A quick ? for everyone-I'm shooting a tight neck 22Br (0.252") w/ loaded cartridges measuring 0.250. After firing there's a moderate amount of black carbon around the neck. My chamber mics out to 1.730 and the Lapua brass (sized from 6Br) is trimmed to 1.550" - loads are on the stiff side with 32g VV N135 pushing a 52g Bart's or 53g Sierra HPFB, but still seeing the same thing at moderately reduced loadings of 29gr.
For more background action is XP-100, brass is as above and FL sized, trimmed, neck-turned to .250 and then neck sized from .250" to ~.248 after neck-sizing with a .247 bushing in Wilson hand die.
What's causing this?
Thanks,
Bill

Gas formed powder burn components slipping between the case and chamber neck walls before the case neck expands to seal off.

Since your chamber is (semi-)tight, you are probably not seeing a whole bunch of powder fouling on your necks. I have the same deal on a .223 Rem with .002" clearance between loaded neck and chamber neck diameter. But, it is less fouling than I get in my (sloppy) factory chambers.

The fact you get about the same amount of fouling with your light load, probably means the chamber/brass will seal off even with the light load. Only if that was a problem, a heavier load might have helped. As it is, there seems to be no difference.

- mike

I'd have to agree w/ mho. You need a bit more pressure to get obturation. You can seat the bullets a bit deeper, use different primers, or add a touch more crimp to see if that'll help. I had the same issue w/ a Contender .223 barrel a little while back- all that was needed was more pressure; hope it works for you too.

Ron