Two Essential AR-15 Case Preps

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This is a specially-adapted excerpt from the forthcoming book, “Handloading For Competition,” by author Glen Zediker, owner of Zediker Publishing. Click here to order from Midsouth.

by Glen Zediker

Maybe the headline above oversells two case preps I routinely perform, but if they aren’t “essential,” let’s at the least say they are “worthwhile.” I don’t like telling folks to endure more tedium than is necessary. Time is not only money: It’s also shooting, relaxation, family, and on down the list of activities that substitute for removing miniscule amounts of brass from cartridge cases.

The 3-pronged anvil is supposed to compress; if the pocket isn’t flat, it won’t do so correctly.
The 3-pronged anvil is supposed to compress; if the pocket isn’t flat, it won’t do so correctly.
Inside flash hole deburring: Just do it. It’s too easy not to.
Inside flash hole deburring: Just do it. It’s too easy not to.
This is a round that got placed into the chamber and then the bolt carrier released (how we load the “Slow-Fire” rounds in competition). The firing pin tapped off the primer and left a nice dimple. A primer sitting too high can more easily pop under this sort of abuse.
This is a round that got placed into the chamber and then the bolt carrier released (how we load the “Slow-Fire” rounds in competition). The firing pin tapped off the primer and left a nice dimple. A primer sitting too high can more easily pop under this sort of abuse.
Check each and every round you load to ensure that the primer is below flush with the case head. Just run a finger over it (use your own for reasons I can’t approach here). Uniforming pockets goes a long way to ensure correct, full seating, and is especially important if priming is done using a mechanism that precludes feedback, like a progressive reloading machine.
Check each and every round you load to ensure that the primer is below flush with the case head. Just run a finger over it (use your own for reasons I can’t approach here). Uniforming pockets goes a long way to ensure correct, full seating, and is especially important if priming is done using a mechanism that precludes feedback, like a progressive reloading machine.
I can always see a difference in the anvil prints on cases that have uniformed pockets and those that don’t. Telling…
I can always see a difference in the anvil prints on cases that have uniformed pockets and those that don’t. Telling…
There are a variety of primer pocket uniformers. The author prefers those that can chuck into an electric drill as well as also mount in a screwdriver-style handle. Way less tedious. Run it no more than 1100 rpm. The author doesn’t like doing this job by hand, so he uses an adjustable-depth tool. It’s safer to get a fixed-depth unless you have the sort of measuring device necessary to do an accurate job of setting the tool.
There are a variety of primer pocket uniformers. The author prefers those that can chuck into an electric drill as well as also mount in a screwdriver-style handle. Way less tedious. Run it no more than 1100 rpm. The author doesn’t like doing this job by hand, so he uses an adjustable-depth tool. It’s safer to get a fixed-depth unless you have the sort of measuring device necessary to do an accurate job of setting the tool.

However, for reasons I’ll hit upon, a couple of actions on the bench make things better, and one makes things safer. The first is a primer-pocket uniforming tool; the other is an inside-flash-hole deburring tool.

The tasks these tools perform only need to be taken once.

When a domestically-produced cartridge case is made, the primer pocket and the flash hole are formed, not cut. The primer pocket is done with a swaging process, and the flash hole is punched. The primer pocket and headstamp are normally produced at the same time with a punch called a “bunter.” I also call it a “blunter” because that’s the result: cross section a case and you’ll see that the bottom of the primer pocket is not square; it looks a little like a cereal bowl. The flash hole is normally punched separately.

A well-designed primer pocket-uniformer’s job, in my view, is mostly to put a 90-degree corner on the pocket bottom, so the bottom is flat. Primers are flat, coincidentally. And this is why it should be done. A uniformer also cuts the pockets to the same depth, which is also within the correct depth range; or, at the least and depending on the combination of the primer pocket and the tool itself, ensures that a minimum depth has been created. That’s between 0.118-0.122 inches for Small Rifle primers.

