To construct a bombard the smith first made a wooden cylinder called a mandrel of the same length as the desired piece, and of the same diameter as its bore. He then forged a number of iron bars of nearly rectangular section which he fitted closely together around the mandrel. Next he made a number of rings of internal diameter slightly less than the external diameter of the bars as assembled on the mandrel. He then heated the rings to white heat and fitted them over the bars so that on cooling they contracted, holding the bars very tightly together.

Before fitting the rings the smith in some cases poured molten lead into the gaps between the bars to prevent the escape of propellant gas. With heavier types the rings were usually placed end-to-end. The term gun barrel derives from this method of construction because the bars and rings correspond to the staves and bands respectively of the common wooden barrel. In small pieces instead of bars the smith sometimes bent a sheet of iron into a cylinder, welded the edges together and shrunk rings around it, as above. Bombards, especially those on which the rings were placed end-to-end, were thus pre-stressed in much the same way as built-up guns made five centuries later. Whether or not the makers realised this fact is not known. As the piece was built up on a mandrel it was open at both ends. Hence lighter types were made breech-loading, the makers fully realising the advantages of this system over muzzle-loading. The breech was in the form of a separate chamber, into which the powder charge was loaded, the open end being closed by a wooden plug. Then the shot was loaded into the barrel, the chamber inserted behind it, and kept in place by a wooden wedge. A fresh plug was required for each round.

Smaller guns were usually supplied with one or more spare chambers which enabled a high rate of fire to be maintained for short periods. Heavier pieces of large calibre were muzzle-loading, a single chamber being screwed on to the barrel or otherwise attached to it. Mons Meg's chamber is screwed on to the piece.