Jeff White said:
Don't worry about the carbon on the tail of the bolt. The high temps and pressures there blows it off on it's own and deposits more. You're going to do more damage to the finish by scraping it off then any gain you will receive.
I shoot my ARs a lot in training. Once a year the bolts go into the SLIP 2000 Carbon Killer tank and get cleaned off.
Yeah, even around my department there seems to be something of a split decision on the scraping method! I never used to do it with my personal weapons, as I too feared potential damage to the finish on the bolt. But, I was given a brand new department rifle to use when I signed up for our rifle course a few years ago (not everyone here carries a rifle here... dept politics and all), and a couple of our range officers were advocates of the scraping technique at that time.
So, given that I didn't own the rifle, I figured I'd give it a try! By the time I switched to carrying my own personal rifle a few years later, I had probably put about 10,000 malfunction-free rounds through the department's Colt rifle. As far as I can tell, scraping never really did any damage to the bolt. But, obviously I can't honestly say whether or not it took some years off of the finish (it didn't
look like it was damaged, but who knows what was going on at a microscopic level).
Admittedly, I try to be careful when I scrape. I'll use the plastic dental-like picks as much as possible, and only use a razor blade to handle stubborn spots. Even then, I am careful not to scrape to vigorously.
I wish I could find the definitive answer on this subject, if one actually exists. There seem to be a few schools of thought on this issue, and I'd certainly like to pick the "right" choice, if there is one! Some folks swear up and down that you'll get malfunctions if you don't regularly remove the carbon from the bolts, but I can't say that I ever experienced one of these malfunctions back in my non-scraping days. Other folks maintain that you'll surely ruin the finish if you do scrape, but I haven't really exerpienced that yet either. Moreover, some manufacturers apparently claim you'll do damage by scraping (see post #12), while other companies make tools specifically for this task...
Maybe none of it matters at the end of the day? Either way, I spend far too much time trying to figure out the best ways to clean these rifles... They are complex, and contain lots of hard to reach places! But, they are a heck of a lot of fun to shoot, and they are a great piece of equipment to have available at work!