An action timer start switch, a breech block with an integral action timer start switch and a system for measuring internal ballistic data in a test weapon system are described herein. The action timer start switch includes a through-hammer adjustable contact having a first contact element and a contact switch supported on the breech block having a second contact element. The first contact element is adjustable for contacting the second contact element when the hammer is a position to generating a primer strike. The first contact element may be set by simply positioning the breech block in the in-battery position with the hammer in the ignition position and moving the adjustable contact relative to the contact switch through the breech of the universal receiver.