AP projectiles was significantly reduced if they broke through a thin plate or dense wire net, because the projectiles become unstable in their trajectory and their tip would also be damaged. This method was very effective against contemporary light anti-tank weapons, like the Soviet 14.5 mm PTRD-41 anti-tank rifle and 45 mm M1937 anti-tank gun, the British 57mm Ordnance QF 6-pounder, and the US 37mm gun. Some armoured vehicles used nets of wooden logs at a certain distance from the hull as makeshift spaced armour to protect the vehicle from magnetic mines, thrown shaped charges and grenades, and occasionally suicidal methods (e.g. the Japanese lunge mine). This method occurred on US M4 Sherman and Soviet T-34 medium tanks among others. Against high explosive anti-tank rounds Most of the Cold War spaced armour was designed against medium-to-low caliber kinetic munitions, (e.g. 30mm autocannon and 76mm HESH rounds), especially vehicle side skirts. Most of them were made of RHA plates (Centurion), or thick reinforced rubbers (T-72), and worked in the same way as did WW2-era ones. This light armour also detonates explosive warheads prematurely. High-explosive anti-tank-type warheads (HEAT) use a focused hypervelocity jet of copper or steel to penetrate armour. To be effective, HEAT warheads must detonate at a specific distance from the target's primary armour to ensure maximum penetration. Thus early detonation greatly reduces the penetration of HEAT ammunition. This requires a distance of 1.2 meters even for an early 100mm projectile, thus conventional skirts are effective against HEAT only at very low angle of incidence. The use of add-on spaced armour skirts can sometimes have the opposite effect and increase the penetration of some shaped charge warheads. Due to constraints in the length of projectiles, some designs intentionally detonate closer than the optimum distance. In such cases, the skirting effectively increases the distance between the armour and the target, and the warhead detonates closer to its optimum stand-off. To increase effectiveness of skirts against HEAT weapons some mid-cold-war tanks (early T-64s) had gill-style armour. It contained a few short skirts on the side of the vehicle which are opened in open terrain at an angle of between 30–45°, increasing the space between the armour and the plate. It was effective (mass-to-efficiency ratio), but easily detached from the vehicle so it did not spread widely. A special version of reactive spaced armour is slat armour. It uses the power of the