Thread: Case trimming tolerances

The main issue is don't let your lengths get too long. The published lengths do have a safety margin though. I have either measured or know from the reamer used drawings what my true trim too lengths are and they are all longer than the table lengths. However, the table lengths safely accommodate a "one size fits all" philosophy so unless you know what you are doing stick to the table lengths.

What causes lengthening? True lengthening is caused by brass flow forward and will vary with cartridge type, load used. My Ackley for example needs virtually no trimming as minimising brass flow is inherent in the Ackley design.

I have seen the effect of letting cases "grow" too long. We had a guy in our fullbore club popping primers at the range one day and saying "what's causing this?" Someone had a vernier and measured his brass and it was massively over length for a 308. What was happening was the tip of the case neck was so far forward it was jamming where the parallel section reduces, and this was restricting the ability of the case to expand and release the projectile on firing. Hence extremely high pressure peaks and blown primers. He just never knew about good reloading practices.