Possibly Dunc, but every time, and without the firer noticing that its not a smooth load?
All types not grouping I could except, different types grouping slightly differently the same, but a two foot board to capture a group at 100, as against a 1-2cm group is strange.
If the tips were getting damaged on the feed ramp to that extent, then the user would feel a difference, if screws were lose or the crown/throat damaged then that would affect all rounds.
The only other thing I can think apart from user error, is a bad batch, but in my limited experience with the hmr, this is not in