How to: Build a replica Cannon Part Two
Updated: Nov 21, 2020
The Gun Barrel
The core element, the cannon's barrel, is so named because the earliest guns were constructed with iron staves bound securely by hoops as was the case in coopering a cask or barrel. The other key piece is the gun carriage. The challenge for this project is how to build both elements as light as possible but robustly enough to survive the rigours of transportation and repeated use. What follows, in Part Two, is one possible solution for constructing a replica gun barrel, the named parts of which are shown below:
Step One A 110 mm diameter plastic down-pipe, cut to a length of 1220 mm, formed the core of the barrel. The smooth bore was intended to allow loading and dry "firing" drills to be carried out by "gun crews" of schoolchildren. Any object (faux powder charge, faux shot, sponge, rammer, etc.) cannot exceed the internal diameter of 100 mm.
Step Two Typically guns taper along their length being widest at the breach, where the increased mass of bronze or iron acts to contain the overpressure caused by the exploding propellant, and narrower at the muzzle. To create the tapered effect a series of 12mm plywood discs were cut to different diameters, as shown below. Each disc had a 110 mm diameter hole cut so they can be centred along the plastic pipe.
Step Three The "cascabel" or breach endcap was formed from a series of solid plywood discs cut to the diameters (Ø) shown right and glued together. The exception was the first 220 mm Ø disc which had a 110 mm Ø centred hole cut to accept the pipe and securely glue the latter into the breach endcap. A round wooden newel post cap formed the "knob". The whole breach endcap was sanded to produce the appropriate rounded profile.
Step Four Remembering to glue the discs in situ before the endcap, the result created a central barrel with reinforcing ribs as shown below. Note that the tape ensured each rib stayed in place as the glue dried. The next step was to add two horizontal cylindrical protrusions for use as barrel's mounting or pivoting point. These "trunnions" were formed from 65 mm Ø plastic pipe, each being 130 mm long and capped at one end with a wooden disc insert. The opposite end was profiled to fit flush with the larger "barrel" pipe and then glued either side, centrally at the barrel's midpoint.
Step Five With the internal structure completed, the next step was to create the cannon's exterior barrel shape. The original intention was to have an outer shell of fibreglass but, with no previous experience of using fibreglass, it was decided to take a simpler approach, namely papier mâché. A woven adhesive webbing was stapled to the discs to produce a cone onto which layers of papier mâché could be added. The first layer used commercial blue paper towel soaked in a mix of 1:1 PVA glue and water. While this was successful it is not recommended as the paper towels tend to delaminate when very wet and the resulting finish is not as smooth as would have been liked. Subsequent layers used torn strips of newspaper as these will accept the adhesive much better, are more robust but still malleable, and produce a smoother finish. Alternate the layers to form a cross-ply for added strength. From the image above right the internal ribs are still visible creating a "wavy" profile. Building up the layers between the ribs eventually solves this problem and allows a smooth exterior to be created with the final papier mâché strips laid lengthways.
Step Six Given the amount of explosive energy released when fired, cast cannon barrels were reinforced at points along their length with iron bands, known as reinforcing bands or astragal and fillets, as shown in the drawing of the Winchelsea gun below. The next challenge, therefore was to create the bands.
The chosen solution was to encircle and glue in place sash cord over which papier mâché strips were laid to produce the desired effect (below). The over all effect approximates the reinforcing bands typically found on a gun but are not particularly representative of barrel astragal and fillets.
The drawing below provides not only the cannon barrel's dimensions but the approximate position of each reinforcing band:
Step Seven With the barrel reinforcing bands and trunnions finished, a vent or touch-hole was the last addition (see right). A 4mm Ø counter-sunk hole was drilled through a piece of plywood (approx. 100 mm long x 40 mm wide x 5mm thick). The plywood "vent field" was glued between the endcap and the first barrel band and disguised with papier mâché. To allow the use of a "priming iron" to simulate the piercing of the powder charge, the 4 mm drilled hole was extended through the barrel body.
And finally, the complete barrel assembly was finished with coats of matt black spray paint.
Next... With the barrel complete, the next step is make a supporting carriage. So, in Part Three we will explore "How to:" build a lightweight, portable wheeled gun carriage.