"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.