I posted the question on the M14 forum of how to remove the tar, and I was contacted by a gentleman that suggested using a used bore brush wrapped with fine steel wool, driven by an electric drill. He said he had been doing it for 30 years. So, I got some steel wool, and peeled out a strip of it around 1/16 inch in diameter or a bit larger, and then wrapped the wool around the brush from the tip towards the shank, clockwise looking from the cordless drill to the tip. I use a pair of plastic dipped pliers that are used for pulling spark plug boots off spark plugs to hold the cases, as the fingers get tired if you don't. I squeeze the trigger of the drill as it sits on the bench with my left hand, and feed the case over the brush/wool with my right.
If the wool is clean and new, it will clean the tar out and polish the inside of the neck. You will have to play around with how much wool to wrap around the brush, so the mass isn't too big to go in the case, but is large enough to really scrub the tar off.
The tar builds up on the steel wool, so you need to remove the wool now and then and rewrap. The brush/wool also removes a majority of the residual powder grains in the cases, but I use a small egg shaped carbide burr by hand to reach inside the cases and rub out any residual powder.
All in all, it goes fairly fast and leaves very clean military grade primed cases. By the way, the guys on the M14 forum said the primers are hot. I plan to run them over a mandrel next, and then all of them through the Giraud case trimmer for length tuning.