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Velocity increases in overbored barrels?

Discussion in 'Shooting Related Threads' started by skeet_man, Apr 6, 2009.

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  1. skeet_man

    skeet_man Well-Known Member

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    I know i've read before that with the .780 and .800 baker big-bore barrels, you could achieve a higher velocity compared to what would be achieved with a .730 bore barrel using the same shells. Would the same hold true (but on a smaller scale) for a .750 bore barrel? I'm currently shooting what Hodgdon says is a 1145 fps load, and would like to cut back on the powder a touch, to the amt they list for a 1100 fps load. However, i'm thinking that since I have an overbore barrel, if I loaded a book load of 1100 fps, they may end up being 1145fps through my gun due to the overbore. I have a chronograph, but haven't tested my idea yet, just curious if anyone else has. Gun is 30" barrels w/ screw chokes (i use .005s).
     
  2. Neil Winston

    Neil Winston Well-Known Member

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    As I wrote in in the above link, page 5, when I tested the speed of many 870's, big and small.

    "The two “overbore” barrels (diameter 0.745 inches) on veteran
    Competition 870’s produced speeds of 1189 and 1187 fps, at the
    upper range of their standard-bore stablemates. The two modern factory
    overbore barrels with extra-full Remchokes™ were significantly
    slower at 1168 and 1171 fps. There was no tendency for the overbore
    barrels to shoot faster as is commonly asserted."

    Neil
     
  3. Hap MecTweaks

    Hap MecTweaks Well-Known Member

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    When using an overbored barrel, your choices of wads diminish with increased size. Some wad bases just won't seal the pressure behind the charge as a standard bore will. Try some old felt/cardboard wads in an overbore and see how that works out for you? I'd prefer a multiple wad choice rather than one or maybe two that may work.

    Usually, it's tough to make blanket statements about shotguns and loads. As much as they are alike, there are differences with exceptions in some too. Hap
     
  4. zzt

    zzt Well-Known Member

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    skeet, I have no experience with big bore barrels. However, I do have limited experience with moderate overbore barrels (.740" and .745"). After they were enlarged, velocities were about the same. Maybe a touch higher, but that could be due to a different lot of powder, different day, slightly different temp, etc. It might even be due to the vastly improved surface finish of the bores. I don't know, but I'd be surprised if you got any where near the 45fps gain you are hoping for.

    Let us know how your testing turns out.
     
  5. chipking

    chipking TS Member

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    Neil Read your write up with interest. From what I gather the statement would be true based on results obtained using an inductive chronograph in fact the statements in another post about higher velocity in open choked barrels was also correct based on the results from an inductive chronograph. Yet with the newer technology of the light triggered chronograph we get just the opposite results. Things that make you go HUMMMmmm.

    --- Chip King ---
     
  6. Neil Winston

    Neil Winston Well-Known Member

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    zzt, though it seemed to me it should work, my single test of surface finish as a contributor to shot-speed was negative.

    I had a Korean hammer gun, 28 inch, which I bought for the test because it looked knurled inside. I tested the speed and it was, as I expected, lower than usual.

    I polished it out with paper on wooden shims, just like the English say they do, then followed with a lot of fine and ultra-fine emery and crocus cloth, ending with the shiniest bore I've ever seen. The speed, however was, unchanged.

    One test isn't conclusive, but I've quit polishing them by and large. I still think smooth is better, but even that has its critics, Gough Thomas being one, as I remember and a startling test by Oberfell & Thompson recounted on pages 196 and 197 of their book, including a test of "grooving" a choke tube which sounds like what Ljutic does to me. And yes, it "worked."

    Neil
     
  7. Neil Winston

    Neil Winston Well-Known Member

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    No, chipking, that test was done with two (light operated) 35p's.

    The inductive chronograph told a different story, as it so often does. There, the larger-diameter bores were clearly faster, but you have to keep in mind that the "shape" of the charge can make that machine report different speeds, and in this (single) case I'm more convinced by the 35p's. But, on the other hand, they may really be faster too.

    <a href="http://photobucket.com" target="_blank">[​IMG]</a>


    I covered this in my report:

    "An earlier experiment with an inductive chronograph produced
    contrasting results. In that test, the overbore barrels were faster, and
    the magnum chambered tubes slower, than any of the standard
    “trap” barrels they were compared with. I cannot account for this
    difference."


    The results of the 3-inch chambers might give owners of K&P new guns something to think about as well. In this case, I can't see how "shape" could produce this difference.

    Neil
     
  8. Hap MecTweaks

    Hap MecTweaks Well-Known Member

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    I owned a side hammer Remington number 9 that I won a lot of card turkey shoots with that had the worst looking barrel internals I'd ever seen! Rusted pitting from black powder was my guess. It did put a lot of shot in the 4X4 cards but would hardly dent a license plate at 40 yards! Prior to that, I had a 20 ga. Mossberg that was the worst shotgun ever. It shot a fair pattern but never made a clean kill with it,, ever!

    This is where the slick bores shine in my opinion, it changes (something?) and improves the clay and game performances we take for granted with all shotguns.

    Hap
     
  9. PerazziBigBore

    PerazziBigBore TS Member

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    I have to agree with Neil on this.. In my many testings of overbored.. and bigbore barrels.. up to .802.. I have failed to see any difference over standard deviation of a regular shell.. I have however.. seen quite a difference in some of the patterns..

    On another note.. If you load light shot weights with slightly slower burning powders..you can wreck havoc with alot of loading data when going to a overbored.. or bigbore barrel.. as the resistance is needed for proper combustion. Sometimes this results in the need for additional powder.. or crimp to get complete burning of the powder charge.

    Many times.. the metal removed in making a overbored barrel is used to create a better profile for chokes..and the resulting patterns can be "tuned" for optimum performance..

    I personally think think that shot string becomes part of this equasion. Going from a .670 bore with 1 oz.. to a .730 bore with the same shell.. to .780.. to a .800 bore will show very different patterns.. AND a visual on target breaks becomes undisputedly a factor on judging what is happening..And.. yes.. everyone owns at least 1 exception to the rule..

    These are my thoughts on the subject.. All Good.. Mike
     
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