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Author Topic: Trim-to Length 308  (Read 4941 times)

Offline MVW

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Trim-to Length 308
« on: 09:28 PM, 03/23/12 »
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  • I notice in the Lyman's 48th edition that it gives the SAAMI max length of 2.015 for 308 WIN, but recommends a trim-to length of 2.005.  A number of folks have said that 2.005 is too short.

    I am wonder how folks manage case length, so that it doesn't exceed the SAAMI max.

    Thanks,

    Mike



    Offline GNERGY

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    Trim-to Length 308
    « Reply #1 on: 09:41 PM, 03/23/12 »
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  • I neck size mine and only so it gives me .003 neck tension. I took the neck sizer die and honed the inside of the neck area to give the desired tension and then made the expander smaller so it doesn't touch the neck when I withdraw the case.
     I know they make bushing dies but I am a do it yourselfer ( cheap )
    Tarey

    Offline Charlie Watson

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    Trim-to Length 308
    « Reply #2 on: 09:46 PM, 03/23/12 »
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  • I trim all of my .308 brass to 2.005 and have excellent accuracy. Whatever you trim within spec should be good to go if you shoot all the same case length.
    US Army Vietnam 1969-70
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    Offline DennisH

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    Trim-to Length 308
    « Reply #3 on: 12:46 AM, 03/24/12 »
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  • Out of 150 2nd time fired cases, I take random measurements.  Whatever the shortest one is will be my base for trimming!

    I use to trim all @ 2.005, it is the recommended trim length per Lyman's.

    Lapua states:  51.00 mm  or 2.008" as per their reloading guide!

    http://www.vihtavuori-lapua.com/pdfs/Rifle-Reloading-Data-2006.pdf

    I have several 308's, the brass stays with the gun it's shot out of until the brass is no longer useable.  I don't swap brass between guns.
    Dennis Haffner
    Pro Team, McGowen
    www.mcgowenoutlet.com

    Offline Steve Blair

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    Trim-to Length 308
    « Reply #4 on: 01:25 AM, 03/24/12 »
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  • I measure my chambers with these handy Sinclair Chamber Length Gauges and trim to .010" less, when needed.
    http://www.sinclairintl.com/.aspx/pid=32925/Product/Sinclair-Chamber-Length-Gage

    Most chambers, whether factory or custom, are well over SAAMI maximum. Here is a sample from some of my .308 rifles.


    Pierce                   2.034
    Borden                  2.035
    Savage McGowen  2.038
    Savage Shilen       2.039
    Steyr SSG69          2.040



    Sleepygator

    Offline NorCalMikie

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    Trim-to Length 308
    « Reply #5 on: 07:43 AM, 03/24/12 »
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  • Like Steve said, get the proper gauges, measure your chamber length so you'll know when and if you need to trim your brass.
    All chambers are a little different.
    Another example: 6BR Max trim length, 1.560. Measured "actual" chamber length with the Sinclair gauges is 1.585. That way I know how close the brass is and if I need to trim. ;)
    OZ never gave nothing to the Tin Man, that he didn't already have.


    Offline Charlie Watson

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    « Reply #6 on: 07:58 AM, 03/24/12 »
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  • There has been a lot of discussion about longer than spec brass, but can anyone really show any improvement in accuracy with this process?

      I don't buy off on the throat erosion argument as your throat is going to erode no matter the length of your case. Worse case.... You may have your chamber measurements wrong and if your case contacts the rifling before the bullet is entirely disengaged, you'll have more to worry about than the length of your case.

    How about it..... Got any accuracy test data?
    US Army Vietnam 1969-70
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    Offline DennisH

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    Trim-to Length 308
    « Reply #7 on: 09:48 AM, 03/24/12 »
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    I measure my chambers with these handy Sinclair Chamber Length Gauges and trim to .010" less, when needed.

    This is one use  for the Chamber Length Guage, what other measuements can this guage be usefull for?  Can't you determine "freebore" with this guage as well?

    sleepygator:  why not just trim .003 or so in this area instead of trimming .010?
    .
    Just asking and learning,

    Dennis
    Dennis Haffner
    Pro Team, McGowen
    www.mcgowenoutlet.com

    Offline jsn

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    Trim-to Length 308
    « Reply #8 on: 10:23 AM, 03/24/12 »
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  • Charlie--couldn't say about the accuracy myself.  The throat erosion won't be altered, and the chamber erosion (22 short fired in a 22 lr type) probably wouldn't occur because the barrel life wouldn't be that long anyway.  You are correct about the possiblity of of a case neck pinch going over pressure--I only go long on fire formed cases , the headspace must be correct.  Bumping shoulders or full length resizing must be done precisely.  All measurements are known and adhered to.

