Primer Seating Depth Uniformity and Accuracy

Each Wednesday, the U.S. Army Marksmanship Unit publishes a reloading “how-to” article on the USAMU Facebook page. Yesterday’s post covered primer seating depth. This article offers many useful tips — including a clever way to measure primer seating depth with ordinary jaw-type calipers. Visit the USAMU Facebook page next Wednesday for the next installment.

Primer Seating Depth — Why Uniformity is Important
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.

SMART TIP: How to Measure Primer Seating Depth with a Set of Calipers
A zeroed, precision set of standard calipers will also measure primer seating depth. (You don’t really need a custom tool.) 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.

Brass and Primer Defects Can Cause Seating-Depth Variances
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.

Measuring Primer Seating Depth with Purpose-Built Gauge
The next question, “How do we measure primer depth?” happily can be answered using tools already owned by most handloaders. [See tip above on how to measure depth with calipers.] At the USAMU, we have the luxury of purpose-built gauges made by the talented machinists of the Custom Firearms Shop. 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.

Primer Seating with Progressive Presses
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 number and 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.

Similar Posts:

• L.E. Wilson Case Gage Depth Micrometer Review
• L.E. Wilson Case Gage Depth Micrometer Product Review
• Accuracy One Precision Primer Gauge — Superb Quality
• New Accuracy One Precision Primer Gauge
• Check Primer Pocket Diameter and Depth with Handy Tools

Tags: Dial Calipers, Hand-loading, Primer, Progressive Press, USAMU