Primer Pocket and Flash Hole Uniforming Basics

The U.S. Army Marksmanship Unit (USAMU) has published a series of reloading “how-to” articles on its Facebook Page. This post explains how to uniform primer pockets and remove burrs in flash holes. These brass prep operations can help ensure greater consistency, shot after shot. Visit the USAMU Facebook Page for other, helpful handloading tips.

Primer Pocket and Flash-Hole Conditioning

This week, we’ll address a question that frequently arises: “Do you uniform primer pockets and deburr flash-holes?”

As we tailor our handloading methods to the specific needs of each instance, the answer, not surprisingly, is “occasionally!” Generally, the USAMU Handloading Shop does not uniform primer pockets (PP) or deburr flash holes (FH) of our rifle brass. That’s not to say we’re against it — rather, it reflects the very high volume of ammunition loaded, the fact that very few cases are ever re-loaded for a second firing, and the types of brass we use. However, as a need is perceived, we DO deburr flash holes (of which, more later.)

As to the type cases we use, many thousands of our long-range 5.56x45mm cases come to us from the arsenal with the primer of our choice pre-installed and staked in per their usual practice. Obviously, we could not uniform either FHs or PPs on this live-primed brass. However, after careful sorting, inspection and preparation, we do obtain match-winning results with it. Regular readers have seen photos of some of the tiny 1000-yard test groups we’ve fired with weight-selected domestic brass which had neither Primer Pockets uniformed nor flash holes deburred.

Figure 1 shows a fired, deprimed 7.62×51 case with primer residue intact. In Figure 2, the primer pocket has been uniformed to SAAMI specs. Note the shiny finish — evidence of the metal removed to uniform and square the primer pocket.

Shooters who reload their brass several times may decide to uniform PPs and deburr FHs, especially on their “300-yard and beyond” brass. Unlike us, they will be using their cases many times, while the operations are only needed once. Also, most handloaders only process a relatively moderate amount of brass compared to our 20-thousand round lots. Having high quality Long Range (LR) brass helps. Many of the better brass manufacturers form their flash holes so that no burrs are created.

Still, it does pay to inspect even THESE manufacturer’s products, as occasional slips are inevitable. Very rarely, some of these makers will have a significant burr in, say, 1 per 1000 or 2000 cases, and it’s worth catching those. Recently, we began processing a large lot of match brass from a premier manufacturer, and were startled to find that every case had a burr in the FH — something we’d never before seen from this maker. We then broke out the FH deburring tool and went to work.

For those who do opt for these procedures, note that various tool models may have adjustable depth-stops. Pay attention to the instructions. Some flash hole deburring tools which enter the case mouth, not the primer pocket, depend on uniform case length for best results.

Does It Really Make a Difference?
It can be difficult to truly verify the contribution to accuracy of these procedures, particularly when firing from the shoulder, in conditions. Members of this staff, as individual rifle competitors, do often perform these operations on their privately-owned LR rifle brass.

One could ascribe this to the old High Power Rifle maxim that “if you think it helps, then it helps”. Another thought is to “leave no stone unturned” in the search for accuracy.

However, an extremely talented World Champion and Olympic Gold/Silver medalist commented on his own handloading (for International competition, which demands VERY fine accuracy). He noted that he did seem to see a decline in accuracy whenever he did not uniform FH’s, deburr FH’s and clean primer pockets before each reloading; however, with the wisdom of decades’ experience, he also remarked that “It could have been that I just wasn’t shooting as well that day.”