What's the ideal trimmed length of a cartridge, relative to the actual chamber length?

Discussion in 'Reloading' started by kai, May 19, 2019.


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  1. kai

    kai Well-Known Member

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    I bought several Sinclair Chamber Length Gauges to measure my various rifle chambers.

    https://www.brownells.com/reloading...s/sinclair-chamber-length-gage-prod32925.aspx

    The Brownells website says: "The differences between published trim lengths and the actual length of your chamber can be quite significant. You could be over trimming your cases by .030 inch or more.

    I discovered that my rifle's chambers are roughly .030" to .040" longer than the SAMMI (trim to) length."

    What is the ideal trimmed length of the case relative to the actual chamber length?
     
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  2. tbrice23

    tbrice23 Well-Known Member

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    I measure all my chambers. I've seen factory chambers. 050" over SAAMI max. The shortest by far is WSM chambers, .015 to .020" of max SAAMI.

    I make a note of the max chamber length for each new barrel. I keep them all trimmed to the same length but will continue to let them grow up to .015" of my actual max chamber length.
     
  3. tbrice23

    tbrice23 Well-Known Member

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    BTW if you own a borescope make sure you keep carbon ring accumulation under control if you grow your necks long.
     
  4. MagnumManiac

    MagnumManiac Well-Known Member

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    I measure my chambers and trim to .015" below that number.
    The only time I trim to SAAMI is when a chamber is short.
    Most reamers, whether SAAMI spec or not, have .030" - .040" clearance cut into them as a safety feature. I have a 25-06 chamber that is barely .020" over minimum SAAMI cartridge length, that is .010" over trim-to length. This caused me some problems prior to measuring the chamber.
    I would say .010" clearance is absolute minimum and .015" is absolute maximum for me.

    Cheers.
     
  5. Mikecr

    Mikecr Well-Known Member

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    I don't trim until cases get within 5thou(my preferred) of chamber end. I don't like it while clearance is beyond 10thou. This, because it contributes to back flow of gasses, delaying neck sealing (as seen by neck sooting) and building of carbon rings.
    Both of these can affect ES in hunting capacity cartridges -more so than normal variance in neck lengths. So where you find that your case mouths are already >10thou do not trim them yet, unless you just know for sure they're never gonna get anywhere near close.
    Fire forming will likely pull necks back further, while FL sizing will likely cause cases to lengthen, eventually, possibly, reaching goal end clearance for you to set.

    With tight neck clearance and tight end clearance there is no carbon ring formation, and no carbon sooting on necks. This is achieved with a planned setting of chamber parameters for it.
    For my tight chambers and relatively modern/improved cartridges, I size so little that cases don't grow and no trimming is needed beyond my plan (once, after 3rd fire forming).
     
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  6. kai

    kai Well-Known Member

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    Thanks for your comments gents.
    I do have a bore scope and will check for carbon and I'll also let my neck lengths grow longer before trimming.
     
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