I had a lilja 308 barrel extension on a 260 that wouldn’t feed well at all. I ended up buying a triune precision extension and chambering a new 260 barrel that has been flawless since.
I have a Wera torque screwdriver for ring caps, and the Borka kit for everything else. With the lower torque numbers for the scope ring caps, a screwdriver style is a lot easier to use.
Take a permanent marker and cover the entire case and bullet. See if it’s contacting anywhere when chambered. I just had the same issue with a 260 barrel. The neck shoulder junction was too big causing tough extraction.