Now, there are differences among manufacturers in primer-cup heights. They’re small, but tend to be consistent brand-to-brand. A uniformed primer pocket pretty much eliminates the chance of a shallowish primer pocket combining with a tallish primer to create a primer that’s not seated beyond flush with the case bottom.

And all primers should be seated below flush! The actual amount advised or warranted varies with the source, but I give it a minimum of 0.006 inches.

So, after uniforming a primer pocket, the primer should be sitting “flat” on the pocket bottom (more in a bit); ultimately, this means all primers in all cases are seated fully. Measurement of the amount below flush with the case bottom doesn’t really matter; just that the primers are seated fully.

The reason I said “more in a bit” is because primers have an anvil. It’s the three-pronged sort of spring-looking piece on the bottom of a primer. (“Top” or “bottom” is a matter of perspective…) When a primer is seated, the anvil feet compress. Using a hand-held seating tool, you can feel it. They are supposed to compress and be sitting equally on the primer pocket bottom.

There are two reasons this is essential. One is a matter of performance. If the primer is not seated flush against the pocket bottom, then some force from the firing pin or striker is redirected toward fully seating the primer. It’s a softer hit, in effect. This leads to inconsistent ignition, and, to a smaller degree only worried about by the fastidious, differing initial vibration nodes.

The other reason I say this is essential for AR-15 ammo (or for any ammo destined for use in a rifle with a floating firing pin) is assurance against a “slam fire.” Out-of-battery discharge. Ugly. When the bolt carrier sends the cartridge home into the chamber, the inertia can cause the firing pin to continue forward and “tap” off the primer. It’s not supposed to happen, but it dang sure does. The mechanism intended to prevent this is faulty. A primer that’s sitting a little high gets tapped harder, and if it gets tapped hard enough: BLAM. It’s more of a problem with M1As, but I have seen them in ARs, more than once.

Inside-flash-hole deburring is too easy. Of course, you’ll need a tool, and there are several that all work well. When the flash hole is punched, there’s a burr turned up on the inside of the case. These vary in height and scope, but without a doubt interfere with ignition. It’s also possible that a die decapping pin can fold one such that it obscures the hole. Just get it gone. Takes virtually no effort.

It makes a noticeable difference on target, especially in small-capacity, small-diameter cases, like .223 Rem. Reason is clear: the flash from the primer enters consistently and therefore spreads consistently to get the propellant burning. A tall, narrow column of medium-burning propellant is a tougher chore to ignite, or that’s what I think.

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15 thoughts on “Two Essential AR-15 Case Preps”

  1. This would be more helpful if you could actually read the entire caption of each slide. These are all great tips. Getting recommendations of specific tools related to primer pocket prep and seating tools (for different calibers) would also make it more helpful.

  2. MidSouth, Thanks for putting this info out there,for my self fairly new into reloading [2yrs] any an all info i can read will help me become proficient in my new found sport. Again thanks ,BillT

  3. I have been running through my 45 brass doing just this thing. I have tended to clear the brass hole with a similar amount of cutting as your picture of the side by side (before/after) picture. in lyman documentation they tend to indicate that you should only take off the bare minimum and not do any amount of counter sinking.

    Another thing I noted is that some brass I was doing had a very small flash hole (Winchester for example) and others like Federal and Blazer had considerably larger flash holes. Probably twice the total area. I would assume the smaller holes would result in a thinner deeper flash when the primer went off. The larger and any counter sinking would result in a shorter but wider flash when the primer went off. I was wondering if anyone ever looked in the differences in primer flash holes and if one was better than another.

    I would think that a narrow hole may work better with fully charged (powder) case, while a wider or counter sunk hole would work better with a partially filled case such as when powders like TiteGroup are used. Any comments on any of this. Thanks

    1. Smaller holes are better… This has been pretty well proven in Benchrest. Most commercial PPC cases have smaller-than-normal flash holes, to the point that there’s a separate uniformer for PPCs. This isn’t an option, as far as I know, with other catridges; but was done the original PPC blueprint. For those not familiar with this cartridge, PPC (Pindell-Palmisano design) is regarded as the most accurate cartridge on the planet (there are variations of it, but it dominates off the bench). I have an AR15 chambered in 22PPC and it’s very good.

