Police may soon be in possession of a rifle which can be programmed to fire bullets at different speeds, enabling officers to 'stun' or 'disable' a violent or dangerous criminal who would otherwwise have been shot dead.

Currently being developed in the U.S., the new weapon is being specifically designed for use by the police and other peacekeeping agencies where the rules for engagement are restrictive but the need for public and officer safety demands immediate action.

The rifle is fitted with an electronic firing mechanism and guided by a laser range finder, while the speed of the bullet is varied according to the amount of aluminum-based gunpowder that is fed to the cartridge.

Researchers at the Oak Ridge National Laboratory say that the technology is based on the reaction which occurs when molten aluminum comes into contact with water. The aluminum oxidizes, releasing hydrogen and explosive energy, potentially four times greater than TNT.

The use of a rifle by police in urban situations is, however, limited. "Armed situations tend to be played out at very short ranges," said Superintendent Norman McKenzie of the Metropolitan Police Firearms Unit at Scotland Yard. "We need a non-lethal weapon which is sufficiently quick-acting and accurate to immediately disable the target. It is no good giving him 15 or 20 seconds to react before he is stopped. The problem we have is that at very short ranges even a baton round can be lethal never mind a slow moving rifle bullet."