ONE-WAY Bullet Proof GLASS:

 You truly can learn something new every day eh ...

'Bullet Proof Glass' does not really exist - because it is neither "bullet proof" nor made of glass nowadays. "BALLISTIC GLASS" will resist some small arms fire depending on specification.

I have fond memories of a large sheet of this material that screened the inner entrance to my Christchurch Manchester Street 'Dungeon' indoor range way back in the 90's. - There were several instances when it withstood punches thrown by "street people" denied access .. which is of itself remarkable - as stopping bullets is very different to resisting punches.

BALLISTIC GLASS nowadays is a laminate of multiple layers of  ACRYLIC (poly-methyl-methacrylate) - POLYCARBONATE - PC .. that will be at least EIGHT to TEN TIMES thicker than window glass and much heavier. Plus it's VERY EXPENSIVE. - The layers are stuck together by very thin sheets of POLYURETHANE adhesive.

My take on this is that the hard fragile clear acrylic lamination deforms the bullets on impact while the polycarbonate layers catch them.

As the name implies, “one-way” (or “unidirectional”) ballistic glass stops bullets heading one way, but let’s them through in the other. For example, a soldier at a checkpoint could be protected from a spray of small-arms fire, but able to shoot back without leaving the safety of his post.

At first reading, this seems pretty counter intuitive. After all, most structural surfaces are basically symmetrical: a cinder block wall stops a bullet regardless of which direction it comes from ., and crashing through a windshield is just as injurious if you’re coming or going. Spalling is what makes one-way glass’s counter-intuitive behaviour possible. - If you build an asymmetrical layer cake with a hard brittle acrylic (or glass) OUTER LAYER to distort incoming rounds for the catcher layers ... but the inner surface of the softer polycarbonate will then let outgoing bullets pass through to knock off spall of acrylic splinters and hit outside offensives.

The secret to making one-way glass is using layering and lamination to control how the material spalls and absorbs shock. In this way the glass forces bullets heading in one direction to waste significantly more energy than those headed the other way.When the people behind the barrier return fire, their bullets do not have to contend with the hard acrylic: they pop through the softer polycarbonate and burst out of the acrylic, throwing off shards and emerging largely undeterred.

VERY useful stuff .. But it is very heavy - very thick - very expensive - and it degrades very rapidly from sunlight UV - needing regular replacement.

Make sure your installer puts it in facing the right way eh ..

Marty K.