Primers are taken for granted by many reloaders. Based on my years of reloading experience and dealing with thousands of handloaders, the majority of problems are traceable to a cavalier approach to priming. There is no question in my mind that most reloading bench accidents involve primers, especially when used with automatic primer feed units. Some folks tend to forget that primers, by in tended design and application, must be explosive! If the reloader does not proceed with utmost caution when an automatic primer feed jam occurs, especially during the disassembly and clearing of the jammed unit, a serious accident can occur. One moment of inattention that results in a bit of tapping or forcing to free the jam could cause an explosion. Carefully read the primer manufacturer's packaging warnings. At least one manufacturer specifically cautions against the use of automatic primer feeds. If you use an automatic primer feed follow all of SAAMl's cautions including adequate operator shielding from the primer magazine and the routine cleaning of primer "dust" from all loading tools.
The best procedure is to place primers manually, one at a time, onto the priming arm. Some reloaders deviate from this safe practice believing they will eliminate any chance of primer contamination by not handling each primer. But a thorough hand washing and drying, done immediately before the priming operation, is all that is necessary to eliminate any possible handling contamination. There are several methods used to seat primers. One that is often preferred is to seat primers by "feel." This method involves pressing the primer into place until you feel the anvil bottom in the primer pocket, and then placing ever-so-slightly more pressure on the primer so as to have the anvil lightly pressed into the priming pellet in order to stress it slightly. Stressing the primer pellet causes it to become appropriately sensitive. Without this sensitizing, primers may not ignite consistently.
Despite the popularity of the "feel" method of priming, it is not an ideal way to obtain uniform primer seating depths. The actual feel of a primer being correctly seated will vary with the pocket diameter and depth as well as the actual diameter and overall height of the primer itself. It's not easy to feel proper primer seating. The motion of the primer must be transmitted through the primer seating plug, all of the tool linkage, the tool handle, and finally to the reloader's hand. Many reloaders believe that they have mastered this "feel" process. How ever, more often than not, when they agree to measure and record 100 successively seated primers they are at a loss to explain the seating depth variations. In numerous tests I have conducted, the average tolerance held by the feel method is plus or minus .005 inch. This is opposed to the tolerance of plus or minus .002 inch recommended by several primer manufacturers. The ideal seating depth is .005 inch below flush for most primer brands. Only CCI suggests a differing depth, .001 to .003 inch below flush with the case head. The use of small hand-held priming tools will sometimes allow individual reloaders to improve upon the usual wide range of seating-depth tolerances. However, there is a better way to seat primers and that is by using a mechanical stop for the priming stroke. Be aware that the effectiveness of this method can vary dependent upon the amount of looseness or "slop" in the loading tool. Some tools are simply tighter than others.
Almost all of the various press-mounted priming units can be adjusted to take advantage of the mechanical stop in the press linkage. Of these, those that mount at the die station, often called ram prime units, have repeatedly provided the best test results with respect to uniform seating depths. Most reloaders find it relatively easy to maintain a .003 to .007-inch seating depth with these. I have carefully chosen several presses for prime-only operations leaving the ram prime units in place full time. I have one press set up for small pistol and small rifle and a second for large rifle primers. The third handles large pistol primers. The three presses set up for primer seating are in recognition of the fact that small rifle and small pistol primer pockets are nominally the same depth and differ from large rifle primer pocket dimensions. Large pistol primer pockets use a depth similar to small primer pockets and hence the need for a separate press. I leave the priming punches for the appropriately-sized primers permanently mounted. By careful selection of shell holders I have eliminated most of the need to adjust the units in order to obtain the correct seating depth.
A little trick for ensuring that primers will generally be seated ,squarely in the primer pocket is to seat the primer half way, back off the tool handle a tad, and then rotate the case approximately 180 degrees. Then complete the primer seating stroke.
There are special tools such as those sold by Lyman, Sinclair, and others that will allow the reloader to ream primer pockets to a uniform depth and to create sharp comers in the bottom of the primer pocket. There is no question that primer pockets so reamed will allow for the maximum uniformity of primer seating depths. However it should be understood that during reaming, primer pockets can easily become oversized in diameter, which can result in them leaking gas upon firing. Reaming primer pockets for uniform depth and shape requires meticulous care, and such reaming will not generally show any accuracy improvement unless the shooter/rifle load combination is already capable of half minute-of-angle accuracy.
Some feel that magnum primers are capable of some special characteristics. In the search for accuracy it has been proven that the mildest ignition source, compatible with proper ignition of the propellant charge, is best. Thus the use to a magnum primer, with its heavier priming pellet weight, is actually detrimental to accuracy in loads that can be uniformly ignited with a standard or match-grade primer.
A customary approach that works well in most primer selection situations is not to use magnum large rite primers for powder charge weights of 60 grains or less. Heavier charges, up to 75 grains of propellant, might be checked for accuracy with both standard and magnum primers. In this range I have often found that Winchester's standard large rite primers deliver the best accuracy. For powder charge weights in excess of 75 grains I invariably use magnum primers. The use of magnum primers should always be accompanied by a careful test program to see what, if any, accuracy changes occur.
The reloader should keep in mind that a change in primers can noticeably change peak firing pressures. When testing different primers remember to reduce the powder charge and carefully work back up to the maximum load. I have found that magnum primers often demand a bit less powder.