The stun gun administers a five-second electric shock

Halfway through the experiment which began in April 2003 a force spokesman said the trial of the gun was going well.

Northamptonshire's force is one of five taking part in a trial of the M26 Advanced Taser.

It has been designed to be a safer alternative to conventional firearms.

An independent interim report for the Association of Chief Police Officers says in six months the device had been fired six times in total and brought out at 30 incidents.

In each incident where the taser was fired, a person was arrested without serious injury either to themselves or to the police officers involved.

Two darts

Concerns had been raised about the reliability of the gun which delivers a 50,000-volt electric shock.

It is designed to temporarily incapacitate suspects to allow officers to arrest them.

The taser, which has been in use in the USA for more than 20 years, fires two darts into the suspect using compressed air.

When the darts hit, this completes an electric circuit and a five-second 50,000-volt charge is released down the cable, causing the suspect's muscles to contract uncontrollably.

The trial is also taking place in London, Thames Valley, Lincolnshire and North Wales.