Carbon ring?

[lawrence97]

I was wondering if any of you have experienced the same with any of your 284's.
I noticed a slight resistance when chambering a round and upon careful extraction of the round I noticed some scratching on the bullet immediately forward of the neck. I know it isn't the lands because those leave 5 square marks around the bullets and my seating depth is set to jump a tad. I also know that my fired and resized brass goes into the chamber smoothly.

The scratching of the bullet will go away if I clean the barrel with a brush and strip the barrel down but it reappeared towards the end of my last match with about 70 rounds down the barrel.

My barrel is set up with a .313 neck, my necks have been turned and my loaded rounds are .308 at the neck which is the .004 to .005 clearance that most suggested I use for the 284. I am running .001 neck tension and my velocities range from 2903-2920 fps with 180 berger hybrids.

Any advice or your experience with this issue would be greatly appreciated!

 

[jonbearman]

What powder are you using? Some are a bit dirty and maybe when you clean you are only reducing the carbon ring instead of removing it completely causing it to build up quicker.I use Iosso bore paste on a tight fitting brush with a patch wrapped around the bristles and rotate it by hand to remove the hardened carbon completely.How much clearance do you have from the end of the case to the end of the neck area of the chamber.If you trim too short you are inviting the carbon to build up where the neck doesnt fit close enough to the end of the neck in the chamber. Too much gap equals carbon build up quicker.Imho.

 

[lawrence97]

32" barrel with rounds loaded up to 54.8 of H4831SC and Berger 180 hybrids.
As for clearance from end of case to end of neck, I do not have the reamer print at hand.
And for trimming I base the trim length of of what the majority of cases measure and make them all equal. They all seem very consistent and I have not had to trim much and the brass isn't stretching a lot at this point with 3 firings.

 

[Lapua40X]

I noticed a slight resistance when chambering a round

Is the resistance prior to, as the round enters the chamber, or only during the actual dropping of the bolt handle on bolt closure after seating the round in the chamber?

 

[fdshuster]

lawrence97: For a cost of about $7, invest in one of the Sinclair chamber length gauges, (item#749-000-784SE, 7mm, page 35 in catalog 2013-B), and measure your chamber length. They all vary and sometimes by a lot !. I've seen as much as .040" difference between actual chamber length and the so-called trim-to-length shown in the loading manuals.

As jon said, the larger the gap in front of the case mouth the more buildup there will be of a carbon ring. It's no different than firing a large number of 22 shorts in a 22 long rifle chamber. The accumulation of unburned powder, bullet lube, and lead shavings will prevent the chambering of the longer 22 long rifle cartridge. The only difference here is your buildup is carbon instead of powder,grease & lead.

When I spec my chambering reamers, I make it a point to include chamber O.A.L., that being about .010" longer than most trim-to-lengths, and I keep the gap no greater than .010" less than chamber length. I never have a buildup of a carbon ring in front of these chambers: minimal gap = zero carbon ring. "Big" gap = a carbon ring.

I have to add though: it sounds like you have a factory chambering, and if so, you may have a long chamber length, and with brass that is not stretching very much, it will be a long time before they can stretch long enough, so you may not have a choice in the matter. All you can do is use the IOSSO/brush/ twist method of keeping that area clean. To not keep it clean is to invite higher pressures when the bullet is chambered on top of the carbon ring.

 

[SmokinJoe]

Is cleaning more often an option? If so, sounds like that would probably be your best option. Does accuracy seem to deteriorate as this comes along? If not, don't worry about it. Also, check your freebore diameter on the print. Sometimes the freebore is a little too tight. If this is the case a throating reamer should be able to fix it; you are only talking .001" or less, but that could change the shooting characteristics of the rifle a little.

 

[lawrence97]

The resistance is prior to dropping the bolt. I usually have to push harder or give a quick tap and then I can drop/close my bolt.

A 284 match reamer was used and the rifle is a full custom. Definitely nothing factory. I believe the freebore used was .207 for the berger hybrids. It was a reamer that MT guns had and that Bruce shot well with.

As for accuracy I have not noticed any decrease. Its just the feeling of resistance that bugs me when chambering a round. I did check, and the resistance is not enough to affect my seating depths with .001 neck tension but I do have to put some pressure when pushing the bolt forward!

Thanks for the info.

 

[lawrence97]

FDSHUSTER:
If I have a tight (.313) neck would the sinclair gage still work or is it designed for a saami spec neck?
I thought about it but thought it might not slide in all the way due to the tighter neck...

 

[artbosco]

I was wondering if any of you have experienced the same with any of your 284's.
I noticed a slight resistance when chambering a round and upon careful extraction of the round I noticed some scratching on the bullet immediately forward of the neck. I know it isn't the lands because those leave 5 square marks around the bullets and my seating depth is set to jump a tad. I also know that my fired and resized brass goes into the chamber smoothly.

The scratching of the bullet will go away if I clean the barrel with a brush and strip the barrel down but it reappeared towards the end of my last match with about 70 rounds down the barrel.

