In the early breech-loading gun no matter how carefully the smith shaped the end of the chamber and the mouth of the breech into which it fitted, he found that efficient obturation, ie prevention of escape of propellant gas over or through the breech mechanism, was extremely difficult to achieve; some gas inevitably escaped, even in a new piece. As wear progressed more and more escaped, reducing efficiency as well as making life uncomfortable for the Gunner. The problem of obturation was not effectively solved until late in the 19^th century.

Until trunnions were invented in Flanders c.1450 no early guns were fitted with them. Despite their obvious advantages gunmakers long continued to omit them from wrought iron pieces because they complicated construction.

By the end of the 15^th century ML guns of cast non-ferrous alloys were fast superseding those of forged iron. A few BL pieces were made of these alloys but ML types predominated. No more forged iron guns were made after about 1520. Although not easy to load, ML guns had no obturation problems, while the bores of cast pieces were trued ona boring machine after manufacture which was not the case with forged pieces. Thus cast pieces tended to be more accurate. Furthermore, trunnions could be cast integral with the piece, ie did not have to be made separately and somehow attached to it.