I usually shoot the first 1 i see. Around here if you let it walk the next guy shoots it. And no matter how long you boil the horns they're still hard. LOL
I never tried it but i think if you took a resized case that's .001 or .002 long and colored the neck with a marker it should scratch the marker off. Or peel some carbon off. I could be wrong.
No idea on new rifles. I stopped trimming my cases to the trim to length. I trim to max length to cut down on the blow by. The outside of my necks have less carbon on them since i started doing it.
Try seating your bullets longer. My 6.5 prc load lost speed . I seated the bullets longer and got the speed back. Do you have a lot of carbon on the outside of the neck?