Thread: Case trimming tolerances

How’s it going gentlemen

Was doing a bit of loading this afternoon, and have always wondered what sort of differences you’ll be getting out of performance with case trim lengths.

To my limited knowledge an OAL gauge is the way to go to get the best projectile seating which gives you what you ideal case trim length would be.

But for myself who loads to shoot more for fun what sort of trim length tolerances start to make a big difference ?

I’m pretty knew to loading so any tips would be good too.

Cheers


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just buy LEE trimmers..it makes life oh so much easier.

Here,

I just trim every load. It's consistant if nothing else.

Greetings,
In the US and using inches the trim length is .010" or 10 thousands of an inch a convenient round number. I have always worked in metric since the mid 1970's so have settled on a tolerance of 0.01mm which is about 0.004" and trim every load. An article in Handloader years ago ran a test on how much difference in trim length made and the answer was not much until the necks became too short. The lengths do have to be the same though. If you only have a couple of calibres then, as Mickey said, the Lee case trimmers will make things simple. If you need to trim much off the cases each time then you are probably sizing the cases too much and case life will be short and you need to go looking for the reason.
As far as advice goes hit and miss approach will usually give hit and miss results. Keep your cases in batches and load, trim, anneal or whatever the whole batch at once so the number of firings is the same for each case. Also keep a permanent record of all your loading additional to marking the boxes the rounds are stored in with the loads contained.
Regards Grandpamac.

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.

Originally Posted by zimmer

The main issue is don't let your lengths get too long.

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.

hmmmm I recal a certain Gent with an obscure wee 6.5mm that was having issues until he listened and gave necks a wee trim back too.
seriously the Lee trimmer is so simple to use,its hard to cock it up....I do know of a fella who got one and started trimming his .270w case and thought,gee the neck is getting short..then mate showed him to use the OTHER bit of the trimmer on the base of case
many years ago it was...young and keen I was LOL.