Large-calibre artillery

The formal definition of large-calibre artillery used by the United Nations Register of Conventional Arms (UNROCA) is "guns, howitzers, artillery pieces, combining the characteristics of a gun, howitzer, mortar, or multiple-launch rocket system, capable of engaging surface targets by delivering primarily indirect fire, with a calibre of 75 millimetres and above". This definition, shared by the Arms Trade Treaty and the Treaty on Conventional Armed Forces in Europe, is derived from a definition in United Nations General Assembly Resolution 46/36L, which set a threshold of 100mm. Several grammatical changes were made to that latter in 1992 and the threshold was lowered in 2003 to yield the current definition, as endorsed by UN General Assembly Resolution 58/54.

Historically, large-calibre weapons have included bombards and siege guns.

Medieval weapons
In the context of late medieval siege warfare a "giant gun" (Riesengeschütze in German) is a stone-firing bombard with a ball diameter of more than 50 cm. These guns were either manufactured by forging together longitudinal iron bars, held in place by iron rings, or cast in bronze with techniques generally similar to bellfounding. Known examples include the Pumhart von Steyr, Dulle Griet and Mons Meg (all iron) as well as the cast-bronze Faule Mette, Faule Grete and Dardanelles Gun.

At the beginning of the development of these guns was the desire to increase the effect of the projectiles. To this end, master gunners first simply used larger powder loads. These, however, exerted larger pressure on the existing cannon and could make it burst, causing the death of the irreplaceable gunner with his crew (and even kings). In addition, it was observed that, due to their higher velocity, stone balls were shattered by the impact on the walls rather than smashing them. Thus, the mass of the cannonballs and, consequently, of the ordnance, also, continually increased, culminating in giant cannon like the Pumhart von Steyr which fired a 690 kg ball. Apart from the anticipated improvement in penetrating power, other factors such as prestige and a potential deterrent effect also played an important role.

For all their manufacturing quality the guns were only moderately successful. Their military effectiveness turned out to be disproportionate to their overwhelming logistical demands and financial costs. For the cost of a single gun, two or three large bombards with a reasonably smaller calibre (in German Hauptbüchse) could be produced whose firepower was enough to shatter any medieval wall, in particular when it was concentrated in a battery. Due to their less bulky dimensions and higher rate of fire, these artillery pieces could be more flexibly deployed and caused more destruction in any given length of time. Furthermore, the transition from stone to smaller, but much more devastating iron balls meant that super-sized bores became unnecessary. The caliber of a 50 pound ball, for example, could be reduced from 28 to 18 cm when using an iron projectile instead.

In the Ottoman army, large bombards were in use by the early 15th century. The Ottoman army successfully deployed large bombards at the siege of Salonica in 1430, and against the Hexamilion wall at the Isthmus of Corinth in 1446.

At the Siege of Constantinople in 1453, the Ottomans deployed a number of cannons, anywhere from 12 cannons to 62 cannons. They were built at foundries that employed Turkish cannon founders and technicians, most notably Saruca, in addition to at least one foreign cannon founder, Orban. Most of the cannons at the siege were built by Turkish engineers, including a large bombard by Saruca, while one cannon was built by Orban, who contributed a large bombard. Orban was from Brassó, Kingdom of Hungary, before working for the Ottoman army in 1453. The extant Dardanelles Gun, built by Munir Ali in 1464, is assumed to have closely followed the outline of the large bombards used at the siege of Constantinople. A similar super-sized bombard was employed by the Ottoman navy aboard a carrack of possibly Venetian design at the Battle of Zonchio in 1499.

In the second half of the 15th century, further development in siege technology concentrated on the Hauptbüchse, and bombards largely disappeared from the leading artillery arsenal of the dukes of Burgundy.

Modern weapons
In India, a large forge-welded iron cannon was built during the reign of Raghunatha Nayak (1600–1645), and was then one of the largest cannons in the world. Artillery was used by Indian armies predominantly for defending against besieging armies.

With the new metallurgical methods and precision engineering of the Industrial Revolution, a revolution in armaments, including artillery took place. In the 1860s, the industrialist Sir William Armstrong, who had already built one of the first breech-loading rifled artillery pieces, constructed a 600-pounder 'monster gun' of then extraordinary size at the Elswick Ordnance Company in Newcastle. The gun was a rifled muzzle-loader of 22 tons that fired shells of up to 600 pounds (270 kg) and could pierce 4.5 inches (11.4 cm) of iron armour. Armstrong identified them as "shunt" guns, but they were soon popularly known as "monster" guns.

