Help me understand brass length and carbon ring

[spin-one]

Help me understand how having your brass length as long as your chamber will help mitigate carbon rings? I have a Teslong bore scope and know how to use it. I used the Sinclair chamber gage so I know my chambers in all three of my semi custom rifles are longer then the recommended trim length. I verified this by putting a resized case in the chamber and looked at it with my scope. So how would having the brass close to the end of the chamber help prevent a carbon ring, wouldn't the ring just be a little further forward?

thanks
Scott

 

[Bob Wright]

Yes, narrower anyway. The closer you get the neck to the throat you would have to really manage that length, so some risk here keeping trim length of brass correct. I just stick to the manual trim length and check for a ring to be cleaned up as necessary.
Curious, did you get that info somewhere?

 

[dok7mm]

The majority of carbon rings form right at the end of the chamber neck where the throat is a smaller diameter than the neck area. It leaves a circular ridge that grabs the carbon and is harder to clean with normal barrel cleaning.

The powder residue becomes thicker and harder with every firing due to pressure and heat. If not cleaned properly, the result will a carbon ring forming over time.

A borescope is necessary to check for carbon deposits, ahead of the ring forming. Once you have the hard ring, it's the devil to remove. It takes soaking, brushing and often abrasives to break down the carbon.

I trim most all my brass .005" short of my chamber length and rarely have had any carbon rings. Beware that you have your exact chamber length before you trim and I always trim after sizing. I use the Brownells OAL gages on every chamber, except my tight neck rifles.

I also shoot several 40* shouldered cartridges that never grow enough to trim at .005" and I'm constantly cleaning carbon from the mouth of the chamber neck.

My experience has been that trimming too short may increase the chance of carbon rings. Of course, powder type, shooting style and barrel cleaning probably has an effect.

 

[spin-one]

Yes, narrower anyway. The closer you get the neck to the throat you would have to really manage that length, so some risk here keeping trim length of brass correct. I just stick to the manual trim length and check for a ring to be cleaned up as necessary.
Curious, did you get that info somewhere?

I have read on this site on several occasions that keeping brass near chamber length will help control carbon rings.

 

[spin-one]

The majority of carbon rings form right at the end of the chamber neck where the throat is a smaller diameter than the neck area. It leaves a circular ridge that grabs the carbon and is harder to clean with normal barrel cleaning.

The powder residue becomes thicker and harder with every firing due to pressure and heat. If not cleaned properly, the result will a carbon ring forming over time.

A borescope is necessary to check for carbon deposits, ahead of the ring forming. Once you have the hard ring, it's the devil to remove. It takes soaking, brushing and often abrasives to break down the carbon.

I trim most all my brass .005" short of my chamber length and rarely have had any carbon rings. Beware that you have your exact chamber length before you trim and I always trim after sizing. I use the Brownells OAL gages on every chamber, except my tight neck rifles.

I also shoot several 40* shouldered cartridges that never grow enough to trim at .005" and I'm constantly cleaning carbon from the mouth of the chamber neck.

My experience has been that trimming too short may increase the chance of carbon rings. Of course, powder type, shooting style and barrel cleaning probably has an effect.

Thanks for this helpful information

 

[L.Sherm]

Ask yourself if your brush is gonna know the difference between scrubbing .005 area vrs a .015 area.

 

[Mikecr]

Trimming way short absolutely does contribute to carbon ring formation.
And so does running excess neck clearances.

The bullet is a semi-plug holding back pressure. Gas flow is pretty limited until the bullet has traveled, but at very high pressure.
Carbon in the gas gives it mass, and therefore momentum with any direction of flow.
So where does it flow?
Well on subject -it flows backwards where it can.

If the gas can make the sharp 180 turn to flow backwards, it will, until the neck seals.
Excess chamber end clearance can provide plenty of room for gas to make the turn, and it carries carbon (with momentum).
We see carbon left on shoulders, necks, and built up in the chamber (where case mouths are stopping flow).
This is not 'good'.

With reasonable neck clearance of ~2thou, and trimmed to chamber end clearance of ~5thou, and the fastest powder that fills the case by SAAMI max pressure, there is no backward flow. No carbon on necks, much less shoulders, and no carbon piling up at case mouths.
The necks seal faster, not just due to less clearance, but because there is less pressure making it's way between necks and chamber neck.
This is 'good' (for low ES/SD, and consistent tune).

So in our world that isn't ideal, which is worse: tighter trim clearance that is not all the same, or looser clearances that are all the same?
-When necks are not all the same length, there is potential for tension variance.
-When chamber end clearance is causing sooting of cases, there is potential for inconsistent neck sealing.

So how much is the trim length variance, and how much tension variance would it lead to?
Let's go big and suggest a trim length variance of 10thou. Some cases with 5thou clearance and some at 15thou clearance.
Is there significant tension force provided with 10thou length of neck? Look at 10thou on your calipers.
Even if we could measure tension, which we can't, I'm sure we could not measure the difference provided by only 10thou length of neck. And I'm sure nobody could see it on target.

On big clearance, I'm sure there are competitors out there who would suggest that they've taken every shortcut invented, and hold records with it. I couldn't counter that.
But when my hunting capacity cartridges have made me proud, tight grouping, low ES, I've had no more than ~1/16" sooting right at case mouths. And that's where I could control things. I picked the powder, the neck thickness & clearance, trim length.
This has led me to deferring trimming until I'm within 5thou of chamber end. Sometimes I never quite get there, and never trim the cases. But where I could, and did, I believe it was a good move.

If your brass is stretched with every reload cycle, and you have to trim somewhat frequently, then you might as well trim back at 10thou clearance to be conservative. You don't want to keep this a concern, or forget about it and run out of clearance.
Just do what you can to get fastest sealing otherwise.

 

[L.Sherm]

Theres some pretty good shooters finding out shorter trim length is helping accuracy.
My nephews 6.5x06AI that I put together using Lapua 06 brass .015 shorter than chamber length is a prime example, its on of the most accurate rifles I've shot.
Test it yourself to see is all I can say.

 

[spin-one]

I have gained some valuable information on this subject since I first posted my question. I understand better why having brass meet closer tolerances in the chamber may help mitigate carbon rings. All three of my rifles have a longer chambers .037 - .034 and .035 than I was trimming my brass to the Recommended SAMMI length. I guess I will let the brass grow and only trim to keep them near the same length. The good news is they all shoot good.