I guess I never even thought about it?
Most reloaders never do, but why don’t we? We have a post EVERY. SINGLE. DAY. about bullet jump, headspace tools and gauges, and COAL, and we have threads all the time about small base dies, or about bullet bases protruding below case necks and eating into powder capacity… AND we see threads about carbon rings, formation rates and round counts, how to clean them, what powders make them worse than others… but why do folks just accept a trim length in a reloading manual and never give it a second thought?
Taking a moment to think about it now:
I trim my brass back to spec
What is “spec”?
SAAMI “spec” for case length is NOT a reference dimension, and is typically defined as a maximum length and a negative tolerance. Chamber depth to the neck is a reference distance, with no tolerance listed.
Reloading manuals typically list the SAAMI max length as the “case length” and then list a trim length which is 10 thou there under… which SAAMI negative tolerance is typically 12 to 30 thou, which isn’t what we see in the reloading manuals - just 10 thou.
So what is spec? Do we KNOW our chamber is cut to exactly SAAMI reference depth? Do we think 10 thou is the right clearance? If we bump shoulders 2-3 thou and we seat bullets anywhere from 10 thou jam to 80 thousandths jump, do we really think “spec” for trim length is “eh, maybe 10 thou shorter than a number on a SAAMI drawing?
If we think 10 thou is a safe clearance which never wedges into the chamber neck and minimizes case mouth eddy deposition in the chamber neck, then why don’t we measure the chamber neck and trim brass to 10 thou under? Or if we never see case length growth more than 1-2thou per firing, why don’t we trim to ~5thou under chamber neck length? If fired brass is shorter than sized brass and we know it’s getting shorter in the rifle, could we just trim back 3 thou shorter than chamber neck length? If we set our headspace dim, and know that reference length between chamber shoulder and end of case neck, why can’t we make an intelligent and knowledgeable decision about where our case necks should be trimmed?
Or just trim to 10 thou shorter than SAAMI minimum because that’s what the book says…