Now if I remember correctly you had to trim necks back .020 shorter to get enough of the bullets bearing surface in the neck.Round holes or not, won’t tell you the whole story at distance or transonic flight. Angle of entry can fool you.
What generally happens long before oblong holes is that the group opens up. Example shooting 1 MOA groups every 100 yard intervals then all of a sudden you cross into trans sonic speed and it’s 2 MOA and growing.
You notice this shooting so called sub sonic, it’s really trans. With a lot of bullets they are much more accurate under 1000 fps than over. Light bullets, as in air guns in particular.
Do you think a different reamer spec is in order, or possibly something that can be overcome with more bullet refinement?
Carbon ring worries with the shorter case of any concern?
Just a couple thoughts