Reamer design for 338 Norma Mag for 250gr Berger

Discussion in 'Gunsmithing' started by Machann, Mar 5, 2013.

  1. Machann

    Machann Active Member

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    Hi guys

    Can some perhaps assist me with a reamer design for 338 Norma Mag for 250gr Berger? I have no luck getting hold of the guys at PTG or Manson to assist me.

    Regards
     
  2. J E Custom

    J E Custom Well-Known Member

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    Why not just buy a SAMME reamer and adjust the round for the chamber instead of building
    the chamber around a Bullet.

    If the barrel does not like the Berger your stuck. with a standard chamber that has been tested
    by the designer you have the best chance to have an accurate rifle with several different bullets.

    I prefer the extended magazines that allow a round to be seated long instead of having to single
    load them.

    Just an opinion,

    J E CUSTOM
     

  3. Machann

    Machann Active Member

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    Thanks JEC!

    Please help.

    The reason why I'm asking is because according to me the difference in length between the 300gr and 250gr Berger is .22 which wil require me to seat the 250 gr bullet outside the optimum range in the case or have a massive jump? If you consider the Lapua and Sierra and Berger they are all approximately the same length and result in the same COAL. My assumption is that you will still be in range for the the other 250gr bullets? Please, I'm no expert and some expert help will really be appreciated

    Thanks
     
  4. J E Custom

    J E Custom Well-Known Member

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    I don,t consider my self an Expert ether, just with 50 + years experienced and know that Free bore is not the enemy, chamber dimensions are. I don't have a SAMME spec. for the new 338 Norma mag but if memory serves me you should have about .200 thousandths free bore.

    The 250 grain bullet has a long enough body to reach the lands and still have plenty of bullet
    engagement in the neck the 300s will have to be seated deeper if you shorten the free bore.

    I don,t want to talk you out of a custom reamer, just warn you about the problems that come with it.

    For velocity, free bore is your friend, for accuracy with free bore the chamber, especially the head space
    is also your friend.

    You don,t have to touch the lands to get accuracy.

    J E CUSTOM
     
  5. shortgrass

    shortgrass Well-Known Member

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    +1 at that!
     
  6. sp6x6

    sp6x6 Well-Known Member

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    why not the 300 gr. for which it was designed.And will out fly it and out energy it at long range
     
  7. Machann

    Machann Active Member

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    JEC I would consider +50 years of experience a expert:).

    The amount of experience you guys are blessed with over there is huge compared to what we have over and thanks for sharing. We don't have the powder over here to push the 300gr bullets fast enough, will be able to get 2650fps max out of the 300gr vs. 2950fps on the 250gr. That is a 7MOA drop difference at 1500m.

    I’m looking to build this rifle as accurately as possible. An old bench rest legend over here designs all his reamers so the bullet kisses the lands with the base of bullet bearing surface between 0.02-0.07 above the intersection of the case neck and shoulder. Thus my thinking behind the reamer design. If you seat the Berger 250gr correctly you have close to a +0.22 jump in a chamber designed for the 300gr Berger kissing the lands. Is this too much jump to get optimum accuracy out of the rifle?


     
  8. J E Custom

    J E Custom Well-Known Member

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    There are some that think that the bullet has to touch the lands to be accurate and some that don,t.

    The logic behind touching the lands is sound for small cartridges like the 6mm PPC (Or most bench
    rest cartridges) very few bench rest cartridges use over 50 grains of powder because they are for shorter ranges. Only the long range (1000 yard Bench rest use more) and these rifles have the same requirements as Long Range hunting has (Velocity, Ballistics and accuracy).

    This type of shooting has a different set of rules than short range bench rest. Short range bench rest
    bullets are normally a Flat base design, cases are loaded down for best case life and accuracy, Barrels
    are normally short and heavy because velocity is not an issue. All of this is just the opposite for a long
    range rifle (Long barrels, boat tail bullets with high BCs,heavy loads of powder for velocity, And yes,
    Free bore to achieve high velocity without excessive pressure).

    Years ago bench rest shooters had to try many rifle/ barrel and chamber combinations to get a winner.

    Many of these guys became barrel makers because of poor quality barrels and chamber reamers were
    not as good and most had small home type Lathes to work with., So in order to have a consistent load/
    chamber combination, seating the bullet against the lands helped to center the round.

    Now with good barrels,reamers,equipment and a good Smith this is not as important as it was in the
    past.

    I felt it necessary to bore you with the details of the different needs and my reasoning because of
    the different requirements of long range hunting.

    Don,t get me wrong, there is nothing wrong with seating the bullet against the lands except the velocity
    and pressure issues and the ability to use many different types of bullets.

    I have 4 personal rifles that I am happy with the accuracy (5 shot groups of .034, .053, .077. and
    .092 and they all have free bore over .185 to .756 they can handle max loads with no heavy bolt lift
    (High pressure) and the trick is a perfectly concentric chamber with little or no head space,good
    load quality, great barrels and a little luck.

    I hope this has not talked you out of anything, I just wanter to give you my reasons.

    J E CUSTOM
     
    Last edited: Mar 6, 2013
  9. Machann

    Machann Active Member

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    Please JEC, you are not boring me with anything and thanks for sharing!

    Suppose the best place to go have a look is the 1000yrd bench rest and how those guys do it and thanks for pointing out the differences. Guess what bench rest discipline our legend shoots.

    Thanks for helping JEC. Got a 338 Norma Mag reamer print on. http://http://www.elr-resources.com/Jimmie%20Sloan%20338%20Norma%20Mag%20USA%20(3.602%20OAL)%20Fin%2032448.pdf
    From my calcs it looks like I'll have a 2.6mm jump if I use the 250gr bullets. Doesn't sound to bad given the advice provided.

    Regards
     
  10. sp6x6

    sp6x6 Well-Known Member

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    A friend of mine shoots 1000 bench, Penn rules.Actually a few of them.They shoot on or in the lands.The one guy has 6-8 world records,his mentor.that I used to shoot archery w/has several also.That is w/6mm dasher.They have also shot 300wsm,their reamer design for comp.The one guy just built a heavy 300 wsm w/promise.Their load prep. and method would take a book to describe .
     
  11. sp6x6

    sp6x6 Well-Known Member

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    I got a box of 250 elite hunters by mistake, so Im going to shoot a few. On my comparator the 300 is 1.970 and 250 is 1.860, difference of .11 , on ogive not as much as it lookes when side by side