Alternative Priming SystemsFor military applications, with bigger cartridges, the standard system with which handloaders are familiar proves inadequate to provide uniform ballistics and maximum performance. For such cartridges, two alternative approaches are generally used.
The first system, used in intermediate chamberings, is the flash tube. This device simply delivers the primer flash to the forward end of the case, so that ignition initiates first near the bullet base and progresses backward. Advantages of this system are twofold; first, essentially no energy is wasted in accelerating unburned propellant; second, the burn tends to be more uniform from one shot to the next because the propellant mass is relatively stationary (it is trapped at the rear of the case).
The second system, used in the largest chamberings, is the spit tube. This system uses a hollow, perforated tube that extends from the primer to approach the bullet base. Designers often fill this tube with a booster priming charge but the system does not depend upon that characteristic. The advantage of this system is that the entire propellant column ignites at the core so that, as it combusts, total reacting surface area tends to increase – automatic progressivity. Combined with the correct priming charge and the right propellant burning rate, this system can provide significant velocity increases and shares the consistency advantage of the flash tube.