On the previous page, we assumed that the origin of rifling of the barrel has some erosion present, and as a result, the rotating band must travel 2.00mm prior to the band engaging the rifling. For this projectile, 2.0mm of free run is not very significant due to the travel required to reach appreciable velocity. However, if we boost the free run to 20.00mm, the implications of free run for this projectile become more important. Since this projectile is crimped to the cartridge case, the initial position of the projectile with respect to the rifling when the cartridge is seated in the chamber is fixed. Erosion of rifling at the origin of barrel travel will cause a delay in the projectile rotating band engaging the rifling. The means the projectile will have an appreciable velocity prior to the rotating band engaging the rifling. The result is a torque spike just as the rotating band engages the rifling, shown on the left hand side of the image below.

An entry to the Projectile Free Travel input slot, along with clicking on the ReCalculate Resistance Profile button, will modify the resistance pressure vs. travel, and cause the rotating band calculations to assume there is a travel delay in rotating band contact with the rifling as previously mentioned. For projectiles fired in barrels other than full gain twist (those with a zero degree starting angle), a spike in the torque vs. time/travel profile is seen. If the projectile velocity upon contact of the rotating band with the rifling is sufficiently large, the torque spike generated may exceed the strength of the rotating band. The torque spike may either fail the band material at the surface of the land tops, or at the interface between the rotating band and the projectile body. The result is the rotating band will not transfer torque from the rifling to the projectile and the projectile will exit the barrel with less than desired spin. This reduction in spin may cause the projectile to be gyroscopically unstable at muzzle exit. The delay in rotating band engraving also has an effect on the peak pressure generated as shown in the figure below:

The result of free run to the rifling is a projectile which exits at lower muzzle velocity than desired and may have insufficient spin to provide gyroscopic stability. It is left to the user to compare the effect of varying free run distance on the interior ballistic performance of the projectile, the torque and stress levels which the rotating band and seat can tolerate, and determine what amount of forcing cone erosion represents a barrel which should be retired from service.

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