Re: brass triming and primer selection for semi auto
I trim all new or 1x fired brass I get! Then I trim just enough to maintain consistant case lengths.
Primers, I use standard large rifles.
I have found certain rifles have more room and I can allow the brass to be longer than the suggested trim to in the manuals. However, doing so may not be ideal for the accuracy of the loads. Some of the rifles I have shoot better with shorter trims than the suggested trim to. A lot of which is determined by the bullets you shoot and how much free bore your rifle has.
Get a Max trim to gage and you can determine just how long your brass can be!