DEAR LABBY Q&A

Submit a question to the Ballistic Experts in our Lab

Dear Labby Q&A

  • This field is for validation purposes and should be left unchanged.

If you would like to submit a story related to shooting, hunting, or handloading, we would like to read it. Please submit your story here. If it is accepted, your story will be printed on our site, with your name in the by-line.

Submit your Story

  • This field is for validation purposes and should be left unchanged.

LETTERS TO THE EDITOR

Letters To The Editor

  • This field is for validation purposes and should be left unchanged.


SIGN UP FOR OUR NEWSLETTER!

  • This field is for validation purposes and should be left unchanged.


Free Trial Subscription to
Handloader or Rifle Magazine

  • This field is for validation purposes and should be left unchanged.

“Cratered and Pierced Primers” in the .308 AR Family

By Ron Colvin

A few years ago my brother Daryl and I decided to explore the world of the AR in a .308 caliber. Being avid Dillon 550 reloaders we started exploring the capability of the .308 in the AR along with verifying the loads in a couple of bolt guns.  We soon found that our .308 DPMS style AR-10’s would have “cratered primers” as well as “pierced primers” especially as we increased the pressure and velocity.  There were no issues in the bolt guns.  “Cratering” and “pierced primer” is commonly recognized as an indication of high pressure loads.   Using chronograph data and QuickLOAD Ballistic software we determined that we were well below the published SAAMI maximum for the .308 of 62,000 psi.  We would see the “cratering” at pressures of 48,000-55,000 psi and “pierced” primers at anything greater than 55,000-59,000 psi.  (“Pierced primers” has also been attributed to excessive firing pin protrusion—not the case here.)

“Cratering” is the extrusion of the brass primer cup material around the firing pin and into the firing pin aperture. See Figure 1 and 2.

Figure 1.
The red arrow indicates the firing pin and Firing Pin Aperture.

Figure 2
Cratered and Pierced primers.

In the DPMS style .308 AR bolt, the firing pin aperture is ~.080” in diameter.  As we went up in velocity and therefore pressure, the extrusion continued until a small cup shaped piece of primer material the diameter of the primer aperture in the bolt was sheared (blanked) out of the primer leaving a hole in the primer.   This blanked out “cup” is roughly the diameter of firing pin aperture. We often found this “cup”, in the firing pin aperture or in the firing pin channel.  If the “cup” was in the firing pin aperture it often prevented a firing pin strike on the next round.   Dry firing usually dislodged the “cup” sometimes launching it into the bore or into the barrel extension, at times preventing chambering of the next round.  This required close inspection and verification that the “cup” wasn’t in the chamber or barrel.    At times we had to disassemble the AR bolt and dump out “cups” from the firing pin channel.   See figure 3 below.  A number of times the “cups” locked-up the firing pin.  The “pierced primer” usually vented high pressure gas into bolt as evident by smoke coming out of the bolt area during firing.  The venting of these high pressure propellant gases into the bolt can cause “gas cutting” erosion of the primer aperture and firing pin channel.  The vented gases can also slam the firing pin into the AR firing pin retaining pin bending it.

Figure 3
Pierced/blanked out “cups: shown on graph paper with .1” squares.

Light hammers and mainsprings in 1911 pistols can also cause cratered and pierced primers.  These “light” components result in the firing pin not staying pressed into and supporting the primer indent in the primer cup until the chamber pressure diminishes to the point that the primer cup material can no longer be extruded into the primer aperture.  The same can be true in the AR platform if you use reduced power hammer springs and a reduced weight hammer.

We could not resist the attraction of the immensely popular 6.5 Creedmoor in the AR platform.  Upon converting our .308’s to 6.5 Creedmoor with 1 MOA guaranteed Ballistic Advantage barrels, we again found the same issues with primers as with the .308 since we were using the same AR bolts.

“Cratered and pierced primers” is not uncommon even in bolts guns.   The most common solution is to reduce the firing pin aperture and firing pin diameters.  There are vendors that provide the service of reducing the firing pin and aperture diameters (“bushing down the diameters”) in bolt guns such as Gre-Tan Rifles.  A simple analysis shows that a .0625” diameter firing pin aperture is 42% smaller in cross sectional area than the standard AR10  .082” aperture and will reduce the linear force the same 42% on a primer cup over this aperture from 317 pounds to 184 pounds assuming a 60,000 psi chamber pressure.

JP Enterprises is one among others that provide a solution for this issue.  JP describes their solution as a “high pressure” Enhanced Bolt™ for the .308 AR.  It has a .060”-062” diameter firing pin and firing pin aperture (see Figure 5 and 6).

Figure 5
Small vs. large diameter firing pin.

Figure 6
Small vs. Large diameter firing pin aperture.

This offering is noted for fixing the majority of the problems that have been encountered with the primers in the AR 10 platform aperture as we also found (see Figure 7).

Figure 7
Note the difference between primers fired with the larger firing pin versus those on the bottom using a JP High Pressure Bolt.

Ron Colvin 1-7-19