Recently got a left handed Tikka T3 in .223. It was purchased second hand but never had been fired and I bore scoped it and it was pristine and shiny no trace of carbon chamber or bore.
So I have shot it and worked up a nice load cleaned it and was planning on shooting a hundred or so rounds before another cleaning.
So after maybe 60-80 rounds playing shooting steel made a trip to Roswell NM and took the Tikka as my son had asked me to bring my LabRadar to verify DOPE on 308 they have for PD so I figured I would shoot it at some fresh steel that I had not already killed.
Rounds would NOT chamber. Fired brass would not chamber.
I didn’t really have time to investigate so just put it up and today cleaned the barrel which cleaned quick and easy. Man that N133 burns clean! Pulled out bore scope and barrel was nice and shiny clean BUT at end of neck in the chamber was a carbon ring that looked pretty serious. Got a pistol brush and after some scrubbing got it out now brass and rounds chamber again.
I have never encountered this. It’s Lapua brass so it’s probably a little thicker neck than most 223 brass but that seems a little odd.
I am still pondering this ....just thought I would throw it out there and get some thoughts.
Just for information on things I am pondering.
I did trim some of the cases on a Garaud after FL sizing because they were shot in another rifle but also I sized about half of this 150 pieces in a Lee Collet die and didn’t trim as they were fired new in this rifle.
We were regularly hitting my coyote steel target at 940 yards with this rifle a week ago with 60gr Sierra MKT so it was shooting great.

Used but never fired?

Fixed it.

As I'm reading your thread, I'm thinking the brass was too short. Some would have to be too short to cause a carbon ring enough to to not allow other brass to chamber. It's kind of like shooting a bunch of 38 Special in a revolver, then wanting to switch to 357 Mag. You have to clean out the carbon build up just in front the case mouth on the 38's where the brass will be in the chamber on the 357.

Way out of my league here but my first guess would be that you trimmed them too short on accident. I’m sure that’s unlikely but it’s the first thing that comes to mind.

Learned this on my 45/454

Like they said short brass would be my guess as well.

Spot checked about 10 or so pieces mostly ones I trimmed and they are spot on 1.750
Of the ones not trimmed they were slightly longer but not much....No really short short ones. Thinking it’s a combination of trimmed brass to minimal length with brass that’s a little on thicker side in necks and my neck must be minimal sized circumference but a tad long in length of neck area? Guess I won’t be trimming brass for a while and see how that affects the issue.