Reamer Prints and the carbon ring

[Nomad47]

Dave Kiff made a couple comments on another topic which prompted a couple questions. Dave mentioned crimp, expansion, and the carbon ring. When I use a dimension in my questions, it is in reference to Reamer Print #1303 for the 6SLR cartridge.

1. Is the crimp formed on the end of the case neck during firing? Does the crimp happen at dimension U?

2. Dimension U is 2.0618". To allow for .005" expansion, would case OAL need to be less than 2.0568"?

3. Does the carbon ring form between the end of the case and U? If not, where does the carbon ring form?

 

[Dave L Kiff]

NOMAD47 could you pull up my three posts on the other thread , I am on a grinder the rest of the day and cant take the time to post a lot . I think there is some valuable info there if you can understand it. It really explains a lot .Thanks Dave

 

[BoydAllen]

Between the end of the case neck and the bottom of the angled transition to the freebore, is an area where powder fouling can build up. If shorter cases are followed by longer ones, the longer ones are forced over the thickness of the fouling that was left in front of the shorter ones, crimping th e case mouth slightly. This points to the need for frequent trimming, and close control of shoulder bump. Additionally the corner that is formed at the larger diameter point of the angled transition is hard to clean, because brush bristles are bent by the smaller freebore diameter so that they miss the corner. IMO a good deal of this can be avoided by trimming for more clearance to the end of the chamber, which does not cause accuracy problems, and is only an issue if bullet and throat length combine to create a situation where there is less than the desired amount of bullet shank in the case neck. In the past, I thought that keeping this gap to a minimum was important. I have since learned better. (Search for Jack Neary on Youtube and watch all six segments.)

Getting to the carbon ring, what I have seen in a bore scope is a deposit at the very start of the freebore. This material is not the same as easier to remove powder fouling, and once it gets a foothold, generally requires the use of one of the abrasive cleaners too remove it. Not all barrels form carbon rings to the same degree, and certainly there are differences between powders. Sometimes the most accurate powder may not give the easiest to clean bore, but for competiton, unless the fouling becomes an accuracy problem in fewer shots than are allowed between cleanings, this is not an issue. On the other hand, for something like varmint shooting, a powder that gives good accuracy measures easily, and is relatively clean burning is probably a better choice than one that gives slightly better accuracy and requires more frequent cleaning.

 

[X Ring Accuracy]

NOMAD47 could you pull up my three posts on the other thread , I am on a grinder the rest of the day and cant take the time to post a lot . I think there is some valuable info there if you can understand it. It really explains a lot .Thanks Dave

No Dave,
You are the phone MOST of the time talking to guys like me......

 

[mikecr]

In contrast, I see it differently.
Carbon forms at case mouths(and in chamber & on necks in general) as gases blow by still unsealed necks. This is seen as carbon on the outside of necks sometimes all the way to shoulders, and carbon buildup at chamber end. It's caused by excess area in the chamber neck area, due to high neck clearances, including that from excess trimming. It's countered by fast powder/high pressure loads, and less unsealing chamber neck area.

With fitted necks, or trim length ~5thou from end, carbon doesn't build up, because it goes down the bore instead, given instant neck sealing(so gasses can't go backwards). I experimented with this a bit until there was zero sooting of necks and found lower ES with it.
I had to COW fireform to do this as necks pull back in forming shoulders, and this is when I did final trim.
I don't normally go through all the trouble, but I don't trim until after FFing or beyond 10thou off chamber end when I have influence over it(custom chamber). When I still get much in neck sooting, I know my powder is too slow.

Getting carbon out there is very difficult, and I keep an eye on it with a borescope, and automatically watch for any shiny areas on the face of case mouths on ejection(after shooting a lot). I'm paranoid about carbon..
Oh yeah, I don't FL size cases, my cases don't grow, and I don't have to re-trim them.
If you FL size and your cases grow with each cycle, then you'll probably have to trim in excess or you'll go nuts about this issue. And it won't directly 'hurt' accuracy either way, but I see no reason to recommend excess clearances -ever.

What point blank BR shooters do with a 6PPC and N133, is less useful to the shooter using a 243Win and IMR7828.
A tiny 6PPC run at extreme pressures seals instantly, even while trimmed way back, due to all the FL sizing that must be done to re-use cases. A bigger 243 with excess trimming, and unable to run at extreme pressures(more than once or twice), might soot half way down the shoulders before sealing.

 

[BoydAllen]

Carbon rings are what is called "hard carbon" which is a whole lot tougher than ordinary powder fouling. A lot of the best benchrest shooters have shot T powder and have had to use abrasive cleaning methods to stay ahead of hard carbon deposits that are not a problem with 133, and at the pressures that a warm loaded round with a unturned neck runs at, sealing differences are minimal. I shot a lot of unturned .220 Swift never with 133 and so even though I refer to my PPC shooting a lot, I have lots of experience with other powders and thick unturned necks.