had been made since medieval times, the essential engineering problem was that the mechanism could not withstand the explosive charge. It was only with the advances in metallurgy and precision engineering capabilities during the Industrial Revolution that Armstrong was able to construct a viable solution. The gun combined all the properties that make up an effective artillery piece. The gun was mounted on a carriage in such a way as to return the gun to firing position after the recoil. What made the gun really revolutionary lay in the technique of the construction of the gun barrel that allowed it to withstand much more powerful explosive forces. The "built-up" method involved assembling the barrel with wrought-iron (later mild steel was used) tubes of successively smaller diameter. The tube would then be heated to allow it to expand and fit over the previous tube. When it cooled the gun would contract although not back to its original size, which allowed an even pressure along the walls of the gun which was directed inward against the outward forces that the gun's firing exerted on the barrel. Another innovative feature, more usually associated with 20th-century guns, was what Armstrong called its "grip", which was essentially a squeeze bore; the 6 inches of the bore at the muzzle end was of slightly smaller diameter, which centered the shell before it left the barrel and at the same time slightly swaged down its lead coating, reducing its diameter and slightly improving its ballistic qualities. Armstrong's system was adopted in 1858, initially for "special service in the field" and initially he produced only smaller artillery pieces, 6-pounder (2.5 in/64 mm) mountain or light field guns, 9-pounder (3 in/76 mm) guns for horse artillery, and 12-pounder (3 inches /76 mm) field guns. The first cannon to contain all 'modern' features is generally considered to be the French 75 of 1897. The gun used cased ammunition, was breech-loading, had modern sights, and a self-contained firing mechanism. It was the first field gun to include a hydro-pneumatic recoil mechanism, which kept the gun's trail and wheels perfectly still during the firing sequence. Since it did not need to be re-aimed after each shot, the crew could fire as soon as the barrel returned to its resting position. In typical use, the French 75 could deliver fifteen rounds per minute on its target, either shrapnel or melinite high-explosive, up to