Flash Hole Deburring - May 2013 Reloading Press | Top Rated Supplier of Firearm Reloading Equipment, Supplies, and Tools - Colt

Flash Hole Deburring - May 2013 Reloading Press

Flash Hole Deburring

Now that you’ve selected the right type and quality grade of brass for your high-accuracy handloads, it’s time to start prepping it for loading. That means performing some steps that might not sound very exciting but that pay off, especially if your goal is to create the most consistent, precise rounds possible.

Case Inspection Basics
A simple visual inspection is important. With cases that have been fired at least once before, it’s vital. The more times the cases have been fired, the more attention you need to give to the visual inspection. Are the case mouths nice and round, and undamageed? Are there any split case necks? Dents, dings, bulges or wrinkles in the case body? Even with brand-new, unfired cases, it’s a good idea to give them a once over. Brand new cases are mass-produced products, and manufacturing slip-ups do sometimes happen. For example, it is rare, but possible to find an unfired case that doesn’t have a flash hole in it!

Flash Holes - Punched vs Drillled
As with many processes, it is sometimes good to start at the bottom and work your way up systematically - in this situation, the bottom of the cartridge case. Flash hole deburring may sound monotonous, but if your goal is to produce the most consistent, precise, accurate rounds possible, it’s a critical step in prepping your brass.

When brass cartridge cases are made, the flash holes are either punched or drilled into them. Most American-made brass, especially bulk brass, has punched flash holes. When the punch pushes through the brass to create the hole, it often leaves little burrs of metal hanging on the edge of the hole. Some premium cases, like Norma, have drilled flash holes, which are typically neater than punched holes. But if you’ve ever used a drill to put a hole through wood or metal, you know that drills can also leave little burrs of material hanging off the exit side of the hole.

Why Should I Care?

See the small burr of brass on the inside edge of the
flash hole of this brand-new brass case?


Same case after deburring: an unobstructed flash hole that allows unobstructed passage of the spark.

Why should you care about those little burrs of brass on the edges of the cartridge flash hole? If you’re reloading a bunch of ammo to blast for fun at the range, or to shoot prairie dogs, or hunt deer in thick woods, they probably don’t matter that much. But if you are trying to make very high-accuracy match ammo, you need to care quite a bit. There’s good evidence that those burrs along the edge of the flash hole cause uneven flow of the primer’s spark, leading to inconsistent powder column ignition that in turn causes velocity differences, ultimately leading to vertical stringing of your groups.

Does it matter if your bullet hits a deer’s vital area a few tenths or a few hundredths of an inch higher than you aimed? No. But in benchrest shooting, groups are measured down to the thousandth of an inch, and a shot off by only a few hundredths of an inch would be like a wild pitch so off-target that it hit a batter in the on-deck circle.

When To Deburr?
The good news is that you typically need to deburr only once in a case’s life. New, unfired cases will have burrs, as they are a natural result of the case manufacturing process. Previously-fired cases may have burrs, especially if they are from factory ammo that has been fired only once, as the burrs will still be left over from manufacturing. But once you’ve deburred a case, that’s really all you need to do. Burrs don’t regrow on the edge of the flash hole simply because a case has been fired. Deburring is a “one and done” project.

Deburring Tools
Deburring tools fit down through the case mouth and are designed to trim off only the burr without removing too much “good” brass. A deburring tool indexes one of two ways: by the cartridge case length or off the web of the case. A case-length piloted tool like the Sinclair Piloted Flash Hole Tool (#749-002-941) has an adjustable pilot that allows the tool to index off the case mouth. This type of tool should be used only after the case has been checked for overall length, and trimmed if necessary.

Sinclair Piloted Flash Hole Deburring Tool entering the case mouth.

The trick to using piloted flash hole tools is to grab a correctly trimmed case, lightly deburr the flash hole, and then lock the pilot into place on the stem of the tool. Now it’s set for use on all your properly-trimmed cases in that particular caliber.

The Sinclair Generation II Deluxe Flash Hole Tool (#749-003-822) has a recessed cutter that bottoms out on the web of the case and keeps the tool from removing too much brass or enlarging the flash hole itself. This tool works fine on cases that haven’t been trimmed. The Generation II Deluxe tool can also be unscrewed from its handle and chucked into an electric drill or a powered case prep system, like the Sinclair Power Center (#749-008-418), to help you deburr cases even more quickly. If using a drill, just be sure to use a low speed setting for this operation.

Just a light touch is all that’s needed to deburr the case properly.

No matter which style of tool you choose, be sure to deburr lightly. One of the most common mistakes reloaders make is to get too enthusiastic about deburring, and remove entirely too much brass from around the flash hole. Just touch it. All you want to do is trim off any burrs that might be sticking up, and chamfer the mouth of the hole just slightly. Do not use a deburring tool like a reamer!

When it all goes correctly, the process is simple and fast and requires minimal effort. Here’s a video demonstration of the whole brief process, start to finish, using a Sinclair Piloted Flash Hole Deburring Tool.

Are You Doing It Right?
Eventually, you’ll develop a feel for how light a touch it takes to effectively deburr a case. You’ll still need to check your work visually, just to make sure. There are two main ways to do this.

One method is to hold the case up against a bright light source, and try to look down inside the case through the mouth. If you focus your eyes just right, look for a uniform, shiny ring around the edge of the flash hole. If you see that shiny ring, you’re deburring the case.

Another method is the whip out a Hawkeye Borescope (#749-008-461) and look right down there at the flash hole, up close and personal, with a 360° field of view. Yes, Hawkeye Borescopes are pricey. But so are custom competition rifles, high-quality shooting rests, benchrest bags, premium brass, high-end bullets, and travel expenses to matches. How much, exactly, are bragging rights worth?

On To The Next Steps
Now that you’ve selected your brass and properly deburred the flash holes, it’s on to the next brass preparation steps. Many competition shooters like to use a flash hole reamer (#749-005-418). A reamer inserts through the pocket primer end, not the case mouth, and opens the flash hole just a bit, making them all the exact same size for an even higher level of consistency, which helps produce precision. Whether or not you choose to ream the flash holes, it’s always a good idea to then use a primer pocket uniformer (#749-003-709). Next month, we’ll look at how to properly use these tools to help you make the most consistent, accurate loads possible.

Roy Hill
Brownells/Sinclair Copywriter