The best use for air marshals' ammunition is no use.

But when the undercover officers who board passenger jets to prevent hijackings or extremist attacks have to open fire, it needs to be 100 percent effective, and as safe as possible.

After five years of development, researchers at the University of Defense in the Czech city of Brno say they can offer a solution that meets those requirements.

The research was commissioned and fully financed by the Czech Interior Ministry that is in charge of the secretive air marshals' program established in the Czech Republic in 2004 as a reaction to the Sept. 11 attacks in the U.S.

There are such ammunition development programs in other European countries. While the Czechs haven't invented anything new, researchers say they have improved existing hollow point bullets.

The interior of a plane is a specific environment that isn't suitable for common ammunition. After a target is hit from a short distance, conventional bullets can cause catastrophic damage to the aircraft, or kill an innocent passenger.

To prevent that, the Czech researchers focused on developing projectiles that get deformed once hitting a target, significantly reducing the chance of passing through.

The result of testing in an underground lab is 9mm ammunition with a brass bullet, weighing about 5 grams (0.18 ounces). It leaves the gun at a speed over 500 meters (1,640 feet) per second, higher than the 350-400 meters per second that is common for this caliber, providing a sufficient wounding capability.

On impact, the bullet expands its surface, imparts all its kinetic energy into the body and remains in it.

"By developing this 9mm ammunition, we've reached its technical and ballistic limits," said Lt. Col. Ludek Jedlicka from the Department of Weapons and Ammunition at the Faculty of Military Technology.

With the bullet ready, the next step for the researchers is to get it on the market.

Air marshals units in neighboring countries and elsewhere have already expressed an interest in testing the ammunition.

Katerina Rendlova, a spokeswoman for a police unit that deals with foreign issues, said Czech air marshals were cooperating on the development of the new ammunition but declined to elaborate.