If using mixed brass they can very in size even when trimmed. Say you set your trimmer at a certain length. You have a Federal 9mm case you used to set the trim length. However. You have 10 different manufacturered brass 9mm cases. You trim then all to what you set your Federal 9mm case length to.
Problem is every manufacturer may have a slightly longer or shorter case head. Some may have a thicker rim and wider extractor groove. Some may be thinner rim and slightly less wider extractor groove. It can have many variations with the different manufacturers. So when you set up and trim all the different cases According to one manufacturers case they will be all over the place after Trimming some shorter some longer.
Now you without thinking about that jump to loading. You take that Federal case or some other case and set up your seating die. Already every case that is different than the federal case trim length will be off slightly. Add to that the possibility of bullets being slightly longer or shorter then you start running into problems with COAL. Been there done that and found out what was causing the problem which I stated above. It works the same way for rifles as well.
I've found almost all cases differ slightly from one manufacturer to another.
Jump to 338 Lapua Magnum. Then all the above really matters.
Problem is every manufacturer may have a slightly longer or shorter case head. Some may have a thicker rim and wider extractor groove. Some may be thinner rim and slightly less wider extractor groove. It can have many variations with the different manufacturers. So when you set up and trim all the different cases According to one manufacturers case they will be all over the place after Trimming some shorter some longer.
Now you without thinking about that jump to loading. You take that Federal case or some other case and set up your seating die. Already every case that is different than the federal case trim length will be off slightly. Add to that the possibility of bullets being slightly longer or shorter then you start running into problems with COAL. Been there done that and found out what was causing the problem which I stated above. It works the same way for rifles as well.
I've found almost all cases differ slightly from one manufacturer to another.
Jump to 338 Lapua Magnum. Then all the above really matters.