The Passion Cannon

The movie gun was made huge for spectacle, and resulted in the unusual situation of the film's stars (Cary Grant, Sophia Loren and Frank Sinatra) being effectively "co-stars" with the big movie prop.

In the Forester novel, the gun is an 18-pounder cannon (i.e., it fired an 18-pound iron cannonball). and was described as "having wheels five feet in diameter, and the barrel thirteen feet long"; the movie gun has eight-foot wheels and the barrel about eighteen-feet long, with the complete gun and carriage being about twenty-four feet long (in the movie, the French Colonel erroneously refers to the gun as a "forty-two foot cannon").

The movie gun is a reasonably authentic re-creation of a Pedrero, though of larger size than any that actually existed in the period represented by the gun's style, about the sixteenth through seventeenth centuries, though the overly large, bulbous breech is very unusual, and would not likely occur on a real gun. The largest actual "pedreros" of record were made in sizes up to a fifty pound projectile; the movie cannonballs were about a foot and a half in diamter, and would weigh around 330 pounds if real; the prop cannonballs are authentically styled, including inset lifting eyes that are clearly visible in the evening scene of preparation of the gun for firing at Avila. Real stone cannonballs of this size would have been lifted to the muzzle of the gun with the same style of gin-pole crane used in the film to lift the cannon trails off the trail-wheel carriage. Such guns were slow to load, and in practice achieved a rate of fire of only about one round per hour. Guns of this size would also consume prodigious amounts of gunpowder, about a full keg or more per shot. The only known real guns of this size were bombards, such as Scotland's "Mons Meg", currently on display at the Edinburgh Castle.

Due to the story needs of the fast-paced firing sequence in the conclusion of the film, technical accuracy had to give way to good production values. The gun in the film was fired repeatedly at short intervals, and the balls were lifted to the muzzle by two men; to make this seem plausible, the movie cannonballs were assigned a fictictious weight of 96 pounds, a reasonable weight for two men to lift.

When Captain Trumbul inspects the gun that is hauled out of the river-gorge in the early sequence, it may be seen to have a bolt-pattern on the trail-end armor plates consisting of two rows of bolts in a square pattern, and a zig-zag pattern up the traces; when the "same" gun is lifted a few minutes later with Jose standing on it, a zig-zag pattern may be seen running from the ends all the way up the traces. "Jose's" cannon also has a different bolt-pattern around the pintle-hole in the trail-end transom, than seen in the view in the pass where the village people run past the gun.

After the "downhill racer" scene, where the gun lies in a ditch in the sheep meadow, the trail-ends have the square pattern, but the pattern up the traces is a "domino" or "Mah Jong" style pattern of two, one, two, one ... and I emulated this pattern for the model, as it is more of a match for the existing hole pattern in the construction straps I used to make the armor.

Whether there was actually more than one full-size gun barrel is a question we may never be able to answer; the gun is very elaborate, and unless it was made from a mold (there do appear to be mold marks on the gun in the Flamenco scene where the camera pans over the muzzle), it would be difficult, not to mention the expense, to build duplicates, and I'm not sure it would be necessary.

In the Avila evening set-up sequence, when Trumbull sets the elevation of the piece there is an excellent view of the muzzle profile; this appears to differ slightly from other close-up views of this area of the gun barrel. Also, in the sequence where the gun is raised from the gorge and the missing wheel "replaced", one can plainly count five bolts in the hubcap of the wheel. A few minutes later, when Captain Trumbull falls down in the mud, there are 6 bolts in the hubcap, and this wheel is seen in all subsequent close-up shots. These minor differences are some of the clues I've seen to differentiate the two props.

There was, apparently, a fourth gun, a reduced-scale model. The model was used in the sequence where the gun "shoots the rapids" and ends up in the mud.

The gun has a steel tube in the fiberglass barrel, with no compensation for the tube's central balance point, resulting in the gun being muzzle heavy; this may have been deliberate, as the effect was used in the "elevation" of the gun in the film, but is more likely accidental. The "elevation ratchet" on the trail is a fake; the scene when Trumbull and Miguel inspect the wrecked gun in the gorge shows that the ratchet has no actual connection inside the trails.

The actual elevation in the film was done by loosening the carriage tie-rod nearest the wheels; a rope looped around the pomiglion is wrapped around this tie-rod.

Real guns from these early periods were elevated with a quoin, a wooden wedge, placed on the breech transom (the "bed"); the oversize breech of the movie gun caused the breech transom to be moved forward of the breech, hence it may not be used with a quoin.

