the entire weapon. For this Instructable I will focus on the process of building the breech blocks.
Step 1: Designing the Breech
We decided it was worth it to have moving parts. 3d printing couldn't provide the necessary strength alone, not and stay under budget, so the evolving design used metal stock and plumbing parts as a sturdy skeleton, with 3d printings to carry the higher levels of external detail.
Many of the mechanical details simply had to be worked out in situ. Because of the lead time for 3d printing (at least 10 working days from completed and accepted model), I just had to count on building the actual working parts while the 3d parts were in production, and hope I could cut and glue them to fit them in place when they arrived. More or less, this is what happened.
The first pass was a massing study; I modeled up the various shapes using primitives (boxes and spheres) in order to get a sense of what looked right. One of the first problems was that the centerline of each barrel lined up with the wall of the tube that ran into the butt stock. That meant the assembly basically "hung out" the sides.
As I developed concepts with the client with a lot of back-and-forth emails, we went back and forth on the concept of exposed hammers. We finally went with them, although they are not strictly period, as by the time we finally decided the 3d order had already been sent. Conceptually, at this point, aluminum hammers rode on a brass rod, with a pair of aluminum "spurs" sitting inside the receiver where it might be possible to link them to the trigger (this turned out to be a stretch goal that was way waaaaay too stretchy).
Since we also had never clarified if these were cased grenades or caseless rockets, whether they had a rim or extractor groove, or whatever, I made some arbitrary cool-looking claw hardware on the inner face. If we didn't like it, we could always cut it off.
Step 2: Mechanicals
The primary piece is a chunk of aluminum stock. It is grooved to ride along two brass rails on the inside of the receiver, and it is drilled out for a brass tube that slides freely along the buffer rod; keeping it aligned but allowing the breech to open. Two bolts stick through in one direction to capture the barrels, and a spring and rod keeps it aligned to the rest of the breech block when the entire assembly is pushed back on the buffer spring.
The point of all this complexity is so the buffer spring works...everything from barrel on back can be pushed back...but rearward pressure on the wooden forend (taken from a Winchester Model 93 shotgun) pushes the face part of the breech assembly back into the breech block and allows the barrels to be pivoted up for reloading.
The other part of the complexity is that the parts were not aligned for mechanical efficiency. The force from the forend had to make a z-bend, pass through the buffer rod, and sneak up in the last remaining gap between the barrels to get to the breech assembly. Had the original design been more "in-line" the mechanical details would have been simpler!
Step 3: Some Day My Prints Will Come
They looked gorgeous. And of course I had to cut them to shreds.
Turns out Alumide is pretty tough in larger thicknesses, I got the only injury I got on this project when my razor saw slipped once.
I drilled through the hole to give me an alignment tool, and used razor saw and x-acto knives to cut away where the existing mechanism had to sit. Then epoxied the whole mess together.
Step 4: Cleaning and Filling
Not trusting 5-minute epoxy, I added another layer of my new friend, Apoxie Sculpt. Apoxie was used not just to fill gaps and strengthen the connection, it was used to build up the final shape of the assembly.
Step 5: Cleaning, Priming, and Detailing
Cutting the Alumide was slightly more annoying. I found out at this stage that the silly (but cool looking) claws interfered with closing the barrels properly, and I trimmed them off.
Most annoying was cutting into the aluminum. The inside face of the breech assemblies is dished, and to maintain that shape I had to dish out the aluminum. That took several annoying evenings with a Dremel and high-speed cutting bits. There were also a number of small alignment errors that I essentially hid by grinding down and using Apoxie to bulk back up.
In our back-and-forth, we decided to have non-functional hammers anyhow because they added some technical-looking detail. They are aluminum, and ride on brass rod inside a brass sleeve which is epoxied into the assembly. They move maybe an eighth of an inch.
Step 6: Painting
What you don't get to see is the round of prime, patch, sand, prime, patch, sand, prime, patch, sand. Especially, cleaning up and rebuilding the shape on the inside of the breech faces took ages. The final round of patching were done with Tamiya White modeling putty, sanding with 220 grit wet-dry and priming with auto body primer. The Alumide had a slight texture that contrasted with the other materials, and it took several coats of primer, sanded between, to hide that.
For the inside face, I tried first using rub-and-buff to get a metallic look and then doing washes on top with acrylics. The intention was to look like bare metal that had been exposed to the combustion products inside the chamber. That's why there is a pattern of burnt powder around the primers. Unfortunately, although that looked great from certain angles, it on the whole looked too much like paint for my taste.
The second attempt was a variation of the outer treatment, but on a blue-gray base instead of black. It only looks "okay," but the breech faces are almost invisible in the final prop anyhow.
The outer treatment is something I picked up at the Replica Props Forum. Over the primer I added two coats (painting always in multiple thin coats of paint instead of single heavy coats) of flat black epoxy for strength.
On top of that a very light "chipping" with a dry brush dipped in silver paint. I was going for nearly-new on this prop, so I held off on the heavy weathering. I also did the chipping early in the procedure so it would be toned down by the later coats.
Then out with the acrylics. I mixed up a wash with burnt umber and raw sienna cut with M. Graham matt acrylic medium. This was applied with a brush and lots of water to get it down into the cracks, and followed by a damp sea sponge to break up any paint lines. Some of it was allowed to pool and dry anyhow to get the characteristic look of old stains.
The final coat was hand-rubbed graphite. What worked this time was to file a General's "fat stick" 2B graphite stick over a sheet of paper to catch the graphite powder, then get that all over my fingers and lightly rub all the exposed surfaces. That was followed by a vigorous buff with a soft cotton cloth, and a light wipe with paper towel and clean rag to remove excess powder. You can of course order a 2lb bag of graphite at a good art store...or a smaller tube of graphite lubricant from a good hardware store.
I had intended to finish with a sealant of Krylon Crystal Clear or Blair Spray Fix, but both of those spoiled the natural sheen of the graphite, replacing it with a plastic-looking shine instead. So this is the final look; just graphite and paint. It seems to be bearing up to light handling just fine, though.
Well. Next one, I think I'll plan around "hammered" paint from a rattle can. And cut the whole thing out of wood and plastic with no moving parts!
This Instructable has been edited to better mate it up with the newly posted Instructable logging the construction of the rest of the prop. Thank you for showing an interest!