New Barrel Throat and Seating Depth?

Discussion in 'Big Stuff -- 6.5mm, 7mm, 30 Cal' started by Papa Charlie, Oct 10, 2017.

  1. Papa Charlie

    Papa Charlie

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    I have having a new .308 barrel made at Benchmark Barrels. My first custom barrel and I must admit I am excited but also a little nervous about it as well. I shoot F-Class and my bullet of choice is the Berger 185 and 200.20X. When discussing this with Benchmark they indicated that they would use a SAAMI Match Grade ream with a 0.110" freebore. I have read here from many of you that you have your barrels done with a 0.170" freebore. When I brought that up to Benchmark they thought that was excessive and suggested I contact Berger, which I did. they also thought the 0.110" was tight for the 200.20X and suggested I load a dummy round for them to gauge the freebore by.

    Berger suggested I seat the bullet to just above where the bullet tail started to fall off from the bearing surface to just above the point where the shoulder starts from the case bearing surface. I did some measurements and come up with seated depth that produced a bearing surface of 0.250". I choose this because this allows me about 0.115" of free space between a powder load of 39.6gr of Varget and the base of the bullet. Using a Hornady comparator that makes my case to ogive length 2.264".

    So does this seem right. Should I have seated it deeper? According to the SAAMI case measurements the neck is 0.303" If I were to seat it 0.050" more it would also reduce the free space between powder and bullet. I am concerned about potential powder compression.

    So am I stressing too much?
    Should I have them go by the dummy I created, but not to exceed 0.170"?
    Should I seat the bullet that extra 0.050" as a reference?

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  2. Zero333

    Zero333 Gold $$ Contributor

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    0.170" freebore is what you want. Tho the 0.110" will work with 185 VLD & Hybrid.
     
  3. 308BAR

    308BAR

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    IMG_20171017_003426.JPG IMG_20171017_003426.JPG i just recently hand throated a 95 Palma with a .050" fb longer to about 0.110" on a 11t barrel. I compared the 185 (left 15thou jump with 0.050"fb) and 20x (right at lands on 0.110"). You may not get as much powder into the case as the .170" fb but in my measurement and mock up totally workable i might just have to go to a slightly lower load node and charge. Not to knock the the 20x or the .170fb but I've talked with many FTR competitors at the national and the world championship in August. Many could not get the 20x to shoot reliably. Compared to the straight 200 hybrid or 185s. Some rifle shoot it well other not so much. The original design of the 20x was to accommodate "standard" chambers. Also consider throat erosion and chasing the lands. Having shorter fb will allow you too seat the 20x more optimally for a longer time in your barrel. Versus hanging it out in the case mouth with a longer fb.
     
    Last edited: Oct 17, 2017 at 11:36 PM
  4. jsthntn247

    jsthntn247 Silver $$ Contributor

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    I've been saying 110-120 was about right for the 20x bullet. I couldn't get them to shoot in my 170 freebore chamber and I think it was because I only have .110 of shank in the neck with the 20x and 150 in the neck with the regular 200's.
     
  5. Papa Charlie

    Papa Charlie

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    Instead of just going with a .170 freebore, I loaded a dummy round with the 200.20X with 0.250 bearing surface in the neck. Benchmark will throat the barrel to allow approximately 0.015 jump to lands, as best as they can.
    Not knowing what that would result in I also limited them to 0.170 not to exceed.
    Will let you know what that resulted in.
    My go to bullet is the Berger 185 Jug. Have heard mixed reviews on the 200.20X. looking forward to seeing what the barrel likes.
    With the way I loaded the bullets I have 0.114 to add powder from a 40.0 gr load of Varget. So I think I have some working room.
     

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