Wall thickness in 8ths of caliber
VentTrunnionChase
Bastard culverin 7 5 3
Legitimate culverin 8 5-1/2 3-1/2
Double-fortified 9 6-1/2 4

As with culverins, so with cannon. This is Collado's table showing the fortification for Spanish cannon:

Wall thickness in 8ths of caliber
VentTrunnionChase
Cañon sencillo (light cannon) 6 4-1/2 2-1/2
Cañon común (common cannon) 7 5 3-1/2
Cañon reforzado (reinforced cannon) 8 5-1/2 3-1/2

Since cast iron was weaker than bronze, the walls of cast-iron pieces were even thicker than the culverins. Spanish iron guns were founded with 300 pounds of metal for each pound of the ball, and in lengths from 18 to 20 calibers. English, Irish, and Swedish iron guns of the period, Collado noted, had slightly more metal in them than even the Spaniards recommended.

Figure 25—SIXTEENTH CENTURY CHAMBERED CANNON. a—"Bell-chambered" demicannon, b—Chambered demicannon.

Another way the designers tried to gain strength without loading the gun with metal was by using a powder chamber. A chambered cannon (fig. [25b]) might be fortified like either the light or the common cannon, but it would have a cylindrical chamber about two-thirds of a caliber in diameter and four calibers long. It was not always easy, however, to get the powder into the chamber. Collado reported that many a good artillerist dumped the powder almost in the middle of the gun. When his ladle hit the mouth of the chamber, he thought he was at the bottom of the bore! The cylindrical chamber was somewhat improved by a cone-shaped taper, which the Spaniards called encampanado or "bell-chambered." A cañon encampanado (fig. [25a]) was a good long-range gun, strong, yet light. But it was hard to cut a ladle for the long, tapered chamber.

Of all these guns, the reinforced cannon was one of the best. Since it had almost as much metal as a culverin, it lacked the defects of the chambered pieces. A 60-pounder reinforced cannon fired a convenient 55-pound ball, was easy to move, load, and clean, and held up well under any kind of service. It cooled quickly. Either cannon powder or fine powder (up to two-thirds the ball's weight) could be used in it. Reinforced cannon were an important factor in any enterprise, as King Philip's famed "Twelve Apostles" proved during the Flanders wars.