Like trimmers, there is a wide variety of priming tools available on the market. Please refer to the operating instructions for the particular tool you are using. There are, however, some general guidelines to the priming operation that should be adhered to, regardless of the type of tool being used.
Primer seating depth is critical. Depending on the type of firearm being loaded for, high primers (a primer which protrudes above the case head) can cause a variety of problems. In revolvers, a high primer will bind up the action, and may prevent the cylinder from rotating. In an autoloader, either rifle or pistol, they can result in slam-fires. In bolt actions, lever, and slide action rifles, high primers can cause stiff or difficult chambering, and again may pose a potential for a slam-fire. In any action type, high primers will cause ignition problems and poor accuracy. Forcing the primer too deep in the pocket will also result in poor performance, misfires and inaccuracy.
Ideally, primers should be seated to approximately .003″ to .007″ below flush with the case head. This may be checked with a hole gauge or the base end of a vernier caliper. The most common method for checking for primer seating depth (and still one of the better methods) is a quick visual inspection coupled with an educated sense of feel. After determining a proper seating depth using an appropriate measuring device, run your index finger lightly over the seated primer. Pay particular attention to the degree of indentation as the finger tip goes across the case head and primer. You will learn what a properly seated primer feels like, with just a little experience.