The bolt tail seals against nothing. It's no different than having high compression humps on the pistons in your car engine. Those don't get cleaned for hundreds of thousands of miles. Scraping off the carbon is something that drill sergeants and armorers insist be done to use up a soldiers daylight and keep him busy.
IF you can keep a count of how many rounds you've shot, then periodic replacement of the bolt is advised, and since the rings come on it, that detail is taken care of. The AR piston can operate with just 1 - one - ring, and it's been demonstrated. Ring failure is rare, lugs breaking off much more common on high round count bolts.
Just keep it wet and no problems. AR's have been documented and publicized for going in excess of 50,000 round with just one or two cleanings. The majority of the maintenance was oiling the parts and wiping them down with a rag. No scrapers, brushes, nada. But, the extractor was changed, and so was the bolt.
The Army would like to count the rounds, too, but can't. So, they inspect it and look for cracks. That is about as high tech as it can get for the time being.
I was told the carbon behind the rings would stop an AR; I've even told people on this site that. At least I will save time from here on soaking w/ kroil and rubbing w/ 0000 steel wool. 
