If you haven't tried flash hole deburring before as part of case preparation you may want to give it a try. During the manufacturing process, burrs are formed on the inside of the case when the flash hole is punched or drilled through. Sometimes the burrs will be quite pronounced and other times almost unnoticeable. Higher quality brass will not usually have large burrs because their flash holes are often drilled versus punched. Hold a case up to a light source and look at the inside of the flash hole through the case mouth. You can easily see the burrs or rough edges in most of your cartridges.
These burrs can significantly affect the ignition process and variations in powder ignition can create vertical dispersion of your groups. We believe the removal of flash hole burrs is one of the most important steps a handloader can perform during case preparation. Fortunately, flash hole deburring is a step that only needs to be performed once during the life of a case.
A properly made tool will remove all burrs and put a very light chamfer on the inside edge of the flash hole. This process will also insure that all flash holes are uniform in diameter and help provide consistent ignition of the powder charge. The process of flash hole deburring will also obviously let you know of any flash holes that were not punched or drilled. This is not a joke; we have run across cases that have not had a flash hole. Your initial visual inspection should find these but you never know!
Flash holes before and after deburring (exaggerated)
DO NOT EVER DEBURR FLASH HOLES ON CASES WITH A PRIMER IN THEM, THEY CAN GO OFF!
A flash hole tool typically consists of a cutter and a depth stop. If you are using a flash hole tool that does not have a depth stop either at the case mouth or down inside the case at the web, be extremely careful when using it. The depth stop prevents the cutter from cutting too deep or enlarging the flash hole. The intent is to just touch the surface and knock off the burr. The Sinclair Generation II Flash Hole Tool has the cutter preset into the shaft so when the shaft contacts the web the cutter cannot cut any further. The Sinclair Piloted Flash Hole Tool utilizes pilots that index off of the case mouth preventing them from cutting too deep (if setup correctly).
To use a tool like the Generation II Sinclair flash hole tool, loosen the centering cone on the shaft and insert the shaft into the case mouth until the cutter just touches the flash hole. Once you have inserted the tool into the case, lock the centering cone into place so it centers the tool in the case mouth. Then turn the cutter in a clockwise fashion. It only takes a couple of turns to remove any burrs. Sometimes this step will take very little effort and other times you will have to turn the tool with a little more force to complete the cut. You can definitely feel the larger burrs and see them tumble out of the case. The centering cone is not intended to be used to control the cutting depth but simply to help you locate the flash hole quickly.
Cutaway view of Sinclair Piloted and Generation II flash hole tools in use
When you use the Sinclair piloted flash hole tool, all of your cases should be trimmed to the same length. This tool controls the depth of cut with a caliber specific stop collar that mounts on the cutter shaft. To set up the tool, insert the cutter into the case mouth until it comes in contact with the flash hole. Turn it gently to remove any burr and to lightly cut a chamfer on the flash hole. Leave the cutter shaft in position, push the stop collar into the case so that the collar contacts the case mouth and lock the stop collar in place. When using this type of tool you may want to think about deburring the flash holes after the first firing. Some new brass may have inconsistent length, which will affect the deburring operation.
If you are going to debur the flash holes on .17 caliber and .20 caliber cases, you will need a special deburring tool. If you are making a .17/.20 caliber wildcat from a larger caliber, you can debur the flash holes prior to sizing the neck down. If you are working with factory cases, you will need to get a Sinclair .17/.20 Flash Hole Deburring Tool.
For those of you that want to debur cases that have a small flash hole (.060”), you will need a special tool. We suggest using a Sinclair Flash Hole Reamer for these cases. This reamer is used to uniform the .060” flash holes from the outside. The tool has a centering device that inserts into primer pocket and then you advance the reamer while turning to cut the hole to exactly .0625”. This tool will remove burrs but will not chamfer the edge of the flash hole.
There is also a Sinclair .50 BMG Flash Hole Deburring Tool for those loading for the Big Fifty.
If you decide to debur flash holes under power, we recommend using a tool like the Sinclair piloted tool. You can mount the Sinclair piloted cutter in a drill or drill press for powered use (reduce the RPM’s down to a low setting (almost as slow as you can). Be aware of the cutter as you feed your cases, since the cutter is sharp and will tear your skin easily under power.
A secondary benefit of flash hole deburring is that it may help you recognize off-center flash holes, which you will see occasionally in various lots of brass. We have seen entire lots of brass with visibly off-centered flash holes. How these cases make it through quality control at a manufacturing facility is a wonder.
Consider giving flash hole deburring a try if it is not already part of your initial case prep procedure.