Brass much thicker right by neck?

I have been chasing this problem more seriously for the last several range trips, but it first showed up around 100-150 rounds ago, ssentially when i started on the third firing of the brass. Occasionally I would get a round that wouldn't chamber - I measured the case neck of a loaded round and it flared right at the end of the neck. This would happen on a round here or there but has gotten more frequent and drives me crazy...It has gotten more frequent lately, maybe 20-25% or more of the rounds. Some of those that do chamber take a little more oomph to close the bolt - i don't fire those but i do note that it happens. I measure the case neck width of a sized case (whole neck, not brass thickness) halfway up the neck and slowly pull it out of the calipers. I see it increase to about 5 thou above the bushing diameter right at the mouth. When I seat and try to chamber many of the cases stop a half inch or so from bolt close - a hard stop and won't go into the neck. Not even close to chambering.

I went through every step of the loading process to try to find where it was showing up and thought i had it narrowed to the seating die but found that wasn't it. It only shows up when a bullet is seated bc all extra brass is pushed to the outside and won't give if needed bc a bullet is seated - I.e. if I chamber a piece of brass without a bullet it will chamber bc the brass can just give in with no bullet to stop it from giving.

So i started looking at bushings, fired neck diameter, etc this morning. It didn't show up as not concentric, and I measured case neck thickness but not at the very very end...It was a eureka moment. I have no idea why it is thicker there, which is why I am asking the interwebs here, but at least I found the culprit.

If anything i expected the donut to show up and have a thick area near the shoulder and figured I would need to start turning necks to get rid of it. But I have no idea how the brass could be .017 thick in the middle of the neck and .022+ right near the mouth.

This has been super frustrating...any words of wisdom?

Pics and other items that will be helpful below. Pics show .017 mid neck and .024 at very end of case mouth.

MOA Rifles 28 Nosler using Nosler Brass, on the third brass firing.
Berger 195 bullet, .020 off lands. I have chased the lands about .020 since it was new, so not any drastic change. 550 or so rounds fired.
Whidden custom dies to my chamber, sizer is bushing with .314 bushing size.
I anneal with AMP after each firing, don't turn necks, and use STM tumbling for 4 hours.
Case length is still within spec, about 2.575 to 2.580 so i haven't trimmed yet.
I have been hand loading for about 4 years, self taught from watching videos, reading here, and reading articles.

The first thing I noticed is the large flat face on your micrometer stem. I suspect it could be skewing your neck thickness numbers. The stem is tapered to a smaller tip, on my tubing mic, and will give far more accurate measurements.

What is your fired neck diameter vs your bushing sized diameter? It could be that you are sizing your neck down too far with one bushing, causing a flare at case mouth. Just a guess.

dok7mm said:

The first thing I noticed is the large flat face on your micrometer stem. I suspect it could be skewing your neck thickness numbers. The stem is tapered to a smaller tip, on my tubing mic, and will give far more accurate measurements.

What is your fired neck diameter vs your bushing sized diameter? It could be that you are sizing your neck down too far with one bushing, causing a flare at case mouth. Just a guess.

Click to expand...

I agree that the mic is large and it is hard to know precisely where I am measuring but the jump is significant enough that I think it is real. I'll look at better micrometers. I'm bringing the neck down about .005 but i think it is thickness that is the culprit.

I would try flipping the bushing over and see what happens.
Some mics come with a small ball bearing in a rubber cup that fits over the flat face of the mic arm. It allows a more pinpoint measurement.

I have witnessed shoulder thickness moving into the neck more often with Nosler brass. I have not encountered case mouths getting thicker, ever.

I would suggest that you make a dummy round, using a case you know wouldn't chamber with a bullet seated.
Mark up the entire bullet and case with a Magic Marker and try to chamber it. It should show where the problem lies.

I tend to think there may be a carbon ring, as you described an increase in the number of rounds that would not chamber.