By the 1880s he had built guns of over 40 feet (12 m) in length that could fire 1,800 pound (810 kg) shells and punch through an incredible 30 inches (76 cm) of iron at a range of 8 miles. The gun was exhibited at the Royal Mining, Engineering and Industrial Exhibition held at Newcastle in 1887 for Queen Victoria's golden jubilee.

Prior to World War I, the German military was especially interested in the development of superweapons due to the need for the Schlieffen plan to march past a line of Belgian fortifications constructed specifically to stop such an invasion route. During the opening phases of the war, the Germans employed a 420 mm Krupp howitzer (the Big Bertha) and two 305 mm Skoda Mörser M. 11 mortars to reduce the famous fortresses of Liège and Namur. Their low overland mobility made them arrive later than the infantry at Liège, so several infantry assaults were made with heavy loss of life and generally little success. The guns arrived a few days later and reduced the forts at Liège one-by-one over a short period of a few days.

Larger artillery after this opening period was generally limited to railway guns, which had much greater mobility, or naval monitors (two of the British Lord Clive class monitors were fitted with an 18-inch (457 mm) gun, and HMS General Wolfe fired 33 km at a railway bridge in Belgium). All of the major powers involved employed such weapons in limited numbers, typically between 280 and 305 mm (11 to 12 inches) although some larger weapons were also used.

The longest-ranged and longest-barreled of the heavy guns deployed in World War I was the Paris Gun, which was used to bombard Paris from a distance of over 130 km. The gun had a bore diameter of 211 mm and a barrel length of 34 m. It was fired from concealed fixed positions in the forest of Coucy.

The British attempted to develop weapons to counter the Paris Gun, but none was ready for testing until after the Armistice. A 16-inch gun under development by Vickers for a class of never-built Russian battleships was converted and lined down to 205 mm, with the designation "8-inch sub-calibre Mark I". The barrel was 120 calibres long. Testing commenced in February 1919, but after only six rounds were fired a crack was discovered, and the gun was scrapped in 1928. A weapon of similar concept, the "8-inch sub-calibre Mark II", was converted from a 12-inch gun (either Mark XI, XI*, or XII), producing a 205 mm/75 calibre weapon. However, with the war ending before the gun was ready, this weapon was soon scrapped.

Development continued during the inter-war era, although at a more limited pace as aircraft were expected to take over in the long-range bombardment role. Nevertheless, the Germans built a handful of powerful Krupp K5s and the great 800 mm (31.5 in.) Schwerer Gustav and Dora. The latter had been designed specifically to defeat the Maginot Line, firing a 7-ton shell to a range of 37 km. Although their original role proved unnecessary, Gustav was used successfully to demolish several heavy fortifications, most notably those at Sevastopol. Dora was readied for combat at Stalingrad, but was withdrawn before it could be used. Gustav and Dora were the largest artillery pieces (by caliber) ever used in combat. Development may have ended there but for the ever-increasing Allied air power, which limited Hitler's options in terms of re-opening bombing attacks on London. This led to the development of the V-3 "London Gun" or "Hochdruckpumpe", fired from Mimoyecques in the Pas de Calais, about 95 mi away. Two attempts to build underground bunkers for the huge weapons were thwarted by massive Royal Air Force bombing raids, which made further attempts futile. Two smaller prototype versions of the gun were used during the Battle of the Bulge.

During World War II, the British developed an experimental 13.5/8 inch hypervelocity gun named Bruce, which was deployed near St Margaret's in Kent among their cross-Channel guns. It was intended only for stratospheric experiments, primarily with smoke shells. These experiments were important in the development of the Grand Slam bomb. It was used from March 1943 through February 1945.

The "Supergun Affair"
Canadian engineer Gerald Bull became interested in the possibility of using 'superguns' in place of rockets to insert payloads into orbit. He lobbied for the start of Project HARP to investigate this concept in the 1960s, using paired ex-US Navy 16"/50 caliber Mark 7 gun barrels welded end-to-end. Three of these 16"/100 (406 mm) guns were emplaced, one in Quebec, Canada, another in Barbados, and the third near Yuma, Arizona. HARP was later cancelled, and Bull turned to military designs, eventually developing the GC-45 howitzer. Some years later, Bull interested Saddam Hussein in funding Project Babylon. The objective of this project is not certain, but one possibility is that it was intended to develop a gun capable of firing an object into orbit, from whence it could then drop onto any place on the Earth. Gerald Bull was assassinated in March 1990, terminating development, and the parts were confiscated by British customs after the Gulf War.

It has been suggested that the US Navy had developed a supergun (actually a prototype railgun, known as the Electro-Magnetic Laboratory Rail Gun), capable of shooting shells at 5600 MPH (7 times the speed of sound).