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Thread: Coned bolt nose to barrel clearance

  1. #1
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    Coned bolt nose to barrel clearance

    Okay I am by no means a machinist. I pay to have my barrels chambered. I was in a conversation and it came up about that clearance.

    I run BAT actions. From what I heard today, if I heard right 5-7 thou the typical?

    We know the less the clearance the potential of the nose rubbing the barrel. To much and un
    suppported case head. I guess that
    Could be relative to the headspace?

    The main question is, has anyone ever noticed any difference in rifle performance between say having 7thou clearance versus say 2 thou or 10 thou?

    I hope this makes sense what I am asking?

    Calvin

  2. #2
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    not in a hunting plinking rifle.
    competition rifles are in the 5-7

  3. #3
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    The two reasons you stated are the only considerations concerning coned bolt to barrel.clearance.

    Too little little clearance, there is the possibility of the bolt nose contacting the barrel, or getting bound up by some small piece of debris.

    Too much, you uncover the web of the case and risk a ruptured case head. I have personally seen this on a6PPC.

    If you fool with enough actions, you will notice that because they vary in the amount of bolt face-recess, some can get away with more clearance without uncovering the web. Early Farleys had a particularly deep recess, so about .008 was max. The Stiller Drop Ports, while not a coned bolt, were EXTREMELY critic in that area of nose to barrel clearance. In truth, you only had a few thousandths to play with with the 6PPC case.

    As to the accuracy question, as long as the bolt does not get interfered with, the actual number is a moot point......until you blow the head of of a case.

    As to the headspace reference, you can have perfect headspace and still have too much or too little bolt to barrel clearance as they are too separate measurements.
    Last edited by jackie schmidt; 04-29-2021 at 07:11 AM.

  4. #4
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    clearance

    If you go to the BAT website, they provide prints with the info you seek

  5. #5
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    Keith

    Quote Originally Posted by KeithBB View Post
    If you go to the BAT website, they provide prints with the info you seek
    We are aware of the recommendation for clearance, we are more interested in my last question re rifle performance/accuracy.

    Calvin

  6. #6
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    Quote Originally Posted by CYanchycki View Post
    We are aware of the recommendation for clearance, we are more interested in my last question re rifle performance/accuracy.

    Calvin
    Calvin, as I said. As long as it has clearance, and not so much as the web of the case is compromised, it has no affect on accuracy.

  7. #7
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    Thanks

    Jackie.

    Appreciate your input.... nice to here from one willing to share and not hold it dear to their heart.

    Calvin

  8. #8
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    Calvin,
    I have had one instance were the bolt clearance was to tight but the bolt still closed fine. After about 100 rds and nothing working for groups I began inspecting and everything looked good until I noticed a faint imprint of the bolt nose in the closed position on the barrel. Sent it back and had a few thou more relief put on it and it started to behave better. I do not remember what the measurement was but I thought it was in the .002- .003 range.
    John

  9. #9
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    There is a wealth of information here and Jackie is one who has shared allot. This also reminds me of cutting my barrel threads to screw on with somewhat of a real snug hand fit. Until Jackie as well as others here have shared that it might not actually be the best or even the right approach.
    Last edited by Louis.J; 04-30-2021 at 01:09 PM.

  10. #10
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    Excessive clearance

    Here is the aftermath of way to much 4198 and excessive bolt nose clearance.
    Joe
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  11. #11
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    I err on the side of a little more clearance rather than less. As much as we like to think all custom actions are perfect and in alignment but some aren't. So I want a little wiggle room and some space for debris. Everything has a natural resting place. I've learned if you try and manipulate location with tight tolerances you introduce variables that are difficult to diagnosis.
    This is more important on factory actions. I fit the counterbores on Remington's to .730" with .010" end play

  12. #12
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    A pal of mine had a new barrel on his Kodiak actioned HBR rig that was tossing shots. Not much...about a bullet's worth. At the end of the first days shooting, he removed the f-pin assy and was double checking his seating depth. As he opened the stripped bolt, there was a barely audible 'click'. I put some black Magic Marker on the extractor front edge and you could clearly see where the forward edge of the extractor was rubbing on the barrel coned area.

    With another .005 off the coned area, it went from tossing a shot to shooting dots. -Al

  13. #13
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    John Pierce had me “glue” a spot of lead on the bolt cone with a spot of grease, close the bolt of it, and when it measured .004 he said perfect.

  14. #14
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    Here is a novice input/question

    One layer of blue tape is go, two layers is no go. Right or wrong?

  15. #15
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    Along with clearance and unsupported case, another aspect of this clearance is that it meters both gas and debris in the event of a ruptured case, that can escape the gap. Add enough pressure and this too can certainly be defeated but I read this years ago in regard to the Remington 3 rings of steel. I feel it holds mostly holds true on any action design. The biggest area for error is the angle. If I'm off a tad, I'd rather err toward the bolt clearance being less, closer to the bolt nose.(inner)
    Last edited by mwezell; 05-01-2021 at 06:22 PM.

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