Primers are the smallest component in ammunition; the class of ammunition determines how priming is accomplished.
In rimfire cartridges, the priming chemicals are integral with the case. Making the case with a hollow rim provides the space for the chemicals.
Priming compound is a mechanical mixture of lead styphnate, antimony sulfide, barium nitrate, and other chemicals. This combination will create heat and gas when struck sharply. For rimfire cartridges, raw wet priming mix is placed directly in the hollow rim cavity. Binders provide all the holding power that keeps the primer in place. Some rimfire mixes differ from their centerfire counterparts by the addition of a frictionator (helps ignition in the rimfire system), which may be finely ground glass.
The priming chemicals are blended in controlled facilities, often by remote control. Adding water to the mix makes it less dangerous to handle and allows the mass to be shaped. Even with these precautions, transporting primer mix is hazardous; cartridge cases are moved to the primer chemistry facility rather than moving the primer mix to the case loading location.
The priming process steps are as follows:
- The wet mix is forced into small holes in a nonsparking metal charge plate that sets a fixed volume of mix in each hole.
- The filled plate is placed over a tray of rimfire cases; the tray has the same hole pattern as the charge plate.
- A third plate with multiple pins, corresponding to the hole pattern in the charge plate, is pressed through the charge plate.
- The individual disks of priming mix fall into the cases.
To ensure that the mix is uniformly distributed into the hollow rim cavity of the case, both downward and centrifugal force is applied during the spinning operation. A rotating spinner quill enters each case, causing the case to spin at high speed. The working end of the quill forces the wet mix toward the cavity and the spinning distributes the primer mix to uniformly fill the cavity.
Voids in the mix in the rim cavity can cause a misfire. Most manufacturers have stringent visual or electronic inspection procedures that reject any cases that do not have the proper distribution of priming mix.
After spinning, the cases are placed in low-temperature ovens to eliminate the moisture. Before moving to the load line, the cases are tested for primer sensitivity. A sample of dried cases from each lot is selected. The cases are placed in a test machine with a vertical cartridge chamber beneath a steel ball of known weight. The ball is dropped on the cases from a series of heights. At each height setting, the operator notes the percentage of cases that fire. As the height increases, the energy applied to the case becomes greater, increasing the number of fired cases. Eventually, the height reaches a point where all cases fire.
By applying standard statistical procedures to the data matrix of fired cases and corresponding heights, a value is generated that can be compared to published specifications for primer sensitivity. If that value falls within specifications, the cases are released for loading.
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