Don't know anything about your trimmer, so I don't know if it's okay to trim them with primers inserted. I would think so, though. I use a Wilson, but have never trimmed brass that was primed.

As to trim length, the manuals suggest a trim-to length of 2.610" for the .300 Win mag. That's what I trim to with mine. Shouldn't be necessary and probably not advisableto trim to 2.600". The neck on a .300 WM is short enough as it is.