Abstract: A unitary, forward buckling rupture disc is provided that is adapted to be mounted in a holder in which the disc is in closing relationship to a tubing string in an oil well and has greatest utility for testing the integrity of the connection between the sections of oil well tubing strings. The disc is designed to withstand the fluid pressure head in the string and a leak test pressure, and to then burst and fully open under a predetermined overpressure. The concavo-convex bulged section of the disc has a non-circular, continuous score line formed in the concave face thereof. The score line is of continuously varying depth around its circumference and includes an outwardly projecting cam segment which is of greatest depth that ruptures first and is in direct opposition to a lesser depth score line portion forming a hinge area for the burst region of the disc.