Summary

A spring-powered toy cannon inspired by the classic Britains 4.7" naval gun, suitable for playing H. G. Wells' seminal miniatures game Little Wars.

See the sections below for further information.

(The toy soldiers in the photo above are from Earsling's Napoleonics collection, also available here on Thingiverse.)

Print Settings

Printer Brand:

Creality

Printer:

Ender 3 Pro

Rafts:

No

Supports:

Yes

Resolution:

0.12mm layer height, 0.3mm nozzle

Infill:

50% cubic

Filament: eSun PLA Pro Gray

Notes:

All holes are about 0.25mm oversized. If your printer prints holes more or less than 0.25mm undersized, you may have to adjust hole expansion in the print settings, or use larger/smaller pins.

Wheels (2) and Axle Caps (2)
Print laying flat, with the side with the smaller rim facing down. Low support distance helps with getting a clean inner surface. You can skimp on infill for the wheels, since they don't bear heavy loads.

You have two options for the axle caps: the STL with 'lugs' in the name has two lugs which lock into the axle for a sturdier fit. The other axle cap file is a simple press-on version.

Carriage and Hammer
The carriage can be rotated so that both ends are touching the print bed, which saves on filament for supports. The hammer can be printed laying flat. Do not skimp on infill. Both the hammer and the carriage bear large spring forces.
Barrel
Print vertically, smokestack-style. Printing with a brim and draft shield helps keep the nozzle from melting into it/knocking it over on the way up.

Post-Printing

Materials

(2) 2x22mm locator pins
Uxcell sells the correct pins on Amazon: B07M63LPFD. McMaster-Carr item 91595A253 is also the correct size, but spendier.
Leaf spring
Should be about 100mm or 3⅞" long. 0.025" and 0.032" (0.6mm and 0.8mm) thickness yield good power without breaking things (either inside the cannon or outside of it). Prime Line part number B803 is a spring for a screen door which is easily repurposed (cut a bit off the end to fit), and can be had 2/$6 or so. McMaster-Carr 9074K38 or similar is cheaper per unit, but the break-even point is about 10 guns when shipping is included. Note that McMaster-Carr ships 5-foot lengths of spring steel in a 5-foot container, and longer lengths are shipped on a spool. A 50-foot length is probably only twice as much, counting shipping, as a 5-foot length.
#6-32 brass thumbscrew with collar, 3/4" threaded length
McMaster-Carr 92421A151 is the elevation screw the gun is designed for, but any similarly-sized screw will work, even an un-collared one. The important dimension is diameter of the screw head, which in this case is 3/8" or about 9.5mm. (In the future, I'll model a printable screw-gun interface for people who can't get the part in question.)
#6-32 brass nut, 1/4" across (measured face to face), 3/32" thick (optional)
McMaster-Carr 95130A120. Fits into a pocket on the gun carriage, to reinforce the screw hole, but probably isn't necessary given my testing.
#6-32 tap (optional)
You can use the thumbscrew to cut threads into the elevation screw hole, but you'll probably need to turn it with pliers. Using a proper tap on the hole makes it much easier, both to install the screw initially and to turn it when adjusting elevation.
1/16"/1.5mm pin punch
Used to remove supports from within pin holes. You can also eliminate supports from inside the holes in your slicer—none of them are large enough to necessitate supports.

Assembly

Video instructions can be found here: https://youtu.be/0GweHWliP5A

Remove supports.
If using the lugged axle caps, file off any protrusions on the underside of the carriage axles so that the axle caps fit cleanly.
Run the elevation screw through the housing on the side of the gun carriage to create threads, or use a #6-32 tap.
Place the spring over the center spring guide in the carriage. Slide one end under the hammer braces in the forward part of the carriage, then push down on the other end of the spring and slide it backward into the spring pocket.
Install the hammer. Run a pin punch through the carriage and the hammer pin to assist in location, then install the pin from the side opposite the punch, using the pin to push the punch out.
Cock the hammer by pulling it back until it rests against the spring.
Place the brass nut (if available) in the elevation screw housing, then thread the elevation screw in about 2/3 of the way.
Working from the front, slide the barrel breech-first into the carriage, so that the trunnion lines up with the wings on the carriage, and the nub on the barrel engages the elevation screw's collar.
Secure the barrel using a pin. The punch trick helps here, too.
Install the wheels: place an axle cap through the hub of each wheel, then press a wheel and cap onto the end of each axle.

Firing

Pull the hammer back until the breech is accessible.
Load a projectile by pushing it into the circular portion of the breech. The barrel diameter is 6.75mm, or 6.5mm accounting for hole shrinkage. The provided printable shell is 6mm in diameter, which provides a good balance of power and accuracy. 1/4" or 6mm dowel stock should also work well.
Adjust your aim by turning the carriage and using the elevation screw. Note that the force of the hammer will push the barrel downward, so to ensure your elevation is set correctly, push down gently on the front of the barrel until the elevation nub stops it against the screw.
Using the tip of a finger, flip the hammer up.

How I Designed This

The original Britains Ltd. patent on the mechanism for their 4.7"-style guns was extremely helpful.

https://worldwide.espacenet.com/patent/search/family/034361260/publication/GB191501215A?q=toy%20naval%20gun

Otherwise, drawn in FreeCAD.

Changelog

11/29/20

Initial upload; not functional.

12/2/20

Revision 2; moderately functional.

12/5/20

Revision 4; release candidate 1.

Early in the morning on 12/6, I uploaded a replacement barrel for revision 4, to fix the trunnion protruding into the bore.

In the afternoon of 12/6, I uploaded a second replacement barrel for revision 4, to fix some various geometry issues caused by the previous error.

12/9/20

Final revision. I don't recall if there are any changes from revision 4, but I wanted to make sure I had the latest versions uploaded.

12/14/20

Noticed that the uploaded carriage file was incomplete, uploaded a correct version.

I've been toying with a gun styled like an older artillery piece, to fit a bit better with warfare 1865 and earlier, but don't know how long it'll take me to attain satisfactory results with it.

12/30/20

Added grooves to the carriage axle and corresponding lugs to the axle cap, to allow wheels to stand up to side-to-side rocking a little better.

3/15/21

Redesigned the shell to print better with a skirt: chamfered at the base, so there's less to catch on the base.

Spring-powered 4.7" naval gun toy