Chamfering Case Mouths
Bart Bobbitt
case to length. A traditional process is to use a chamfering tool that
has an included angle of about 60 degrees. It's turned in the case
mouth to debur the ridge left by the case trimmer. Chamfering is done
either before or after sizing the case.
I've noted that in looking at the case mouth after the bullet is seated,
that most of the time, some of the bullet jacket has been scraped off and
is piled up at the case mouth. This is easily seen with a magnifying
glass. When I first noted this many years ago, I thought that removing
jacket material, especially nonuniformly all the way around, would
unbalance the bullet. Surely, there must be a better way.
Pulling bullets from traditionally chamfered cases told the rest of the
story. Each bullet was scratched lengthwise from the base to where the
case mouth was on the loaded round. Some bullets were more scratched than
others. As fast as those bullets spin, I knew their unbalance would
not let them shoot too well. Even using a bore brush to clean up the
edge of chamfered cases did not improve them any significant amount.
My reasoning was to have the angle of the chamfer much less. That would
not present so much of a `cutting edge' and might reduce the amount of
jacket shaving. Then I remembered a tool totally unrelated to handloading
operations; an easy-out. I took a No. 5 easy-out and looked at its edges
that normally are used to fit in a drilled-out screw and remove it. Those
edges were pretty rough, so I chucked it in an electric drill and polished
the edges with emery paper; they came out much more uniform and without
any rough edges.
After trimming a batch of cases, I took 20 and chamfered them the normal
way with a Wilson tool. 20 more were chamfered with the No. 5 easy-out.
Under a magnifying glass, the easy-outed cases had much cleaner chamfers
than the ones the Wilson tool was used on. I then loaded the cases and
bullets seated in the easy-outed cases had no jacket shaving; the others
had the normal amount.
I also ran a test with ten different cases chamfered with the easy-out.
Seating bullets in them and pulling the bullets, then checking them for
scratches. There were none to speak of; what there was was small and
hopefully insignificant.
Next step was to test the ammo. I put a scope on one of my match rifles
in .308, the cartridge used, and shot each batch for group at 600 yards.
Group sizes were 3 inches and 5 inches; the larger one was with the cases
chamfered with the Wilson tool. That was a good indication that the
bullets with shaved jackets did not shoot too good.
This happened about 13 years ago. The only problem is that I've been
mentally kicking myself on the back side for not paying close enough
attention to my reloads for competition for the 13 years before that.