be used in the anti-aircraft role, eschewing the traditional dual mount in the hull; the standard ammunition load was also lower, only around 600 rounds, compared to 3,000 of the "M" series. The mechanical systems were a development of the "M" series, in particular the leaf spring suspension which was reliable, but in rough terrain would not allow speeds similar to the more modern Christie suspension or torsion bar suspension. Nevertheless, the good power-to-weight ratio represented a significant improvement in mobility over its predecessors. The armour, quite resistant by Italian standards, was sloped and 60 mm thick at the turret front and mantlet (by comparison the M13/40 had 42 mm), but it was still riveted at a time when most tanks were constructed by welding. Compared to welded armour, riveted armour is vulnerable to breaking apart at the joints meaning that even quite resistant plates can be defeated by rivet failures. The front armour had a compound slope with a best facing of 50 mm/45 degrees. The armour was capable of protecting the tank against early anti-tank guns such as the British QF 2 pounder (40 mm, 1.6 in), but was vulnerable to subsequent anti-tank weapons such as the British QF 6 pounder (57 mm, 2.24 in) that entered service in 1942, and was completely overmatched by the QF 17 pounder (76 mm, 3 in) coming into use in 1943. The P 40 design was reasonably up-to-date, but the tank was without some modern features such as welded armour, modern suspension, and a cupola for the commander. The P 40 was designated as a heavy tank in Italy, not because of its weight, but because of its intended role in support of the widely used medium ("M") tanks on the battlefields. In weight, armour and armament it was similar to the medium tanks of the Wehrmacht or other contemporary armies, its armament and protection being roughly the same as the early production American M4 Sherman tank. It was the final evolution of Italian tank designs, that began with the Vickers-based tankettes (such as the CV29 and L3/35) and developed into models such as the M11/39 medium tank, a much heavier construction whose internal design shared many characteristics of the earlier tankettes. Production Some 1,200 tanks were ordered (but the total was later reduced to 500 when development work on the heavier P 43 began), but the start of production was