Manuallyaimed light cannon

Heavy machine guns are still fitted to some light naval vessels, but only on the simplest of mountings with virtually no technical changes since the Second World War. In contrast, several new 20mm, 25mm, 30mm and 40mm cartridges and weapons have been introduced and are used in mountings under local control - that is, with the gunner standing behind or sitting on the mounting in order to aim the gun.

One of the few points they share in common is that they are all air-cooled. The main points of difference, apart from calibre, concern the mountings. Small-calibre cannon are usually on free-swinging mountings with the gunner standing behind them (in which case they may be entirely unpowered or have power assistance to help the gunner aim them). Larger weapons normally have the gunner sitting on the mount, which may again be unpowered, have power assistance or even be fully stabilised.

The sighting system is also important. This may range from a simple open sight to a highly sophisticated device combining a laser rangefinder with a ballistic computer so that all the gunner has to do is keep the sight pointing straight at the target; the sighting system will calculate where the gun has to point in order to hit the target.

The combination of mounting and sighting system does far more to determine the usefulness of a light cannon than the calibre. An unpowered mounting and open sights limit the weapon to engaging nearby stationary or slow-moving targets in reasonably calm seas (especially when fitted to small craft). In contrast, a stabilised mounting and a computer sight will convert almost any cannon into a highly effective surface-fire weapon with a credible back-up anti-aircraft performance. Broadly speaking, unpowered free-swinging mountings tend to be of 20mm calibre, with 25mm calibres seeing some powered assistance and 30 + mm are fully powered and often stabilised.

Very few of the light cannon used in naval mountings have been developed solely for naval purposes. Most are also found in ground mountings, armoured fighting vehicles, helicopters and even fighter aircraft, and they will therefore be mentioned more than once in this book. Description of the weapons can be found in Chapter 2, with data summarised in Appendix 2 (Table 3). It should be noted that naval cannon have a long life and some pre-Second World War designs are still in front-line service alongside the modern versions.

Most of the modern 20mm guns, whether gas/blowback or gas-operated as described in Chapter 2, have a comparable performance, with projectile weights of 120-130g, muzzle velocities in the region of 1,000-1,100 m/s, rates of fire of about 1,000 rpm and gun weights of 80-100kg. The

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