Chamfering Case Mouths
Date: Jun 01 2006
We chamfer case mouths for two reasons: to "clean up" the mouth and to allow bullets to seat more easily. It should be done with any un-chamfered brass, whether fired factory rounds, new brass or just-trimmed brass. But the ways we do it can make a big difference and should change with the type of brass.
New brass is often slightly thicker at the mouth than in the neck just behind the mouth, due to a slight burr left during forming. This is why we should chamfer new brass both inside and out, since a slightly tight chamber can cause pressure or, at the least, accuracy problems if the mouth is too thick. The standard double-ended chamfering tool sold by most major companies works okay but can itself leave a slight burr inside the neck, due to the steep angle of the chamfer. (This is also the case with some of the chamfer-cutting blades for case trimmers that chamfer as they trim.) This burr can hurt accuracy slightly, so if you're one of those handloaders looking for the very tightest groups, another tool can eliminate the problem.
Consequently I've been using a couple of different tools for inside chamfering for the past couple of years. The first arrived as a gift from a friend, who bought it at a machine- tool store. It has a rotary, curved blade attached to a simple handle; give the inside of the neck three or four turns and the job is done. He sent me an extra blade as well, but the first still works great after several thousand rounds. The other tool is a shallow-angle tool with far more blades than the standard chamfer tool, resulting in a gentle cut, from Darrel Holland (PO Box 69, Powers OR 97466, 541- 439-5155). Both of these tools work great on typical, modern rifle cases.
When the chamfering any case that requires crimping the bullet, whether for a handgun or rifle, chamfering should be limited to the absolute minimum. Such cases should first be trimmed to exactly the same length, one basic "secret" of fine accuracy with crimped bullets, and then the burr created by trimming barely eliminated during chamfering, leaving the case mouth with a flat forward surface. This al- lows the brass to dig into the bullet when crimped, getting a good "bite" and is also essential for any rimless round used in semiautomatic handguns, since the case mouth controls headspace. The standard double-ended chamfering tool is perfect for this job. Just give both the outside and inside of the mouth a very light twist to take off the burr.
The standard tool was greatly improved some years ago by the addition of a post inside the outside- chamfer end. This prevents the tool from slipping off the case mouth and jabbing you in the fingers or hand. My first chamfering tool did not have this post, and I was very glad to get rid of the thing a few years later!