My barrel is set up with a .313 neck, my necks have been turned and my loaded rounds are .308 at the neck which is the .004 to .005 clearance that most suggested I use for the 284. I am running .001 neck tension and my velocities range from 2903-2920 fps with 180 berger hybrids.

Any advice or your experience with this issue would be greatly appreciated!

How much Freebore are you working with? Are you sure you're really jumping the bullet? Pull your firing pin from your bolt, make up a dummy round, then seat it a few times while rotating the bolt back and forth. If the bullet has no marks then you are probably jumping it.

 

[lawrence97]

Positive on jumping the bullet. If I seat further out I get the very distinct rifling marks on the bullet. Also, after a clean up with wet patches and 5-8 passes with brass brush my current round slips right in without resistance. It's definitely carbon but not sure on the cause. Maybe it's just tight tolerances or I Need to clean more aggressively.

 

[Multitoes]

FDSHUSTER:
If I have a tight (.313) neck would the sinclair gage still work or is it designed for a saami spec neck?
I thought about it but thought it might not slide in all the way due to the tighter neck...

You can always turn down the gauge ; I chucked my .17 unit in a drill and turned it down to under the diameter of a loaded round for my 17 Hornet -- works great.

 

[fdshuster]

lawrence97: Yes, I see now that you have a custom, tight necked chamber, and with that in mind I would definately measure the chamber length, record that dimension & keep it with the barrel. Always good information to have even if that's not the cause of your problem.

The note in the Sinclair catalog does say that with one exception (6ppc tight neck) they will not work with tight necked chambers, but as multitoes said, they are very easy to modify, being made of soft steel. I reduced a few myself by chucking it in a drill press and easily reduced the diameter with a polishing stone.

 

[lawrence97]

I spoke with my gunsmith and the reamer print shows the chamber at 2.180. My fired brass has never been longer than 2.154. I assume that is the problem! There must be a difference in Lapua brass from when the reamer was cut and current Lapua brass ( gold box vs blue box). Now, does that mean the barrel must be recut, chambered, and new reamer ordered? Or live with a very strict cleaning schedule?

 

[fdshuster]

lawrence97: I personally would never have the barrel setback & re-chambered. At least until you've gotten the maximum use out of it at its' present length. That's only a difference of .026". Keep firing your brass and measure the length after sizing, and it will stretch to get you closer to the 2.154" dimension. For me, that would be 2.170".

A quick inspection with a borescope would verify that a buildup of carbon is, or is not, your problem. Does your gunsmith have a borescope?

 

[lawrence97]

I am actually at the 2.154 length right now and I need to get out to 2.180 before I reach max length and have to trim!

Yes, the gunsmith does have a bore scope and said to send him the barrel if I would like. But I am almost 100% sure the bore scope will reveal a nice carbon ring filling the gap between my 2.154 and the chambers 2.180!

 

[SmokinJoe]

Lawrence, if it'll make you any more comfortable, I won everything I ever did with about .025 - .030 gap between the end of case neck & end of chamber neck. Shooting a larger caliber may be different, but if you don't see it on paper, don't worry yourself too much. I think it would be a good idea to get the Sinclair spud, modify it & get an actual chamber OAL measurement. Sometimes there are surprises, so it is just good practice, especially with precision, custom rifles. Keep us posted.

 

[22BRGUY]

Another option that may help is Cerrosafe you can make your own chamber cast with it.

 

[lrgoodger]

No one has mentioned shoulder setback. I find that in almost every instance of hard bolt closure, the problem is a tight shoulder. I have to set the shoulder back .001" on every firing on my .284. A shoulder measurement tool is a must for setting up your die to make sure you bump the shoulder correctly.

 

[lawrence97]

I did mention that my brass was not the issue due to it chambering fine with fired brass and resized brass. The issue is when there is a bullet and that is when there is resistance when pushing forward on the bolt. When the shoulder is stretched out the feeling is a tad different and you can feel more resistance when trying to close or drop the bolt. I have a resistance prior to that point.

 

[artbosco]

I am actually at the 2.154 length right now and I need to get out to 2.180 before I reach max length and have to trim! But I am almost 100% sure the bore scope will reveal a nice carbon ring filling the gap between my 2.154 and the chambers 2.180!

If, your max chamber length is 2.180", give yourself a safety cushion of .010". So, trim back to your "trim to Length" of 2.160", when you reach your "trim from length" of 2.170".

Keep the area between the end of the brass and the end of the neck chamber clean by using Iosso or J-B paste and by using a brush one size larger, only for cleaning that area.

Put Iosso or J-B [about the size of a large pea] on the "end of the brush" and spin it about 30 times. Do that after every 50 to 60 rounds and it will stay nice and clean in there. The carbon won't have a chance to build up if you do that religiously.

Needless to say, after you've done that, give the rest of the barrel, and that area at the end of the neck chamber, a good cleaning.