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Very true, one is supposed to/has to trim/check bottleneck cases with each firing/loading..For my AR, I trim EVERY case to length, and crimp with a separate Lee crimp die. To crimp or not is an individual decision...but it you do crimp, I find a separate die easier to set. Just my opinion.
Actually, if you are using the Lee factory crimp die, the case length is not as critical as you might think. Now if you are using standard dies which result in a roll crimp, then uniform case length is critical, as with magnum revolver cartridges which need a strong roll crimp to make the best use of the relatively slow burning powder for maximum velocity performance. I use the Lee factory crimp die on my 9mm and .223 ammo and it works well, even if the cases aren't uniformly the exact same length. FWIW, dies for most auto pistol cartridges come with a taper crimp die instead of a roll crimp, at least I know that RCBS and Lee do.Case length, seating depth and crimp are critical in the .40 S&W cartridge. Even more so than the .223, I would say.