Amount of freebore in a 300 RUM

Discussion in 'Gunsmithing' started by Gary Morgan, Jul 13, 2010.

  1. Gary Morgan

    Gary Morgan Well-Known Member

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    May 27, 2007
    Guys my gunsmith is about to chamber my new fluted Krieger barrel in 300 RUM. I want to shoot the 180 grain Accubonds in it with the boattail/bearing surface junction seated to the neck/shoulder junction. He uses a throating reamer to get the throat right where we need it. The problem is that we are unsure if the 300 RUM should be seated in the lands, just touching, several thousandths off or way off the lands. Can someone with some experience with this cartridge give us a good rule of thumb?
     
  2. J E Custom

    J E Custom Well-Known Member

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    All of the "NEW" Magnums have lots of freebore to reduce the pressure to a safe level and
    still get higher velocities. If you run the bullets close or touching the lands pressure will go
    up and loads will have to be reduced loosing some of the velocity that was the attraction
    in the first place.

    I have known many instances where a big magnum was chambered with a short throat and
    heavy bolt lift was encountered and velocity was lost in order to bring the pressures down.

    Example; If you short throat a 30/378 it is unsafe to shoot Weatherby factory ammo in it
    (You will blow the primers almost every time).

    Accuracy can be achieved without short throating and seating close/against the lands buy
    Holding the head space to a minimum (.0005 to .001) and still get good velocities with
    SAAMI pressures.

    I do not recomend short throating "ANY" of the High pressure cartriges because it creates a
    new set of problems and is not nessary to acheave outstanding accuracy.

    On smaller less potent cartriges this is an excepted practice and can work in your favor
    but not on the Big/New magnums.

    Just my opinion.

    J E CUSTOM
     

  3. MTBULLET

    MTBULLET Well-Known Member

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    Jul 16, 2007
    sorry, no "easy" answer to this question. I have had several different 300 RUM, (still use one I built with PAC NOR bbl.) and ALL are different as to what they like, but none of mine had other than SAMI throat. The 300 RUM will errode (ie: wear) the throat rapidly anyway.
     
  4. Gary Morgan

    Gary Morgan Well-Known Member

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    thanks, very helpful.