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Reply with quote  #1 
I am acquiring reloading equipment and have a fundamental question on case trimmers. Some trim to length based off the shoulder or datum line while others are based on overall length. Which is preferred and why? I think the Giraud is shoulder based and the Wilson Ultimate is overall. I am new to this, so please use lay terms.

Thank you.

- Phil
Reply with quote  #2 
I'm not sure I can answer all of your questions, but I can tell you what I do and why.
I use a Hornady cam-lock case trimmer that trims based on OAL.
The first time I shoot new brass I check trim length to make sure it's safe, but unless its excessive I don't trim it. After it is fired it is all formed to my chamber, so the shoulder is at the same distance from the head. I trim every time I shoot it from here on, but I almost always neck size so I never really change the body dimensions. After many firings I do have to full length size, but even then all the brass should be bumped down to the same size. What I'm trying to say is I don't really think it matters if you trim from OAL or shoulder. As long as you do it the same to all the brass the results should all be the same.
All the measurements in the reloading manuals that I have seen as based on OAL.
Reply with quote  #3 
I never heard of "trim to length based off the shoulder or datum line". The datum line is halfway up the shoulder, approximately, on most bottle type cases, and is used to measuire headspace, not trim length. Total trim length is simply measured from bottom of the case to the mouth, or OAL case length. After first firing try to stay about .010" below the factory or SAMMI length, which you can find in any reloading manual.
Reply with quote  #4 
The distance between the case head and shoulder datum is fixed, so it really doesn't matter. Just make sure you trim as needed so that the case dos not pinch in the throat and it's a clean job.
Reply with quote  #5 
Originally Posted by tenring
I never heard of "trim to length based off the shoulder or datum line".

I had understood the Giraud power trimmer and Possum Hollow tools use the datum line. Given that with both, you just hold the case and insert it into the tool, something has to stop it from being inserted too far. Perhaps I am not understanding this correctly.

- Phil
Reply with quote  #6 
Trimmers that work off the shoulder don't really use the datum line. They simply trim to whatever point on the shoulder that happens to make contact. It doesn't really matter since uniformity is what you're after.

Trimmers that work off of the shoulder are so much easier to use. No base, no handle to crank, you don't even need a caliper once they are set. I make one for every cartridge that I shoot.

Reply with quote  #7 
The two best trimmers that I know of (Gracey and Giraud) both index off the shoulder. Must be a reason for it.
Reply with quote  #8 
Phil,You need to know your chamber length.Or buy a insert from sinclair(in catalog)that goes inside your case and then you chamber it in your rifle and it seats itself in the case and then you measure it and then you trim your brass .005-.010 less and then after 2-3 firings you check and see if they grow and then trim again,it may be every 5-10 firings before you need to trim.
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