Loaded some Berger 168's over 94gr Retumbo. Case necks are partially sized and bullets were seated long to touch the lands. Base of bullet is at neck/shoulder junction inside the case.
Below is a pic of the result.
Never seen this before and I am assuming it is due to the small amount of bearing surface held by the neck not allowing the case to expand enough to seal the chamber before the bearing surface exits the case and gas is allowed to flow around the boat-tail and back into the chamber.
I base the above conclusion on not only appearance, but also measurement. Outside case neck diameter on loaded round prior to firing was 0.314. Outside case neck after firing is now 0.313. Internal neck diameter before seating bullet was 0.282. Internal neck diameter after firing is 0.282. Case did not grow in length at all, nor did body diameter change.
You can see in the pic that soot is evident on the case body along its entire taper. There is a definite ring at the case web where the soot no longer traveled rearward.
Am I on track with this? I have ideas for remedies, but wanted to get input from others as well.