I have been noticing a lot of carbon on my brass. I also have a lot of build up in the chamber that restricts lock up of the bolt. This is a JP upper with their bolt.  I am wondering if the gas rings could be worn out? This has a max of 1200 rounds. I only shot 25 rounds the since the last time I cleaned it and it needed to have the chamber cleaned. The bolt and carrier seem to be excessivly dirty also. I have checked the adjustable gas block for proper setting. It cycles fine when clean. I only shoot it off a bench slow fire. The rings measure to .506 loose.
Let me know what you think.
Rick.

Can't hurt to replace them. You might also want to check if they're lined up (hopefully they're not).

A lot of carbon on the brass is usually due to blow back or excessive chamber pressure... It could be related to an over sized chamber or from using 5.56 ammo in a .223 chamber...

You didn't mention what kind of ammo you were shooting.  Sounds like an ammo issue to me.  Reloads that are loaded too lightly can also give you the same problems.  If carbon build up is keeping you from chambering rounds, then I would guess you do not have a nato chamber and should not be shooting surplus through it.

That all said, some brands of ammo are just dirty.  If you want to keep shooting the same ammo, just live with it.

Also make sure the chamber is free of lube before shooting since that will lead to carbon build up in the chamber as well.

Edited to add:  Doubt it is your gas rings giving you problems.  If the bolt doesn't slide out of the carrier under its own free weight, your rings are fine.  Don't worry about rings being lined up, it doesn't matter, ignore the TM on this.

This is just a long shot but something worth checking out. JP uses a stainless bolt carrier which is softer than the phosphate or chrome plated carbon steel used in bolts and such. Is there any scratching on the inside of the carrier where the bolt and gas rings would sit? Basically your gas rings provide a seal so that gas pressure with all the crud floating around with it is blown out of the bolt carrier rather than being introduced to the chamber. If there is some way for the gas to get around that seal, it might explain your fowling issue. I'm probably wrong with my theory and the inside of your carrier is as shiny as new, just something to check out next time you clean.