In the modern era, 'cannon' are automatic firearms of 20mm calibre or above, although as with much military terminology this figure, indeed this definition, is somewhat arbitrary, as shall be seen. Non-automatic weapons, such as field guns or certain anti-tank weapons, are not cannon. A cannon may have more than one barrel, indeed the most common cannon in use today are the 'Gatling guns' installed on many military aircraft, such as the Aden or M61 Vulcan - smaller-calibre versions of which are called 'miniguns'.

The evolution of the word is complicated. The word 'cannon' itself was only briefly used as an official term during the 17th century, the military otherwise calling their artillery ordnance 'guns'. For this reason, soldiers are taught from an early stage in their training not to call pistols, rifles or shotguns merely 'guns', as this misuse inevitably leads to confusion. Today, 'cannon' remains informal. Some large-bore hunting and sniping rifles are of a calibre equal to or approaching 20mm, but are never referred to as cannon, even though their ammunition might be described as 'cannon shells'. Some tank guns of 100mm calibre or more are fed from automatic loaders but are not usually referred to as cannon, although sometimes they are. Therefore, the term is somewhat arbitrary.

By the end of the 19th century a new generation of quick-firing rifled artillery pieces were generally called 'guns' to differentiate them from the ponderous cannon of yore. For the first quarter of the 20th century the term fell out of use, with the exception of the Hotchkiss rotary cannon, an oversized Gatling Gun used by the American navy. In the early 1930s the Swedish firm Bofors combined one of their smaller artillery pieces with an automatic loading mechanism. (Bofors was as this time quite controversial, as the company worked in partnership with Krupps of Germany. Although Germany's government and industrial capacity were restricted in their possession and manufacture of arms, one half of the world still required high-quality weapons with which to attack the other half, which in term required similar weapons with which to defend itself against the first half. Krupps' arrangement with Bofors allowed the company to design and manufacture weapons outside Germany, to much consternation from other governments - not so much for the morality of this trade, but for the competition it posed to Vickers-Armstrong, Oerlikon, Madsen and so forth).

Perhaps seeking to differentiate this new breed of small-calibre automatic artillery piece - or were they large-calibre automatic rifles? - 'cannon' appears to have come into use at this time, especially with reference to the aircraft guns of the Second World War. The most famous cannon derive from this period; the 40mm Bofors anti-aircraft gun, the 20mm Hispano and ShVAK aircraft cannon, and the 30mm Mauser, also an aircraft weapon. Today the most popular cannon are the aforementioned Vulcan and Aden, as well as the 30mm GAU-8 fitted to the A-10 Thunderbolt, the BAE SYSTEMS 30mm Rarden cannon and the Bushmaster 25mm weapon fitted to the American M2 Bradley. Many of the world's armoured personnel carriers are fitted with cannon of indeterminable manufacture. Cannon are more powerful than machine-guns, although they have a slower rate of fire and are thus more suited for use against vehicles than groups of infantry. Nonetheless, high-explosive cannon shells can make short work of defensive fortifications, notwithstanding their powerful psychological effect.