After reading Larry Gibson post on how they would get short and thinking about it, I do believe they get short. Now I'm one of the largest fired bullet collectors you've ever known. A fascination habit carried over from my childhood. Take my 6.5 Grendel using that 140 Saeco. That bullet is dang near 3/4 bore rider. I've shot it to little over 2500 fps. That's about the limit for that small case especially in the AR15. I've recovered fired bullet and the only place there are land marks on the bore riding nose are just right there in front of the first bearing band. The marks go just a little ways up the nose...not far at all, be lucky to make 3/32th's of an inch. So it's squatting some, not a lot.
Yes I have breach seated bullets. In fact my 140 Saeco in my 6.5x54MS because that rifle has a god awful long freebore. The result were mediocre.
Joe