Manufacturing tolerances for both cannons and cannon balls were loose, so, to ensure that most balls would fit into the guns for which they were intended, the bores were deliberately made to a diameter greater than the intended shot diameter. The resulting gap, measured as either a difference in diameter or as an annular area, was called "windage." That word has at least three other meanings in ballistics so I will speak of the looseness of fit as "bore-windage." In our period, the bore-windage wasn't standardized, but was typically 0.25 inches. In 1716, the British adopted the rule that the bore diameter should be 21/20th the shot diameter; a 24-pound shot had a shot diameter of 5.547 inches, windage of 0.277 inches, and was fired from a gun of 5.823 inches caliber (Douglas 71). In 1787 this was changed to 25/24th for the Blomefield pattern guns. The short-barreled carronades could be bored more accurately; bore diameter was 35/34ths shot diameter. The French, in contrast, allowed just 0.133 inches (1/45th caliber for a 24 pounder) for heavy (18+) guns and 0.088 inches for field guns. (74).
It should perhaps be noted that even if shot and bore were a perfect fit initially, they wouldn't necessarily stay that way. The shot would rust; the bore would be fouled. Both were subject to expansion when heated, which I would think would especially be a problem for the gun if it had been fired repeatedly. Douglas (74) suggested that at white heat 24-pound shot expanded by 1/70th diameter, and smaller shots by less.
Guns may also be classified according to their construction, as muzzle or breech loading, and as smoothbore or rifled.
Muzzle versus Breech Loading
The cannon barrel is a tube, open at one end (muzzle) and hopefully closed at the other (breech). To load a muzzle loader, it is drawn in, the bore is cleaned, the powder charge and the shot are rammed in at the muzzle end, and the cannon is run back out the gun port. A breech loader has a loading door at the breech end; this is opened, the charge and shot are inserted, and the door is closed.
A breechloader could have either an integral chamber, into which the powder and shot were placed directly, or a removable chamber (Buchanan 251ff); this would be loaded with the powder and shot and then the chamber placed in the breech. The removable chamber looked somewhat like a beer mug.
The proponents of muzzle loading and breech loading have engaged in a half-millennium long struggle for ascendancy. Just because modern naval guns are breech loading doesn't mean that this was a foregone conclusion, or that the vagaries of technological and economic development in the new time line might not provide a niche for muzzle loaders.
The first naval cannon were breech loaders, and Mary Rose(1545) carried both wrought iron breech loaders and bronze muzzle loaders. By the early-seventeenth century, the main guns of a warship were all muzzle loaders, but her swivel guns were still breechloaders.
Muzzle loaders usually were brought inboard for loading. According to Martin and Parker (193), this was done manually; "the much more efficient process of allowing a gun's own recoil to bring it inboard under the restraint of a breeching rope was not developed until well into the seventeenth century." Smith, Seaman's Grammar(1627) says, "britchings are the ropes by which you lash your Ordnance fast to the Ships side"; in the light of Martin's comment, these lashings were too tight for recoil-aided loading.
The longer the barrel, the less convenient it was to load it from the muzzle end, and high-caliber guns tended to have long barrels. With black powder, there wasn't much advantage to making barrels longer than 10 feet, because the powder burns quickly, but with cordite the lengthening of the gun barrels permitted an increase in muzzle velocity. (Sweet 171).
On the Mary Rose, the gun crews were so cramped that it's been suggested that they engaged in outboard loading; the gunner would sit on the barrel, sticking out the gunport, to reload the piece (Konstam 40). A Dutch painting shows this was still going on in 1602. (Gould 227).
While short-barreled carronades were easy to load, they had other problems; the flash could set fire to the rigging, and the vent fire could do the same to the hammocks. (Douglas 103).
For anysmoothbore muzzle loader, the shot had to fit loosely in the bore, so it could be rammed down. But when fired, gas could escape around the shot and out the muzzle, and it was also difficult to keep the projectile centered as it moved down bore.
Late in the history of muzzle loading artillery, the gas escape problem was reduced by use of a gas check, a thin disk that filled the cross-section of the bore. The first gas check was a papier machedisk inserted between the cartridge and the base of the shot, but by 1878 a copper disk was attached to the base of the projectile. (Ruffert). The centering problem theoretically could have been addressed with a sabot (see part 3), but that wasn't normally done.
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The most obvious advantage of the breech loading system was that the gun could be reloaded from inboard while run out, which potentially increased the rate of fire.
The most enduring design problem with breechloaders, which had a door rather than solid metal at the breech end, was preventing gas loss at the breech. The more powerful the gun, and the greater the strength of the powder, the greater the pressure that this mechanism had to withstand.
In Elizabethan breechloaders, the removable chamber was wedged into the breech. Lucar (1588) warns that the gunner "ought not stand upon that side of the piece where the wedge of iron is placed. because [it] may through the discharge of the piece fly out and kill the gunner." (Corbett 333).
According to EB11/Ordnance, the first successful breech mechanism was that invented by Armstrong (1854). The vent piece (a vertically sliding block) was secured by pressure from a hollow screw. To load, this was loosened, the vent piece removed, and the projectile and charge inserted through the hollow. The vent piece was then replaced and the screw tightened. On the chamber side the vent piece had a coned copper ring that fitted into a coned seating.
Unfortunately, the success was limited. "During the bombardment of Kagoshima in 1863 there were 28 accidents in the 365 rounds fired from 21 guns. On a number of occasions the vent pieces were blown from the guns. The guns were also inaccurate." (Brassington).
Moreover, the rise of the ironclads demanded an increase in punch, and the imperfect seals of the mid-nineteenth-century breechloaders frustrated this. The British navy conducted comparative trials and in 1865 it decided to switch to rifled muzzleloaders! (Hogg 16).
This turnabout didn't last long. In 1879, one of the guns of the HMS Thunderermisfired; the misfire was undetected and the gun was reloaded, making it inadvertently double-shotted. When the gun fired again, it exploded, killing everyone in the turret. This accident couldn't have happened with a breechloader-the gun crew would have seen the unexploded charge when it opened the breech-and the British navy reluctantly abandoned muzzle-loading for good (Batchelor 11). At least for new construction; there were still battleships with big muzzle loaders in active service in 1894 (Clowes 47).
This accident provided the impetus for change, but there were other considerations at work. A new powder that could achieve a higher muzzle velocity had been developed. But if it was used in a muzzle loader, the shell zipped out before the charge was exhausted. In other words, the barrels weren't long enough. But if the barrels were lengthened, then recoil wasn't sufficient to bring the muzzle inside the turret for loading. This was actually done on HMS Inflexible(1876) (Watts 56); the muzzle was lowered to an armored loading hatch and the shell inserted by a hydraulic rammer.