Real guns do not have brakes; they are not intended to go up and down hills. The brakes are there only for the up-down hill sequence of the movie.

See my Analysis of the Gun.

The model is built as an emulation, rather than an exact copy, as I lack the tooling and skills to reproduce certain features, such as continous tires and the more artistic features, namely the king's head and the elaborate dolphins (handles over the trunions). There is also the limitation of using available materials, such as the construction ties which have their own holes on 1-5/8 inch centers and which affects the adaptation design.

The carriage is made of pine or fir re-sawed from scrap 2x4 stock, with hardware-store fittings. 6-32 threaded rods simulate the carriage tie rods, with square nuts and 1/8-inch fender washers. The bright finish was removed by bathing the parts in muriatic acid, which dissolves the zinc plating (when enough zinc is added to muriatic acid until the bubbling stops and it has absorbed all the zinc it can, the result is known in the old-time gunsmithing trade as "killed spirits"; the resulting fluid, either straight or diluted 1:1 with H2O, is known as "acid soldering flux"). The parts have then been allowed to oxidize toward the rust-color on the movie carriage.

The barrel is made in 7 pieces, including the pomiglion, and the two trunnions of 3/4-inch poplar dowel. The muzzle and breech are made of 3 pieces of laminated pine; the pyramid-studded cascabel-ring is probably poplar (the wood was donated "firewood" from a furniture factory), and is mounted to a breech-plug of birch which fits into the breech with O-ring seals.

The main section of the barrel was made of a square-tube of fir; pine would have been a better choice, as fir doesn't turn very well, being prone to splintering and chipping. When making a hollow-tube in this fashion, which is to be turned on it's exerior, it's a good idea to fill in the interior corners with epoxy-based wood filler; I didn't do this and had the barrel come apart once on the lathe when I started it at too high a speed. The barrel on the movie gun was made of fiberglass with a steel-tube liner; this model also has a steel-tube liner, in this case made of EMT electrical conduit, although originally it was PVC TUBLUAR ASTM F 409.

The trunions on pre-1756 guns were centered on the lower line of the bore; the passion gun is similar, with the top dead-center of the trunions on the center line of the bore, Cannons had the trunion center on the lower-line of the bore prior to that time, when John Mueller, Master Gunner at Woolwich Arsenal, re-desiged guns with the trunions on the center of the bore, which reduces the firing stresses on the carriage.

This caught me out, as I made my trunions on the bore center line, which changes the appearance of the gun.

Also, the muzzle should be 3-7/8 inches diameter to the base of the pyramids; I made a mistake in the fabrication and made 3-7/8 inches to the tops of the pyramids.

The wheel is constructed the same as a full-size wooden wheel, by making 5 "felloes" (the segments of a wooden wheel), spokes and a hub. The wheel hub has been updated since this photo, to the proportions on the movie wheels.

Iron "Tyres" as used on the movie prop are problematic, as I have no local access to the proper size steel or iron strips, nor the equipment and skills for welding them. Instead of tyres, I used bolted artillery-wheel strakes, based on illustraions in Gun Carriages (mentioned in the links section at the bottom of this page) fabricated from 3/4-inch Simpson Strong-Ties; the same construction ties were used to make the armor-plating for the carriage trails.

Now technically, I've recently learned that strakes, rather than hoop-tyres, were more widely used on artillery wheels, especially on wheels in general that were over 5 feet in diameter due to the problem of uniformly heating a large metal ring.

The movie gun's wheels have strake bands although since the wheels have no strakes the bands do not wrap around the felloes. The way they are built, with the spokes square-socketed into the felloes is typical of straked wheels; obviously, the movie wheels are intended to look as period-correct as possible, but equally obvious is the fact that the art of straking a wheel hasn't been practiced since the late nineteenth century, hence the movie wheels had to be tyred, as this art is still well known and practiced.

The wheel article linked at the bottom of the page gives a detailed description of straking a wheel, and it's a true "dirty job".

Note the photo in the background, from the motion-picture, with Cary Grant inspecting the "re-attached" wheel. This was the scene with 5 bolts on the hubcap; I made my wheel with a 6-bolt hubcap, as seen throughout the movie. The hubcaps were made from 5/16-inch fender washers, and the "bolts" are actually small wire brads driven into the hub. The hub has a bearing-bushing of 11/32-inch hobby brass tubing, and 5/16-inch cold-rolled steel axle.