Semi-automatic (sometimes written as one word, without the hyphen) firearms are those which can fire successive shots without requiring the shooter to manually chamber new rounds or recock the hammer. "Self-loading" and "auto-loading" are sometimes used as equivalent terms. This enables shooters to fire more quickly, getting off more shots in a given time period, but the additional mechanisms required to give guns this capability may make them more bulky, expensive, difficult to maintain, or prone to failure.

When a round is fired from a semiautomatic firearm, some of the force of the shot is harnessed to eject the cartridge in the chamber, load the next round from the magazine, and return the hammer to a firing position. Thus, the gun is ready to be fired again without the need for any action on behalf of the shooter. Semi-automatic operation differs from full automatic in that the trigger must still be pulled once for each round fired; pulling the trigger of a fully automatic weapon will cause it to fire repeatedly until the trigger is released or ammunition is depleted.

If asked to visualize a pistol, most people will picture a semiautomatic: fully automatic firearms employing pistol ammunition are typically thought of as submachine guns, and single-shot or manual-action handguns are relatively rare in modern times. By contrast, there are a wide variety of semiautomatic pistols on the market, produced by several manufacturers and chambered for all common and some obscure handgun calibers. Most of these guns are, if not direct descendants, heavily influenced by the seminal Colt 1911 or Browning HP designs. Revolvers, with few exceptions, are not true semiautomatics: while after each shot another chamber is rotated into alignment with the barrel, the hammer is not recocked, and must be cocked again, either as part of the trigger action (in double action revolvers), or manually (in single action models).

Most modern automatic military rifles have a selectable semi-automatic mode, and "civilian" versions of such rifles are usually exclusively semiautomatic. True machine guns are by definition automatic; while it may be possible to convert them to semiautomatic operation, as with civilian versions of the .50 cal. Browning HMG, this rather defeats the point. Semiautomatic shotguns, while less iconic, do exist (the semi/pump hybrid SPAS-12 probably being the most famous example), and are preferred for target shooting and human-against-human conflict.

Some semiautomatic firearms can, with skill and aftermarket parts, be modified to fire in full automatic and/or burst fire modes. This may be illegal in some jurisdictions, however, and shooters will often find that any increase in effective rate of fire is more than offset by decreased accuracy, especially if they have not been trained in these modes of fire.

Semi-Automatic is a type of transmission fitted to many cars nowdays (although most manufacturers have their own name for it such as "Sequential Automatic", "Steptronic", "Tiptronic" etc. It comes in two major different versions.

Conventional Clutch

This is the ideal form of Semi-Automatic transmission, in terms of having a minimum of power loss. This system has a conventional friction plate clutch, but instead of having a clutch pedal, a computer controls it being activated and de-activated. The user simply selects the gear (s)he wants to use, at whch point the computer disengages the clutch, changes gear and re-engages the clutch. This is the type of system used on Formula 1 racing cars, and can change gears in less than half a second.

Fluid Clutch

The alternative method is to use a Fluid Clutch, also known as a Torque Converter. This is the system which is often fitted to road cars with automatic transmission, that can be switched into Semi-Automatic mode. In this case, the system - when changing gear - is no different to when running in automatic mode, except that it changes gear when you say, rather than when it wants to. The fluid clutch absorbs the engine / wheel speed difference as normal.

Ways of changing gear

There are typically three different ways to select the gear on cars with Semi-Automatic options.

  • Paddles behind the steering wheel - This is how things are done on Formula 1 cars, and also cars like the Honda Jazz. There are two paddles behind the steering wheel, one on each side. Pulling them briefly towards you activates a switch to change up or down a gear (usually down on the left and up on the right).
  • Buttons on the steering wheel - This is like the paddles option, but instead of paddles, there are buttons on the steering wheel itself. These can either be one on each side, like with paddles, or (such as on the Lexus GS300, two on the front and two on the back. The two on the front go down and the two on the back go up.
  • On the gear shift - Some cars have the selection on the gear shift itself. In all cars which are automatic or Semi-Automatic, you select the mode on the gear shift, often by moving it to the side. On these cars, where the gear shift is used to select the gear, you then usually push it forwards or backwards to change up or down a gear. This is the method used on the Toyota Avensis.

It's important to realise that in all cases, there's no direct link from the selection buttons/paddles/lever to the gearbox. Everything is just a microswitch that sends an electronic message to the management system, which actually changes the gear.

Summary

There's no doubt that the Semi-Automatic boxes with real clutches are amazing devices - hence their use in Formula 1 cars - and that they can change gear faster than any person can. On the other hand, the ones that are just adaptations of conventional automatic boxes are fun to have, and do give you slightly more control of your car, but on the whole, modern automatic systems are good enough for 99% of driving.

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