    Dennis--the diameter of the gauge will only allow measurement to the end of the case neck as reamed--it's too wide to measure freebore.   Most chamber reamers are documented as to dimensions today.  If a factory or unknown build barrel, a chamber cast would get you the freebore info you want in most cases.  Sometimes a chamber cast is used to document throat erosion over time.  Borescope would show you a lot also if you had access to one.

    Here's a recent forum where this was discussed, including a carbon ring buildup which could cause the same effects as a too long trim length:

    http://forum.accurateshooter.com/index.php?topic=3783774.msg36053116#msg36053116

    Offline fdshuster

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    Trim-to Length 308
    « Reply #9 on: 10:23 AM, 03/24/12 »
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  • Dennis: No, chamber over-all-length has nothing to do with freebore.

    Chamber O.A.L. is the actual length of the chamber, and that dimension as cut with the chambering reamer will determine the maximum case length that will fit in the chamber. There are no SAAMI specs for chambers: the length's can be whatever the reamer maker decides to make it. That's the reason you find some chamber lengths that are approximately .010" longer than the listed max. case length, ( 1.760" for the 223 for example), or as much as .040" longer than max. case length.

    The Sinclair chamber case gauge, as posted by sleepygator, cannot provide any information about freebore since it does not extend into the leade area of the throat.

    Charlie Watson: Throat erosion has nothing to do with chamber length. Erosion develops "downrange" from the location of the case mouth.

    My concerns about chamber length are not directed at any improvements in accuracy, but do concern the possible development of a ring of carbon that can/will buildup at the case mouth location in the chamber. Keep that "gap" to a minimum, and I work with .005" on my match chambers & there is no space available to fill up with a ring of carbon. Create a long "gap" of .020" or greater & a ring of carbon will form. I've watched it happen step-by-step with my Hawkeye borescope.

    When it gets out of control you can expect pressures to increase & the results are not pretty. Ask me how I know, after wiping out a match trigger with debris from blown primers, when I was not paying attention & a ring of carbon developed on one of my factory rifles ( Rem 700, 222) that came from the factory with a chamber length of  1.745" for a max case length of 1.700".

    Tarey: We're talking here about chamber length, not diameter, as it relates to neck bushings. ;)
    « Last Edit: 10:33 AM, 03/24/12 by fdshuster »
    Civilian Marksmanship Program/ Distinguished Pistol, 1982

    Offline Steve Blair

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    Trim-to Length 308
    « Reply #10 on: 10:59 AM, 03/24/12 »
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  • How about it..... Got any accuracy test data?


    Frank has it right about carbon rings and it is the primary reason that I only trim when it is within .005 of actual chamber length. Once my cases "grow", they are typically between .005 and .010 of actual chamber length. There is no accuracy advantage of which I am aware except for avoiding carbon ring overpressure. My brass preparation is already lengthy and, at times, tedious. If trimming only when needed will reduce that with no penalty, I'm in. On the flip side, there is no accuracy advantage that I know to trimming well under chamber length.
    Sleepygator

    Offline GNERGY

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    Trim-to Length 308
    « Reply #11 on: 06:08 PM, 03/24/12 »
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  • fdshuster,
    I guess I read it the wrong way. If you full length size your brass and you know how it sizes the neck smaller than it needs to be, then pulls it over the expander ball to give it a certain ID. That is where you get most of your case stretching. I got away from that by neck sizing to just about eliminate trimming of my 308's
    I thought he was asking how you keep your brass from stretching.
    I'll be quiet and go back to my corner.  :-[
    Tarey

    Offline fdshuster

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    Trim-to Length 308
    « Reply #12 on: 06:27 PM, 03/24/12 »
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  • Tarey: Sorry if I came on too strong. Come on out of the "corner" & I'll take your place. :D
    Civilian Marksmanship Program/ Distinguished Pistol, 1982


     

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