  4. very cool, I never put much stock in doing these things but now i’m shooting some long range and I believe I will get some tools and see what happens. thanks for all the great articles!

    1. Scott, I’ve found that it’s the little things that make the biggest differences when it comes to precision reloading. Getting every aspect of your reload as uniform as possible will pay huge dividends down range, I guarantee it.

    2. Even though I said this in the last article, and probably should have included it in this one: depending on the cases, sometimes I wait until after the initial firing to uniform primer pockets just because it’s easier to do (primer pockets expand).

  5. I’ll have to take exception on “Inside flash hole deburring: Just do it. It’s too easy not to.” photo showing the depth the case on left has had the inside of flash hole deburred. Taking off the burr is fine; however, going past cutting it off level with the case web by cutting a “dimple/funnel ” into it may weaken that area as well as funnel extra pressure onto the primer.
    Most manufactures of inside flash hole deburring tools warn against this.

    1. The tools I use are configured to do what is shown in the photo. They’re not adjustable. I have three from different specialty makers that all cut to this same degree. Mine are made by “benchrest” loading tool makers, like Sinclair Intl.

      1. Sorry, posted that before finishing it… I asked the same question, and received assurance that there’s no effective weakening of the case, and that the “funnel” is a good thing… I think that, because this area is in such close proximity to the flash hole that there’s no material structural weakening to worry over. It would be different if the whole bottom got thinned, and that’s why it’s hard to find anyone who can agree with the prep-step of case head squaring (that some Benchrest and Long Range like to do). Now THAT scares me… I’ve done it and took a look, and decided it was a tad amount too worrisome.

  6. I have be deburring the flash holes, and uniforming the primer pockets in my 45 Colts that I use for cowboy shooting for about a year now.
    I’ve found that I get a lot more consistency in my groups and fps’s with the flash holes deburred. I figure I must be getting a better flame dispersion with the light loads I use (4.9gr 700X in huge 45LC case).
    I only take off the hole burr with little to no chamfering as per Lyman tool instructions. I also make one quick turn of the tool in the exterior of the flash hole just to knock edge off.
    Uniforming the primer hole has a benefit I really enjoy that’s not mentioned. I haven’t had a distorted, dented, or high primer since I started uniforming, and all primers seat easily and with exactly same feel case to case. Well worth the effort.

    1. Hmm. Never tried it on the outside… The deburring tool. The reason I mentioned that I recommend non-adjustable pocket uniformers is because they can be trusted not to go too deeply. I’ve had some cases where the primer pockets were not cut fully on the bottom, and others where a whopping lot (well, relatively) of chips came away. Don’t worry, though: if the cutting seems like it’s just too much, it’s because the pockets needed that amount to be what they should be.

  7. It is the little thing that count. When you are shooting close it is not as noticeable. It is all these little things that make shooting distance so much better. Thanks for the write up.

  8. I guess I am going to have to do some testing with my .45 ACP brass that I am deburring right now. If the smaller flash hole is indeed more accurate, then it would stand that any amount of countersinking on the brass should hurt accuracy. The flash pattern from a primer going off would be similar to a garden hose. A small hole promotes a long narrow jet of water where a wider hole and counter sinking should provide a wider but less deep spread of flash. It would suggest that any amount of counter sinking would be detrimental to accuracy.
    I could see that counter sinking would add to consistency though because it may be more effective in starting the powder burning at a similar same rate when the brass is not 100% full of powder.
    I am also interested in the question if a smaller hole provides better accuracy, then why has Federal made a significantly larger flash hole in my .45 brass. There should not be any real differences in cost of punching one hole or another. I will have to ask them this question although I am sure they will come back with the answer that primer flash holes vary across manufacturers and does not make any significant difference. I only want to know because then I might sort my brass by flash hole size to put my most accurate brass with my most accurate loads. Good conversations to talk about if nothing else.

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