Lead ring in 9mm cylinder

[Thumbcocker]

This is a new one for me. I took my RBH with 9mm cylinder to the range. accuracy was good and things were looking up. then my cartridges stopped seating. This was puzzling since I had set up my dies for a plunk test in the cylinder. A look in the chamber showed a small ring of lead on the shelf that the case headspaces on. Just a tiny ring like loading a cap and ball revolver. Boolit was NOE 135 grn sized .358 ACWW and 3.5 of red dot.


I tried to reaccess Beagle's article in cast pics but the site wants me to click on an "I agree" button that is hidden somewhere. In any event what gives with the lead ring?

[Love Life]

Bullet being shaved.

[DougGuy]

You might need to send me the cylinder and have it reamed. What are you sizing the booilts to?

[Multigunner]

Seems to me that when the cartridge is fired chamber pressure would force the case back against the breech to the limit of headspace/headgap.
The bullet not having fully left the case would now be bumped up with a thin ring of lead being forced into the narrow space between case mouth and chamber throat.
As the bullet moves forward this thin ring would be shaved off leaving the ring or at least part of it. After a few firings enough lead and fouling accumulate that the ring won't compress enough to allow the next cases to seat fully.

A harder alloy , lighter charge, or more effective lube should cure this tendency.
On the otherhand a heavier charge might expand the case to grip the chamber wall hard enough to prevent the case from being forced back to expose the gap at the case mouth.

[Outpost75]

Base upset is common in 9mm cast bullets. Besides cylinder throats being undersized, the mouth shoulder where the case headspaces has a sharp corner, which will shave bullet bases unless the edge is broken slightly with a 3/8" ball end mill cutter, turned by hand to chamfer the shelf inside edge. Only slight breaking of the edge is needed.

[Thumbcocker]

Boolit are .358. They will push into the cylinder mouths from the front.

[DougGuy]

Have you mic'd the case mouths after firing? It isn't tight throats if a .358" will push through but if the chamber diameter itself is generous, then there would be some of the headspace ledge exposed to the boolit, which would cause it to shave some lead.

In so many words, if the fired brass is .012" thick at the mouth, and the height of the ledge is .018" from the wall of the chamber, you would have .006" of the ledge exposed which would be the logical explanation for shaving lead and maybe what Outpost75 is suggesting with the ball cutter would be a good solution for it.

I have a 100° 3/8" single flute countersink that would work too.

[Outpost75]

I learned the 3/8" ball cutter trick at Ruger. That was the factory fix for lead rigs, but the official company line is that they build guns for factory ammo and they don't ecommend handloads.
Brownells sells the 3/8" ball chamfer tool to break the wire edge on. 22 rimfire chambers.