Re: Secondary Explosion Effect? Or.....?
In addition to having that significant lip on the inside of the neck, the virgin rem cases are ALSO "wavery" around the neck's diameter, most likely a result of down-necking/annealing the 300RUM parent cases. You definately need to run virgin rem brass through the sizer/expander die [numerous times], BEFORE doing anything else to them, in order to get the neck diameter trued. And I would advise against trimming them. I just pulled a random sample of 5 virgin cases out of a box, and miked them up. They come up between 2.8425 and 2.845, while the spec indicates [max] length should be 2.850. You should strive to be as close to that as you can get, in order to get the best possible seal around the neck, for minimizing the possibility of any high-pressure gasses from getting past/behind it. If that inner lip really bothers you, try chamfering it out very lightly, making sure that the case does not loose ANY of it's length.
If that doesn't solve the problem, the fault must lie with the chamber, such as mis-alignment to the bore. If you still experience the phenomenon after trying all of the suggestions [including using hotter charges, for breaking in the cases], then HAVE THE RIFLE CHECKED OUT BY A SMITH.