A cannon is a large caliber gun, mounted on a carriage, and used primarily as a siege weapon. It’s a period artifact, dating from the 14th Century, but doesn’t appear to have been used as an heraldic charge until much later: e.g., the arms of de Leitan, c.1540 [Nobreza xli]. There was also the “culverin” or “culvering”, a smaller type of cannon but longer in proportion, which seems to have been more for use against troops than walls [Guillim1 225]; and the “mortar”, a short-barreled cannon for lobbing projectiles over walls, found in the arms of von Brösicke, 1605 [Siebmacher 177].
The cannon is mounted in a carriage, mouth to dexter, by default; if palewise, the mouth is to chief. The largest cannon, sometimes called “bombasts”, were mounted in simple cradles and dragged into position for the siege [EB XX:190]; this form is the default for Society heraldry, and is shown in the illustration. Later in period, carriages with wheels were also used; these must be specified in blazon, as a “wheeled carriage” (field artillery, with two large wheels) or a “ship’s carriage” (naval ordnance, with four or more smaller wheels). The illustration on the left is of the latter, taken from ordnance recovered from the Mary Rose, 1545 [Rule 165]. The cannon barrel alone may also be used [Guillim1 225,226], which fact is always specified; it’s shown on the right. For related charges, see gun, pole-cannon.
The Order of the Scarlet Battery, of Æthelmearc, bears: Per fess embattled argent and gules, in chief a culverin dismounted gules charged with an escarbuncle and in base a sheaf of arrows argent.
Gareth of Gutenberg bears: Sable, two cannon barrels in saltire between two halberds in fess and in chief a sallet helm Or.
Alastar the Coursayre bears: Sable, in pale a woman’s head couped and in saltire two cannons mounted on ship’s carriages and crossed at the barrels, a bordure argent.
Angus Olyver bears: Lozengy Or and gules, in pale three cannons reversed, mounted on ship’s carriages, on a chief sable three bezants.