That tight section above the powder after a couple of shots is the typical Tripple Seven "crud ring". Some rifles get it bad and some not so bad. You could try the new Tripple Seven primers from Winchester as they are supposed to be able to reduce or eliminate the ring. Sounds like you pretty much have all the minor problems under control. Good luck with the new rifle. I can't shoot Tripple Seven in my Omega but sounds like your crud ring is not as bad as the one I get.

I shot my gun a total of 11 times, but with the powerbelts loading wasnt to bad. I would run a patch down the barrel between shots. But yeah it felt like a ring of crud like you say..