cone faced bolt

Discussion in 'Gunsmithing' started by trueblue, Feb 17, 2009.

  1. trueblue

    trueblue Well-Known Member

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    What are the advantages of a cone faced bolt.I am not a machinist, but it would seem to be harder to cut a 65 degree recess ( for example) in the breech of the barrel than to just take a .705 counter bore to cut the recess for the bolt face.
     
  2. NesikaChad

    NesikaChad Well-Known Member

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    A cone breech facilitates chambering as it's a uniform feature that acts as a funnel for the cartridge as it enters.

    Machining the feature is pretty simple. Just set the compound on the carriage to the angle and begin making passes until it's at the right depth to ensure the bolt/breech clearance is right.

    Some do lots of kinky math to determine this, being a lazy person I found a way that involves less work. I got some lead wire and I stick a piece to the bolt on the forward lug surface. As I rotate the bolt into battery the wire is smooshed between the lug and the breech.

    Measure the wire with a pair of calipers and I know without a doubt what my clearance is.
     

  3. trueblue

    trueblue Well-Known Member

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    Thanks for the responce, Nesika.
    So, basically the cone acts as a feed ramp. As far as determining clearance, would that have not already been determined when the tenon length was measured and cut with about 0.015 clearance to the lug face ? If not, what amount of clearance are you wanting in this area from the bolt face to the barrel cone face ? Thanks for the help. Just trying to learn, to gain a better understanding of the process of putting rifles together.
     
  4. NesikaChad

    NesikaChad Well-Known Member

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    Sure. but that is assuming you have a barrel that was properly done to start with. What if a customer mails you an action only?

    Measuring the bolt is always the trick and you kinda have to measure each one individually because no two are exactly alike. I can stick a depth mic on the receiver ring with the bolt installed and take all sorts of dimensions to generate a drawing all the while hoping I didn't goof something up.

    Or I can just thread the tennon a smidge long, cut the cone, and take a couple passes to creep up on the desired clearance. If I'm a touch short, push the shoulder forward. If the bolt is too tight, just take another whack off the back of the barrel.

    Fast, easy, lazy, and 100% accurate with nothing more exotic than a pair of 6" dial calipers.
     
  5. trueblue

    trueblue Well-Known Member

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    Nesika Chad,
    So when you close the bolt on the lead wire, what thickness for clearance are you trying to end up with ?
     
  6. NesikaChad

    NesikaChad Well-Known Member

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    Application kind of dictates. (my rules)

    Pretty little bench guns that shoot inside a vacuum and never see a spec of lint get set up at .005".

    I used to run them much looser but a customer fussed at me so I tighten them up now.

    Sporting rifles, across the course match guns, and sniper rifles get set up at .03" cause I want dirt/grit to have room to go somewhere.

    Target rifles that are single shot get set at .015"

    These are my personal tolerances. I pretty much pulled them from thin air and they've all worked just fine. To put things in perspective in case your concerned. An 03 Springfield rifle has enough unsupported case to drive a bus between the breech and the bolt face.


    Chad
     
  7. James Jones

    James Jones Well-Known Member

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    I'm thinking about going with a coned bolt head on my next build to aid in a little bit of strength and added feeding ability when going with a Lapua or WSM case ,is it realy worth the added cost? the way i look at it PT&G only charges $35 for it then why not?
     
  8. trueblue

    trueblue Well-Known Member

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    James,
    I'm not sure, but I don't think PT& G will do a cone face for the 338 Lapua. Might want to check that out.