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Civilian Marksmanship Program (CMP) shared U.S. Army Marksmanship Unit's post. ...

(HL) Welcome to another edition of the USAMU Custom Firearms Shop's "Handloading Hump-Day!" Photo 1 is one of our smaller 1000 yard groups fired when testing a rifle from a machine rest, and illustrates the accuracy we strive for. Today, we'll address FACTORS PERTAINING TO PRIMER SEATING DEPTH AND UNIFORMITY. (**Special note: for those hardy souls brave enough to read it all, there's an 'Easter Egg' anecdote buried within that may well save you points on the firing line -- enjoy the hunt!) The first concern is for safety: for that reason, primers should be seated below flush with the case head. One primary cause of "slam fires" (which includes catastrophic failures from firing out of battery) is "high," or protruding primers. These stand above the case head, are readily felt with simple finger-tip inspection, and may fire when slammed by the bolt face and/or a floating firing pin in feeding. Here at the USAMU, we ensure our rifle primers generally run -0.003" to -0.005" below the case head. Maximum primer depth is -0.006" and minimum is -0.002". Upon inspection, any cases with high primers will be corrected before loading. Aside from improving ballistic uniformity, ensuring the primers have proper compression upon seating also helps reduce possible misfires. These can be caused by the firing pin's expending part of its energy either seating the primer or having to deform the primer cup enough to reach the anvil. Factors affecting variance of primer seating depth include brass maker and lot number -- all primer pockets are not created equal! Another factor is the primer manufacturer and individual primer lot. We've encountered occasional primer lots by top-quality makers that included some primers with slight defects affecting seating. While finely accurate, these primers were out-of-round or had small slivers of cup material protruding which affected primer feeding or seating depth. Has one's brass been fired previously? If so, how many times and the pressures involved also affect future primer seating. Obviously, this is another factor in favor of segregating one's high-accuracy brass by maker, lot number, and number of times fired, if possible. The next question, "How do we measure primer depth?" happily can be answered using tools already owned by most handloaders. At the USAMU, we have the luxury of purpose-built gauges made by the talented machinists of the Custom Firearms Shop (Photo 2). One places the primed case into the gauge, and the dial indicator reads the depth quickly and easily. The indicator is calibrated using a squarely-machined plug that simulates a case head with a perfectly flush-seated primer, easily giving meaningful "minus" or "plus" readings. The gauge is usable with a variety of case head sizes. Fortunately, such tools are a convenience, not a necessity. A zeroed, precision dial caliper will also measure primer seating depth == Photo 3. (Olympic Gold Medalist stunt-hands generously provided by USAMU CFS.) Merely close the jaws and place the calipers' narrow end squarely across the center of the case head/primer pocket. Keeping the narrow end in full contact with the case head, gently open the jaws, and the center bar will extend until it reaches the primer face. Voilà! Primer depth is read on the dial. Taking a few measurements to ensure accuracy and repeatability is recommended until one is familiar with this technique. Methods of primer seating include hand-seating using either hand held or bench-mounted tools, vs. progressive-press seating. Progressive presses may either seat by "feel," subjective to each operator, or by using a mechanical "stop" that positively locates primers nearly identically every time. Testing here has shown that we get more uniform seating with the latter type progressive press, than we do with a high-quality bench-mounted tool lacking a positive stop. Primer stop depth adjustments on our main progressive presses involve turning a punch screw in and out. While the screw is not calibrated, fine "tick" marks added to the top of the press help users gauge/repeat settings by "eye" efficiently with practice. Then, once a sample of primed cases is run to confirm the range and accuracy of depths, the identifying lot # & maker is noted on the press for reference. When it's necessary to switch brass/primer lots, changes are easy to make and settings are easily repeated when it's time to switch back. While all this may seem like mindless minutia, remember that finest accuracy is due to an aggregate of attention to detail of many SMALL factors, as well as the major ones. As one's mechanical accuracy gets better and better, improvements will be found in smaller, harder-to-obtain increments. Whether that degree of effort is appropriate to your equipment, needs, skill and shooting style is purely up to you, the reader. And now, as promised, it's "Easter Egg" time" while not scientifically proven, the following anecdote may give food for thought. In the writer's competitive shooting, he has long made a practice of never attempting to re-fire a cartridge for score if the primer had been disturbed by a light firing-pin strike -- LFPS -- or similar incident. He ascribed this to an abundance of caution, probably verging upon superstition. Until... The incident occurred during ammo development with a machine rest and a very accurate bolt-action test rifle. At the time, this test barreled action had developed a problem with generating occasional LFPS's. When testing 10-shot groups with increasing powder increments at 300 meters on a sheltered range, he fired most of a 10-shot group, which was very tight indeed. However, 1 cartridge had suffered a LFPS/indent. As there were no spare cartridges made with that powder charge, the 9-rd group was recorded and the LFPS rd. was then fired. The bullet went well out of the group by several inches, to 11:00. Hmmm... The next incremental 10-shot group was then fired. Once again, 9 rds grouped very tightly, with one LFPS cartridge set aside and saved for last. You guessed it -- upon firing, it went well out of the group -- at 11:00 -- almost identically to the previous group. Now the author doesn't feel quite so superstitious! This writer has also seen this phenomenon with an accurate .22LR match rifle with ignition troubles. Fliers were traced to, and eliminated by, replacement of a firing pin with a chipped face, plus a worn/weak firing pin spring. As always, we hope our readers will find these tips helpful. There is no "ONE" way to handload, and many have found ingenious solutions while reloading. All good tips are welcome here, as we strive to help our fellow shooters get the finest results. Good luck, and good shooting! #Handloading #CampPerry #LongRange #FClass #USAMU #ArmyShooters #NRA

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