Cylinders That Collect Carbon At The Case Mouth.

It is fairly common to see cylinders with some pretty tough carbon deposits right in the chamfer where the case mouth stops. Removing this "moon rock" can be difficult at times. Leaving it to build up will eventually cause rounds not to seat fully and can impede the bullet pulling crimp and opening the case all the way to release the bullet.

Some of these carbon deposits can be removed fairly easily, but every now and then you get one that just defies every attempt to have this area clean and shiny. Here is one that a customer sent in to have the throats reamed and honed to .4525" and in the process of trying to remove the carbon with a dremel tool and a small, very stiff stainless steel cup brush, I discovered each chamber had a ridge or "corn row" as I called it, which gave the carbon and some leading a perfect place to perch and after a more than difficult effort to remove the carbon, I finally got down to the ridge which we decided would be best served by having the chamber reamed with a finishing reamer, which worked great for removing ONLY the offending metal in the chamber, while keeping the rest of it perfectly smooth and shiny.

This cylinder isn't bad per se, I have seen them with a LOT more carbon buildup, but here's what it looks like:



After laboriously removing the carbon, the reason it is building up in the first place is revealed, a solid ridge of metal all the way around each of the chambers:



After using the Clymer 45 Colt finishing reamer, the ridge is gone and this cylinder should be very easy to clean in the future as there is no roughness or ridge for carbon to cling to and build up:

Very nice job there!

I'm the owner of this cylinder and am quite pleased to say the least. I bought the gun month or so ago in lightly used condition, took it to the range once and wasn't thrilled with the performance, pretty inconsistent groups at all of 10 yards. I decided to have Doug do the throats as my lead bullets were very a very tight fit and figured it was as good a place to start as any. I loaded up a box of fifty rounds, went to the range and shot half to give me a baseline or a "before" with the same result as before. I sent the cylinder to Doug who is wonderful to work with and very quick too and he found the issue, we discussed it and I figured it couldn't hurt to try it. Shot it today with the other half of the original ammo and the groups are half the size they were and more uniform. I'd say it's much better and money well spent.

Could it not be that the cylinder was showing some erosion at the case mouths? Seems like that when I look at the first picture. That type of erosion is very common in rifle chambers.

In my opinion, the ridge was originally caused by a chambering reamer that was dull.

The result is that it pushes some of the steel ahead of the cutting surface. 8)

Jimbo357mag- Highly unlikely that there was throat erosion, the fellow I bought the gun from said he'd barely shot it at all and overall condition confirmed that also this gun is only about a year old. While I'm no machinist, wouldn't erosion make the throats larger to the point they wouldn't clean up with a finish reamer?

ChiefTJS said:

Jimbo357mag- Highly unlikely that there was throat erosion, the fellow I bought the gun from said he'd barely shot it at all and overall condition confirmed that also this gun is only about a year old. While I'm no machinist, wouldn't erosion make the throats larger to the point they wouldn't clean up with a finish reamer?

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A picture I saw one time showed a 357 mag cylinder that had a long and steady diet of 38 specials to the point that there was a huge carbon build up that had to be scrubbed or scraped out of the chambers. After that it was revealed that under the carbon there was actually a ring of erosion that was noticeable. A microscopic picture showed that the chamber was actually damaged. Ever since I saw that picture, which I can't find now, I have shot full length cases in my 357 mag and 44 mags.

Without knowing the history and after looking at the pictures above I was just wondering if there might have been some erosion inside those chamber. Just a guess. btw, glad you got the cylinder fixed and the gun shooting good. That is some nice work.

Jimbo- I remember that too, something about the erosion happening due to the crud ring drawing in moisture or some such. Some folks just don't understand these things are supposed to get cleaned once in a while. With any luck I'll have that .45Colt out again today and make sure I just wasn't having a good day when I got better groups.

I don't know what that guy would be shooting to cause erosion from firing, I have not seen anything like that in over a thousand cylinders that have passed through my shop. Moisture behind the fouling very possible, especially if it was allowed to sit for any length of time.

Most of the really hard carbon deposits once removed reveal a smooth chamfer, about half of them have some tool marks that would help the carbon gain a purchase so they don't have to be rough for it to stick there at the case mouth. Chief's 45 Colt cylinder was the first one that really needed reaming to smooth it up so I will say that his was the exception.