above chart; all copper-jacketed lead FMJ, except 44 mg is lead semi-wadcutter gas-check, and 30-06 is LEAD core soft point. Test standards Bullet-resistant materials are tested using a gun to fire a projectile from a set distance into the material, in a specific pattern. Levels of protection are based on the ability of the target to stop a specific type of projectile traveling at a specific speed. Experiments suggest that polycarbonate fails at lower velocities with regular shaped projectiles compared to irregular ones (like fragments), meaning that testing with regular shaped projectiles gives a conservative estimate of its resistance. When projectiles do not penetrate, the depth of the dent left by the impact can be measured and related to the projectile’s velocity and thickness of the material. Some researchers have developed mathematical models based on results of this kind of testing to help them design bulletproof glass to resist specific anticipated threats. Environmental effects The properties of bullet-resistant glass can be affected by temperature and by exposure to solvents or UV radiation, usually from sunlight. If the polycarbonate layer is below a glass layer, it has some protection from UV radiation due to the glass and bonding layer. Over time the polycarbonate becomes more brittle because it is an amorphous polymer (which is necessary for it to be transparent) that moves toward thermodynamic equilibrium. An impact on polycarbonate by a projectile at temperatures below −7 °C sometimes creates spall, pieces of polycarbonate that are broken off and become projectiles themselves. Experiments have demonstrated that the size of the spall is related to the thickness of the laminate rather than the size of the projectile. The spall starts in surface flaws caused by bending of the inner, polycarbonate layer and the cracks move “backwards” through to the impact surface. It has been suggested that a second inner layer of polycarbonate may effectively resist penetration by the spall. 2000s advances In 2005, it was reported that U.S. military researchers were developing a class of transparent armor incorporating aluminum oxynitride (ALON) as the outside "strike plate" layer. Traditional glass/polymer was demonstrated by ALON's manufacturer to require 2.3 times more thickness than ALON's, to guard against a .50 BMG projectile. ALON is much lighter and performs much better than traditional glass/polymer laminates. Aluminum oxynitride "glass" can defeat threats like the .50 caliber armor-piercing rounds using material that is not prohibitively heavy. Spinel ceramics Certain