p.s. LeadSled, the S&W spring-loaded tool you mention was also designed to attack 'toughened glass' at its 'achilles heel',
i.e., its relative rigidity and inherent tension.
Stanwell,
I did manage to crack an old side window from a light aircraft, suspended between tow bricks.
In any event, I would recommend the welder's chipping hammer, around $12.00 or so from Bunnings, and as effective as the USAF standard canopy breaker --- indeed, a very similar size and weight.
Tootle pip!!