Deliberately throating a match barrel long

Discussion in 'Gunsmithing' started by Tumbleweed, Jan 30, 2013.

  1. Tumbleweed

    Tumbleweed Well-Known Member

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    Hi guys. I will be re-barreling my 300Rum with a 31+" Broughten 5C 12.4 heavy palma 9 twist in a couple of months as well as blueprinting the receiver and bolt. Found my smith for the job, he seems more than qualified. I will be setting this up to run the 230 Hybrid Targets. Here is my question:

    I want some feedback on the 'idea' of deliberately throating the chamber .25-.5" longer than a standard chamber with a match reamer that would allow say .0005-.001" bullet side clearance in the throat portion. I'm sure it would lend itself to very nice velocity above a standard chambering but my question and concern is would I be giving up any accuracy even with the very tight throat? With .0005-.001" throat clearance the bullet should still start out lined up with the rifling straight right? Would the heavy hybrid put up with this kind of jump without giving up any accuracy? I do have a Wyatt's box it my 700 now and it measure right around 3.860". I will probably keep it magazine feedable, so yes that's a lot of jump. I am going all out on this build looking for max accuracy, if I would need to sacrifice accuracy in any way I will not do it. Does anyone have experience with setting up a chamber this way and what kind results did you get? (Accuracy, velocity, ES, pressure, ect)

    P.S. I know a guy that built a 7STW with 1" of freebore along with a special i.d. bore and claims to have been able to run 3600fps with a 175 grain gameking with .5MOA accuracy.
     
  2. J E Custom

    J E Custom Well-Known Member

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    Freebore is not a bad thing like some think If the correct chamber and head space are used.

    However , there is a point that you reach of diminishing returns. you will lose accuracy at the
    cost of velocity.

    Freebore should be bullet diameter so the bullet stays in alignment with the lands as it engraves
    before it completly leaves the case mouth never allowing the bullet to be uncontrolled.

    The length of the freebore however, is a source of major controversy and will never be totally
    agreed on. the one thing that can be agreed on about Freebore is that not enough can cause
    pressure early (Less powder) and lots of freebore can and does increase velocity.

    All of the "NEW" high pressure/velocity cartriges have generous amounts of freebore.

    How much is to much is still up for debate, But allowing the bullet to be uncontrolled at starting
    velocities and durring the engraving process would be to much In my opinion.

    Weatherbys are known for there velocities with factory ammo and on the larger cartriges they
    have any where from .300 to .750 freebore. and if you re-chamber with less freebore you will
    not be able to use factory ammo in the Weatherby chambering because of pressure.

    The people that design cartriges knew what they doing and were trying to get the most of there
    cartrige safely and redesigning a cartrige should be intered into with great caution.

    J E CUSTOM
     

  3. Edd

    Edd Well-Known Member

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    What do you mean when you say "allowing bullet to be uncontrolled"?
     
  4. J E Custom

    J E Custom Well-Known Member

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    If the bullet can leave the neck before it reaches the lands it is not controled by the case or the rifling
    and may engrave poorly'

    Ideally the body of the bullet is long enough to engage the rifling while it is still in the neck.

    One of the reasons that secant ogive bullets can give you problems if not properly set up is
    that they typically have shorter bodies and if they are seated out to far this is one of the problems.

    Look at the bullet you intend to use and then do your calculations for throat dimensions
    based on mag length. bullet engagement desired and cartrige overall length and bullet jump
    to lands.

    J E CUSTOM
     
  5. Tumbleweed

    Tumbleweed Well-Known Member

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    Makes total sense to me. So I would really want a zero clearance fit between the bullet and the so called "free bore" section in front of the rifling. So with the bullet needing to travel through say .5" of this zero tolerance free bore zone before it starts engaging the rifling it should be perfectly centered up to enter the rifling straight right? What could be induced in a match chamber set up this way that would cause a loss of accuracy in and of itself? Sorry if I seem difficult, I just want to get the best answers I can so I can make a good decision. I really don't want to expiriment on this if someone else has done it and made it work with no loss of accuracy.
     
  6. Edd

    Edd Well-Known Member

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    I don't understand this part.

    Why does the bullet need to engage the rifling before it clears the neck?
     
  7. MontanaRifleman

    MontanaRifleman Well-Known Member

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    Because the throat/freebore area is a larger diameter than the bullet. If the bullet leaves the neck before engaging the lands, it is essentially floating uncontrolled and might "wobble", entering the lands slightly cocked.
     
  8. Edd

    Edd Well-Known Member

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    The case has more room to wobble in the chamber than the bullet has to wobble in the freebore.
     
  9. MontanaRifleman

    MontanaRifleman Well-Known Member

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    The bearing surface of the 230 Hybrid is .495" When the bullet is fired, the ogive will engage the lands just forward of the bearing surface, maybe .050 give or take. If you make your freebore longer than that, you might loose accuracy.

    I just ordered and 300 RUM reamer and I am going with a .200" throat which was designed for the CE 200 gr Max Agg bullet. I also have an additional .018" from my trimmed necks to the end of the neck of my chamber for a total of .218" of actual freebore It will also be good for the 230 Hybrid as I will be seat that bullet so the the junction of the bearing surface and boattail are even with the junction of the neck and shoulder of the brass leaving some jump to the lands.
     
  10. MontanaRifleman

    MontanaRifleman Well-Known Member

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    That depends on chamber and throat specs. It also depends on how and to what degree the brass was sized. if there is a misalignment of the brass in the chamber, it would not likely exceed .001, especially if it were a match chamber.
     
  11. Edd

    Edd Well-Known Member

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    There is always the possibility of exceptions but that isn't what is being discussed. A typical freebore spec is .0005". A typical chamber to case clearance is at 6-8 times that.
     
  12. MontanaRifleman

    MontanaRifleman Well-Known Member

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    Typical freebore clearance from what I've seen on prints is .001 Match is .0005

    My Chamber clearance will be .002 all around for .004 total after sizing.
     
  13. Edd

    Edd Well-Known Member

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    Some of them are .001" , a 300 RUM happens to be one of them, but many of them are closer .005". Most of the Weatherby's are .004", including the 300 and 30-378.

    Even in your example the chamber clearance is 4 times as much as the freebore.
     
  14. MontanaRifleman

    MontanaRifleman Well-Known Member

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    My reamer is a match and the throat clearance is .0005, so it is actually 8 times. When the firing pin strikes the primer, it drives the case forward which then centers it up as the shoulder comes in contact with the chamber shoulder.

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