Re: When to trim 7MM Rem Mag brass?
I trim to the trim length in the Hornady manual (which is pretty much the same in all manuals) - 2.490". I never allow it to go over the SAAMI spec case length of 2.500". I check case length after the brass is cleaned and sized and before it is primed.
I control case stretch as much as I can by how I size the brass. I get 4, sometimes 5, reloads between trims if I FL size to bump the shoulders 0.001" to 0.002". If the brass is only getting two reloads per trim I expect it is being sized to bump the shoulders .005" or more. If the sizing die is set up per the manufacturer's instructions it will bump the shoulders on the brass back considerably more than is necessary in the typical chamber - usually .003" to .005" more than necessary.
Why is trimming important? Allowing case necks to be over length can cause serious pressure problems if the case neck is long enough that it gets jammed into the end of the neck region of the chamber at the throat and effectively crimps the bullet into the brass when the bolt cams closed. In that condition it takes much higher pressures to get the bullet moving much like it does when it is jammed into the lands.
That said, if you want to know how long your chamber neck really is, it may be a couple thou longer than the SAAMI spec, you can measure it. Sinclair sells a tool to do this, or you can easily make one if you have access to a lathe.
I doubt pushing case length (really the neck length) a 3 or 4 more thou is going to do anything useful and it can result in damage to at least the brass if it causes crimping and over spec. peak pressures.
"For a successful technology, reality must take precedence over public relations, for nature cannot be fooled." R. Feynman. (Last sentence of the Feynman appendix to the Space Shuttle Challenger Report.)