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"Backbored"

Discussion in 'Shooting Related Threads' started by glenns, Jul 23, 2009.

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  1. glenns

    glenns Member

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    "All XT's are advertised as being factory backbored."

    What is backbored and what is the benefit of it?
     
  2. Carol Lister

    Carol Lister TS Member

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    <blockquote>"What is backbored and what is the benefit of it?"</blockquote>

    It's Klingon vernacular for "over-sized bore"... usually in the range of .735" and up compared to the standard 12 ga. bore of .720"-.730". Any diameter larger than .730" is a backbore.

    I put it right up there with barrel ports. What's the advantage? I have no clue, considering that the increase in bore size adds only ±.028 sq. in. to the bore area at the chamber mouth. Shooting a standard bore and an overbored gun in a blind comparison, I couldn't tell the difference by either feel or result and I'd bet that very few can.

    You can read a discussion about it (along with one on the mystic of longer forcing cones) here: <I>The Technoid on Cones and Bores</i>

    (The first half of this treatise is <I>Here</i>)

    Carol Lister
     
  3. mike b.

    mike b. Member

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    As I remember, regular 12g bores used to be in the .725-.729". (25 yrs. ago) That makes the backbore difference even smaller.
     
  4. Old Confederate

    Old Confederate TS Member

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    The larger the bore the better it will pattern a heavy load of shot. A 12ga. will pattern 1 1/8oz. better than a 20ga. and a 10ga. will pattern 1 3/4oz. better than a 12ga. so in theory the bigger the bore the better it will pattern. Beretta calls .732 overbore and some others call .740 overbore. About 20 yrs. ago I shot skeet with some people that had their guns overbored but they used the orignal chokes and they bragged about how much harder they hit the target. They didn't understand that by opening the bore .10 that is the same as adding .10 to the choke. They thought it was shooting a better pattern when it was really just shooting tighter. Larry
     
  5. Milkbone

    Milkbone TS Member

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    Larry, can you define "better" when it relates to shotgun patterns.
     
  6. Brian in Oregon

    Brian in Oregon Well-Known Member

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    I have a factory backbored Remington trap barrel.<br>
    <br>
    A standard Rem 12ga bore typically .727".<br>
    <br>
    Their backbored barrels are typically .745".<br>
    <br>
    That's a difference of .018"<br>
    <br>
    Just before the choke, the bore tapers back down to standard. This allows the use of Rem chokes.<br>
    <br>
    The Rem chokes that are used, however, are different from standard Rem chokes. These are basically remarked because they take into account that the bore has already been constricted by .018". That means a full choke, which is typically .036" in a standard bore, must be .036" minus .018". This means the special full choke for these barrels is .018". In a standard barrel that's a mod choke.<br>
    <br>
    It has been claimed that the purpose of the backbored barrel is to create a more dense core by shifting pellets from the outer part of the pattern to the inner part, all while maintaining the same overall pattern diameter. From my patterning, this seems to be true. However, the only real way to tell would be to take a standard barrel, then backbore it, and see if there is indeed a change. My patterning has no control test subject to compare to. Still, I believe the idea has merit.<br>
    <br>
    More importantly, I've found, is that the backbored barrel gets better patterns with tight chokes with cheap shells that use soft lead, than a standard barrel. Why? Because though the amount of total constriction is the same (full choke) the backbored barrel. This is because the shot is being squeezed less, causing less distortion to the pellets. in other words, I'm getting pattern quality with cheap soft lead shells through my backbored trap barrel as premium shells in my standard barrel. That's a cost savings to me. Or, if you put premium shells through the backbored barrel, you get even better patterns yet. This is especially true when using extra full chokes, because they really squeeze the shot in a standard barrel.<br>
    <br>
    Note I'm saying "backbored" instead of "overbored". The Remington barrel is backbored, because the bore steps down before the standard type choke tubes. Remington also makes an overbored barrel. Same oversize bore diameter, but it does not taper down before the choke tube section, and uses special, oversize choke tubes totally unique to that barrel. They are not interchangeable with Rem chokes. I realize both terms are often used interchangeably, but that's the Remington terminology.
     
  7. ccw1911

    ccw1911 Member

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    It would seem that if the barrel goes back to normal diameter for even a short distance before the choke tubes the barrel would not need special chokes. Wouldn't the actual restriction be the difference between the the choke diameter and that part of the barrel just before the choke?
     
  8. Old Confederate

    Old Confederate TS Member

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    Milkbone---- I know that better is relative. Better to me is the shot more uniform with no clumps or empty spaces in the pattern. That may not be better for you. Larry
     
  9. Milkbone

    Milkbone TS Member

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    Thanks Larry. I'd sure like to see a pattern with no clumps or empty spaces- never seen one yet.
     
  10. Brian in Oregon

    Brian in Oregon Well-Known Member

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    ccw1911, by "normal chokes", the Rem backbored barrel uses regular pattern Rem chokes, but they take into account that the bore steps down .018" ahead of the choke tube. So whatever choke tube is in use, you must add .018" to its constriction. If you were to screw in a normal full choke tube, it would be .036" plus the built in constriction of .018", for a total of .054". The specially marked choke tubes for this barrel simply eliminate the need to do the math. They marked to take into account their constriction plus the barrel constriction. Also, because of the built-in .018" constriction, the pattern will never open up more than a mod, even if you install a cylinder bore choke.
     
  11. hmb

    hmb Well-Known Member

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    One day I made an aluminum sleeve 6 inches long that fit in the chamber of my K-80 trap gun. The inside of the sleeve was a 20 guage chamber. This allowed me to shoot 20 guage shells in my 12 guage trap gun. It was the ultimate backbored barrel, over 100/1000. Very good patterns and very little recoil. HMB
     
  12. glenns

    glenns Member

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    Thanks for the explanations.

    "Backbored" does sound pretty cool when you're trying to sell a gun.
     
  13. phirel

    phirel TS Member

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    The theory behind back boring and lengthening forcing cones is that lengthening the area where the shot is compressed reduces shot deformation. Also, the enlargement of the barrel reduces deformation of shot as it is traveling down the barrel. Evidence for this theory is scant. But, a couple of the top barrel men, such as Tom Wilkinson, are able to improve patterns. They work with the specific load the shooter intends to use.

    Pat Ireland
     
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