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barrel crown

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  #8  
Unread 02-13-2011, 10:08 PM
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Re: barrel crown

No problem gr8whyt.

Recessed or not, I agree that square is the main thing.

-- richard
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  •   #9  
    Unread 02-13-2011, 10:20 PM
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    Re: barrel crown

    Someplace over 30 years ago, I recall reading that the military did exhaustive testing which showed that 11 degrees was the optimum crown angle for best accuracy. Not by a huge margin, but still significant enough to be the recommended angle of choice.

    However, I also recall that this assumed a perfectly perpendicular cut.
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      #10  
    Unread 02-14-2011, 12:47 AM
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    Re: barrel crown

    Quote:
    Originally Posted by Susquatch View Post
    Someplace over 30 years ago, I recall reading that the military did exhaustive testing which showed that 11 degrees was the optimum crown angle for best accuracy. Not by a huge margin, but still significant enough to be the recommended angle of choice.

    However, I also recall that this assumed a perfectly perpendicular cut.
    +1

    The theory behind the 11degree crown was based on high speed photography testing done by
    the military.

    It appeared that the gas escaped at close to 11o from a squared of crown and that more than
    11 degrees of crown caused the gas to contact the surface of the crown and possibly having an
    effect on the bullet.

    So they theorized that any thing from 0 degrees to 11 degrees had no contact with the surface
    of the crown preventing any effect of the shock wave or gas on the bullet.

    After some firing some crowns have Smoke trails that match the rifling from contact of the
    escaping gas and 0 to 11 degree crowns have little or none.

    I am not sure that the High pressure rounds perform the same as the test done by the military
    but a good clean exit of the gasses does make a difference.

    The gas and unburnt powder can and will erode the crown over time and it needs to be re cut
    when this happens.

    The 11 degree crown has a thicker edge than a square crown (90 Degrees compared to 101
    degrees of the 11o crown)from the bore and should last a little longer. Does it? I cant prove
    it, so the choice is from 0 to 11 degrees.

    This is all theory like lots of other things in ballistics, and the reason that there is so many opinions
    on it.

    I like the looks and it seems to work good so I use 11 degree crowns.

    But I do think that any well cut crown 0 to 45 degrees will work if it is true and clean to the bore.

    If you look at some of the factory crowns they are not concentric to the bore and are there to
    aid in cleaning from the muzzle without tearing the patches.

    Note: never clean a barrel from the muzzle end if possible.

    J E CUSTOM
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      #11  
    Unread 02-14-2011, 10:58 AM
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    Re: barrel crown

    JE Custom - Your reply prompted a memory flash. Don't have those very often anymore...... so I cherish them when they do!

    My recollection suggests that you are correct. Thats how this whole 11 degree thing got started.

    But I also remember now that a group of benchrest shooters reviewed the military data and then wore out a few barrels testing the angles further to see how the military photo results affected accuracy. They found that an 11 degree crown also seemed to produce the most consistent accuracy.

    I don't remember where I saw this. It may have been in the Rifle Magazine, in Precision Shooting Magazine, or in Handloader Magazine. I suppose it could have been in one of the many books I had too - "The Accurate Rifle" comes first to mind. It sure as heck wasn't the internet. Contrary to popular belief, the internet did exist back then but a multitude of BBSs and Compuserve was about as good as it got - and they were pretty poor.

    I threw out all my old magazines when my self appointed management threatened to divorce me. So I tried to find something on the net to corroborate my memory. I could only find the short description in the following link:

    Barrel crowning (Bart Bobbitt)

    Accuracy back then wasn't what it is now - although still damn good. So maybe recent improvements have shown this to be a flawed conclusion. In any event, I think that even if the precise angle does make a difference, it isn't likely important for anything but the finest competition rifle and probably couldn't be detected on the finest long range hunting rifle.

    That said, I know of no evidence to suggest that any other angle is better than the 11 degree crown, so as they say "In the absence of any compelling information to the contrary", I'll stick with what I currently know won't hurt - 11 degrees.

    BTW, I found this forum because I was looking for info on how the OEMs make their crowns. I recently purchased a Browning X-Bolt White Gold. Its never been fired. But it appears to have some crown damage in it that is hard to explain. Other damage I have seen is easy to explain by poor tooling and the like. But this is very different. The metal actually appears to have been melted at the crown edge and there are little tiny beads of metal welded to the edge.

    Here is a photo taken with a digital camera in Macro mode using a bore light. The loose stuff in the barrel is just dust.

    Does anyone know how this happened? My best theory is a lathe centering tool that came loose and spun in the barrel. But its hard to imagine how that could have happened at the factory. I'm contemplating either re-crowning the rifle or re-barrelling it.



    Cheers!
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      #12  
    Unread 02-14-2011, 11:11 AM
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    Re: barrel crown

    Great photo Susquatch!

    And, great info JE/Susquatch.

    My recollection also was something like Susquatch reports. i.e. in the absence of anything better, stick with what I know.

    But, I do like the point JE makes about the potential wear of a sharp shoulder vs thicker shoulder of the 11 degreee crown.

    I wonder how many different bullet designs were tested in the referenced works? I can imagine that if it makes any difference at all, then the tail of the bullet would also be a factor. e.g. flat base vs boat tail

    I have also heard bullet manufacturers comment that making a perfect boat tail with perfect concentricity is much more difficult than making a flat base. As such, it's equally as important as the crown itself for ultimate precision.

    -- richard
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      #13  
    Unread 02-14-2011, 11:42 AM
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    Re: barrel crown

    Not that I am any expert..... but.....

    My personal experience reflects your comment. Flat base bullets provide slightly smaller groups at 100, and boat tails are better at 300. They are about the same at 200. The boat-tail advantage at 300 is probably mostly ballistics related - just my WAG. More retained velocity means less influence from conditions. This also reflects what you see being used in the 1000 yrd events - all boat tails.

    This of course is in a benchrest rifle that routinely puts 5 into 1/8" @ 100 in perfect conditions - not a hunting rifle that an old man like me can actually carry without a forklift. I doubt you could ever see the difference in a hunting rifle.

    Cheers!

    Last edited by Susquatch; 02-14-2011 at 11:46 AM.
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      #14  
    Unread 02-14-2011, 12:05 PM
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    Re: barrel crown

    I know one thing, after getting set up to do my own lathe work, the factory crown is the first thing to go!!! I haven't done many yet but everyone showed a large improvement, I've been cutting an 11 degree just cause it seems to be the universally used crown for accuracy. I wish I had a Hawkeye bore scope to just see what is going on!
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