Lock-Time

In Hatcher's Notebook (pp 396-398) there is a description of some experiments in lock-time performed at Springfield Armory by John C. Garand on a model 1903 rifle. Garand measured the time, in milliseconds, between the tripping of the sear and the end of the firing pin travel in an ingenious way. He devised a rig consisting of the trigger mechanism mounted in a test stand. A rigid wire pointer was attached to the cocking piece. The end of the pointer was in contact with a "smoked disc", a rotating disc coated with lamp black, and the disc was spun at exactly 3057 RPM. The pointer thus made a circle on the spinning disc. When the sear was tripped, the pointer would move across the face of the disc to another position, and when it came to rest it would inscribe a circle of a smaller diameter on the disc. Connecting the outer circle and the inner circle would be a small helical section representing the time during which the lock mechanism was in motion. By measuring the angle of this section, and knowing the rotational speed of the disc, the lock time could be determined.

Garand found that the lock-time of the service rifle was 5.7 milliseconds, and the National Match rifle with the special headless firing pin was 4.9 milliseconds.

Garand then fabricated a special firing pin spring using chrome vanadium steel and square wire. However, when the wire was wound into a spring, the outer diameter of the wire was stretched and the inner diameter was compressed, and the coils of the spring would not lie flat. So Garand had special wire made with a "keystone" cross-section, so that the coils of the spring would lie flat when fully compressed. With this special spring he achieved a lock-time of only 2.2 milliseconds.

For comparison, modern bolt-action rifles have lock times in the region of 2.6 to 9.0 milliseconds. The legendary Remington 3200 shotgun has a lock-time of 3.2 milliseconds.

You can read a discussion of lock-time in shotguns in Bob Brister's "Shotgunning - The Art And Science" here: http://tinyurl.com/d8rj7s

Hatcher's Notebook can be found here: http://tinyurl.com/cn25o4