How Thick Does Plexiglass Need to Be to Stop a Bullet?

When it comes to using plexiglass as a protective barrier, such as for bullet resistance, the thickness required depends on the specific bullet and firearm in question. The calculations can be quite complex, and it is often more practical to test the material to ensure its effectiveness. Mounting methods can also significantly impact the performance of plexiglass.

Factors to Consider

The first step is to determine the type of bullet you wish to stop. Each caliber has a different velocity and energy, which translates to varying levels of penetration and resistance requirements. Here are the key specifications for some common firearms:

Handgun Calibers

45 ACP: 480 to 640 J in a 98.5 mm2; 4.9 to 6.5 J/mm2

.9 x 19: 480 to 675 J in a 61.1 mm2; 7.9 to 11.0 J/mm2

.22 LR: 180 to 280 J in a 22.9 mm2; 7.8 to 12.2 J/mm2

.357 Mag: 800 J in a 62.1 mm2; 12.9 J/mm2

Cartridge Calibers

7.62 x 39 mm: 2050 to 2200 J in a 45.6 mm2; 45.0 to 48.2 J/mm2

5.56 x 45 mm: 1680 to 1860 J in a 24.3 mm2; 69.2 to 76.6 J/mm2

.30-06: 3800 to 4100 J in a 45.6 mm2; 83.3 to 89.9 J/mm2

14.5 x 114 mm: 30 to 33 kJ in a 165.1 mm2; 181.7 to 199.8 J/mm2

Against certain handgun rounds, a thickness of around 1 inch (25 mm) is generally sufficient. However, for more powerful rounds like the 14.5 x 114 mm, a thickness of 18 inches (457 mm) might be needed for protection against fragments from a 105mm artillery shell.

Mounting and Placement Considerations

The mounting method of the plexiglass is equally important in determining its effectiveness. Free-standing plexiglass with legs that allow it to rock back will provide different results compared to plexiglass bolted into a frame, which is then bolted into a brick wall. The stability and rigidity of the mounting system can greatly affect the overall performance of the plexiglass.

Additionally, the total unsupported area should be considered. Side windows on vehicles typically have smaller unsupported areas, whereas windshields and rear windows have larger unsupported areas. This means that the plexiglass will need to be thicker for larger unsupported areas to maintain its protective properties.

Alternatives to Standard Plexiglass

For applications where bullet resistance is critical, using alternating layers of glass and polycarbonate bonded by acrylic resin can offer better protection. This layered approach combines the advantages of glass, which flattens impacts, and polycarbonate, which holds pieces together and prevents them from shattering into the room. However, this solution is generally heavier than standard plexiglass.

Professional Assessment

For a more precise assessment of the appropriate thickness and mounting methods, it is recommended to consult a professional glazier. They can provide specific advice based on the type of bullet and the intended application. The level of ballistic protection required will also influence the glazing specification.

Conclusion

The thickness of plexiglass needed to stop a bullet depends on a variety of factors, including the bullet type, the firearm, and the mounting method. While no material is completely bulletproof, finding the right combination of plexiglass thickness and mounting technique can provide adequate protection. Always consult with a professional to ensure the best possible outcome.