I just cleaned for carbon ring using simichrome using this method . That was the first time I cleaned for the carbon ring...i usually clean every 50 shots or so using KG1 for carbon and BTE for copper.

There must have been something there bc the velocity of my rounds dropped from 3000 to 2940 for the first several firings. In the past the first round after cleaning would be low and come back after one shot, two at the most.

The first 8 shots were all right at 2940 with ES of 13 and SD of 5. After 28 shots the next 7 averaged 2955, So after 35 shots the next 9 averaged 2975 with ES of 23 and SD of 7.5. Those were the last shots I fired yesterday.

What others have written in their responses to the micrometer affecting your measurements could be factual. Did you try measuring the outside of the case neck with narrow section of your dial calipers up close to the neck and then the end of the case neck? Are the outsides of the case neck parallel to each other? Did you measure your brass prior to and after resizing it? What I am trying to figure out is whether it would be best to turn the necks or ream them. Could the brass have been over annealed causing the brass to glow forward?

Sounds like you have a handle on any possible carbon ring. You can also have carbon build up at end of chamber neck, it's usually more difficult to remove because of the step, so it helps to have a borescope to confirm carbon buildup and removal.

I hope the marked dummy rd will steer you to the problem. Good Luck.

Couple of more things...I measured fired brass and there was no increase in thickness at the case mouth and no flare at the mouth.

Here are pics after using a sharpie to check for contact points. Kind of tough to see bc of glare but i circled the key parts. Out of 5 that went through FL body and bushing sizing, neck expander, and seating, 1 wouldn't come close to chambering, 2 were difficult but would chamber, and 2 chambered without any issues.

One the one that wouldn't chamber there is wear on the shoulder from it trying to eject as I extract it. But there is marker rubbed off right around the case mouth at the very tip...very tough to see bc it is a very line line and hard to decipher from glare.

One the two that were difficult to chamber there are two spots on the case mouth that are small but it created a burr you can feel with a fingernail. There is also a ring around the neck where the marker is rubbed off. There is wear on the shoulder but that is from trying to extract...it was always between 12 and 1 of how the case was oriented in the bolt claws.

Nothing on the two that chambered as normal.

Pics...both difficult to chambers look almost the same so I only added one pic. The one without a label is the one that wouldn't chamber - for some reason Apple won't save any text I add to the pic.

Here is the measurement with the neck micrometer...there is some flare of .002 to .003 on this

I have a borescope and didn't see elements of a carbon ring but I cleaned anyway. How would i clean the chamber neck? Same way but don't go into the barrel?

I feel like my head is going to explode...not having someone who taught me anything or having a local resource sucks. I figure everything out as problems come up, not knowing what is causing them or if anything I am doing is causing it. And it could be carbon in the chamber, sizer, seater, annealing, etc.

Bc the brass isn't thicker and it is a flare, I feel like it is something I am not doing correctly with sizing or seating, but those seem pretty straightforward. Theloaded rounds measure .317 and I am using a .314 bushing and expandingto .315 and getting .0025 shoulder bump. And the bushing is floating as I can hear it when I shake. The seater seems straightforward and I am getting the desired length with minimal runout during the whole process based on finished rounds...most are .002 or less with the rest being maybe .004 or less. Maybe I will try a .315 bushing...but at this point I am ready to pull hair out.

I have used a brush, with a patch around it for a snug fit, and a bit of JB. Turned on a short rod with a low revolution drill, for good results.

I would first measure my actual chamber length using the Brownells gauge on a shortened case. (Cheaper than their postage). You never know for sure until you measure it. A trim might be the solution, if you see zero carbon in chamber neck.

That pic shows the mouth hitting one or the other.

That carbon buildup in the chamber neck starts from firing new (short OAL) brass with lower powder loads. It can be hard to see without a 90* angle lens on your scope.

Here is the borescope...hard to get a phone to video while one hand guides the cable and another holds the main borescope unit / video screen.

Not sure if this gives you something to go on.