As a general statement Vihtavuori powders are the cleanest burning powders. Here is the US they are the most expensive powders 1.5x-2x the price of most other powders. We joke about them being powdered unicorn horn.
I see that you are Wet Tumbling your fired brass. This is best way to remove soot/dirt from brass fired through a semi-auto rifle, such as AR15. So your reload should be very clean cases to start.
I also see that the photo that you posted were of fired cases without a suppressor on the rifle. Suppressed spent brass is usually much more dirty and more heavily coated down the entire case length.
There are two potential culprits:
1) Inadequate Chamber Seal:
As others have mentioned, if you don't get a proper chamber seal (case neck not expanding sufficiently to seal off chamber) then gases can sneak back around the case mouth and start heading toward the rear of the case. This can occur from having a generous chamber (oversized) &/or underpowered powder loads in the cartridge resulting in case not expanding quickly enough to seal off chamber. Usually when this happens there is a much more prominent, heavy soot deposit along the neck and shoulder of the spent case. I shoot several Daniel Defense AR's and their chambers are very good (not sloppy loose).
If you haven't tested your reloads over a chronographer then you really should. A great way to determine velocity and also identify velocities that your rifle likes to accurately shoot a particular bullet (nodes). With an AR15 you really want to get that bullet moving fairly fast... loading toward the upper velocities (refer to recommended loads by powder manufacturer). For instance a 55gr bullet should be moving 2900fps-3100fps out of a 16" barrel. Depending upon the load, an 11" barrel should be somewhere in the 2550fps - 2750fps range.
2) Excess Lubricant either in rifle or on cartridges:
I know guys who like running their AR's wet (dunk the BCG in oil, shake excess off and install back in rifle). Adding a thinnest coating of lubricant on the BCG and underside of Charging Handle isn't the worst thing, but it quickly becomes too much especially when you start shooting suppressed. To avoid the mess and extra residue that builds up while shooting suppressed, I moved to nickel boron BCG to eliminate the need for any lubricant.
I think your last post is the key... Excess lube on the outside of your cartridges. The soot pattern. like fingerprints, is result of oil on outside of case being burned. You need to rinse off your cases after resizing them before you reload. There are a number of threads on best ways to get rid of the lanolin from the use of hot water or spraying down cases with straight alcohol and then rubbing down with a towel... After I prep my bulk brass for reloading, I place it back in the wet tumbler for a quick 15 minute tumble with soap to remove the lubricant. This also gets the primer pockets a bit cleaner. Try this and see if it doesn't solve your issues.
