Royal Ordnance L9 is a British short-barrelled 165 mm (6.5 in) gun used for combat engineering, particularly the demolition of defences. Initially called Ordnance BL 6.5" Mk I, it was later renamed 165mm L9 Demolition Gun.

The M135 is a license-built copy of the 165 mm L9A1 gun that was used on the British Army's FV4003 Centurion Mk.5 AVRE (Armoured Vehicle Royal Engineers) tank. Depending on the usage, the gun can be depressed and elevated against targets up to an effective range of 925 metres (1,012 yd).

The main armament of the M728 is the 165mm M135 demolition gun. A total of 30 shells are carried. The demolition gun is for use against obstacles. Effective range is about 900 meters. In a secondary role it can be used against enemy fortifications or vehicles. A 7.62mm M240 machine gun is mounted as a coaxial weapon.

该车装备1门M135式165mm破坏工事炮，炮的俯仰范围为-19°～+20°。 炮塔可作360° 旋转 ，转速为1.6°/s； 俯仰 和方向转动用动力或人工 驱动 。 并列于主炮安装了1挺M219式7.62mm机枪(在生产型车上为M240式并列机枪)，指挥塔上安装M85式12.7mm机枪，俯仰 范围 为-15°～+60°。

Ammunition for the M135 165mm gun is issued in fixed complete rounds. A complete round consists of all ammunition parts needed to fire weapon once. It consists of an electric primer and propelling charge contained in a cartridge case and a fused projectile. The cartridge case is threaded onto the base of the projectile.

1965年投产，1968年列装，配备装甲师、机械化师和步兵师的工兵营，装备1门M135式165mm破坏工事炮，炮的俯仰范围为-19°~+20°。 炮塔转速为1.6°/s，俯仰和方向转动用动力或人工驱动。

2022年5月2日 · 在火力配置上，M728安装了一座旋转式三人炮塔，炮塔转速为1.6°/s，搭载了一门165mm的M135榴弹炮（备弹50发），可发射拥有16千克TNT装药当量的碎甲弹。 （主炮最大俯仰角为-10°～+20°）

二战结束后，英国 Churchill VII AVRE（Armoured Vehicle Royal Engineers）工程坦克换装了Ordnance BL 6.5" Mk I 火炮（Royal Ordnance L9，大名鼎鼎的L7的兄弟），这是一门165mm口径的火炮，以射击HESH为主。 165mm HESH炮弹实物图，左侧为2pdr弹头（40mm口径）做参照物，该碎甲弹重达64 lb (29 kg)，威力惊人，射程高达2400米，主要用于清除障碍物，如墙壁，围栏，路障或掩体，以及建筑物。 随后发展的 Centurion Mk 5 AVRE也使用了装备HESH的L9火炮。

2022年12月13日 · 特点： 165mm火炮、防护性能良好、车体前部装有推铲、车体轮廓较大、高级载具加成。 与M60高大的尺寸相比，165mm M135火炮看起来似乎有些不太协调，但其破坏能力却毫不逊色。

2024年3月12日 · The M728 was based on the M60A1 tank. The 105mm gun was replaced by the 165mm demolition gun cannon M135, which was a modified version of Britain's L9A1 weapon. Changes to the ordnance included omission of the bore evacuator, a manual instead of semiautomatic breech, and the breech bushing was combined with the gun tube.