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Perazzi Bore Size ?

Discussion in 'Shooting Related Threads' started by SMITH47, Jun 13, 2009.

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  1. SMITH47

    SMITH47 Member

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    I see references to Perazzi bore sizes 18.4 and 18.7

    I look on my barrel (recent mx8) and it has 18.6

    is my barrel considered in the 18.7 category ??

    thanks,
    ernie
     
  2. TOOLMAKER 251

    TOOLMAKER 251 Active Member

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    Each decimal point is .004 of an inch. .4 .5 .6 etc.
     
  3. tcr1146

    tcr1146 Well-Known Member

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    The actual conversion is closer to .03937 inches per millimeter. If .04 was correct your bore size would be .744, which I highly doubt. Using .03937 it is approx .732 which is much closer to your real size. Tom Rhoads
     
  4. TOOLMAKER 251

    TOOLMAKER 251 Active Member

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    Tom, yes a millimeter is .03937 of an inch and a tenth of that is .003937 but instead of trying to remember how much choke is on a #7 Perazzi choke tube as .03937x7 it's easier to remember .004 or 4x7= 28 thousanth's choke. So I answered smith47 question of the difference between 18.6 vs 18.7 as .004
     
  5. BigM-Perazzi

    BigM-Perazzi Well-Known Member

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    just divide the metric by 25.4 and you'll get the correct answer...

    For figuring choke constrictions, the .004 x metric # (5,6,7,etc.) works just fine.


    Now, if you want some grade 3 master balls, we can talk .000001

    and I can get you a price....

    Jim Chapman

    <img src="http://i75.photobucket.com/albums/i303/BigM-Perazzi/DSCF0039.jpg"
     
  6. SMITH47

    SMITH47 Member

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    So does the number on the (my) barrell 18.6 indicate the constriction at the muzzle ?

    If the barrel starts at 18.7 and ends at the muzzle 18.6 then i would guess i have .004 (skeet choke) ???

    am i on the right track ???


    ernie
     
  7. TOOLMAKER 251

    TOOLMAKER 251 Active Member

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    No, your off track. The only way to know for sure is to check it with a bore gauge. The 18.6-18.7 figures are the bore diameter, not the choke dia.
     
  8. Steve W

    Steve W Well-Known Member

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    ernie, I think your original question was the bore size in mm. ?

    Perazzi made 18.4, 18.5, 18.6, & 18.7 bores. These numbers were certified by the proof house.
     
  9. SMITH47

    SMITH47 Member

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    Steve W

    That is what i wanted to know ?

    that they made that many different bore sizes ???

    thanks, everyone

    ernie
     
  10. BigM-Perazzi

    BigM-Perazzi Well-Known Member

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    We all had good answers!! You just didn't ask the correct question.... lol

    Jim
     
  11. miketmx

    miketmx Well-Known Member

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    My TMX has 18.4 stamped on the barrel but the measured bore size is .728 and not .724 which it would be by the conversion.
     
  12. TOOLMAKER 251

    TOOLMAKER 251 Active Member

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    Miketmx, My TM-1 and MX-8 are both marked 18.4 also but are .728 (18.5) I guess 1 tenth of a mil was close enough for Perazzi 35 years ago.
     
  13. tsosin

    tsosin TS Member

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    Can anyone comment on the interchangability of a 18.6 and 18.7 choke in the same gun. They both seem to fit the barrel.
     
  14. Neil Winston

    Neil Winston Well-Known Member

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    An interesting aspect of the English/Metric conversion is that one inch = 25.4 mm is an exact figure.

    An apparent assumption on this thread is that the second conversion, 18.4 mm = 0.7244 in is also an exact figure, but it's not. The precision implied by the tenth millimeter figure is (about) plus or minus 0.002 inches. Thus 18.4 mm is between 0.722 and 0.726 but we don't know where along that continuum it lies. I think that explains a lot of the confusion here.

    The rest is, I think, explained by the fact that different equipment is used to measure that diameter. At Perazzi (and probably everywhere else) diameter is measured by "air gauges" and there's no reason to think that the "three-ball" system we all use gives exactly the same results, just as shot speed from a factory chronograph will not be the same as you get "at home."

    Neil
     
  15. BigM-Perazzi

    BigM-Perazzi Well-Known Member

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    Good point Neil. We often forget to take into account manufacturing tolerances added or subtracted from the bore number.... I'm going to guess the Italians are using +/- .05 mm... which is pretty dam* close to your number....

    Jim
     
  16. Steve W

    Steve W Well-Known Member

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    tsosin,

    Perazzi made two type of tubes for their 4th generation chokes. One for 18.4 mm bore, the other for 18.7 mm bore.

    They are interchangeable, but you'll have to calculate/convert the constrictions if you interchange them. If you use the 18.7 tubes in 18.4 bore, just subtract 0.3 mm, or go 3 numbers up when picking your tubes. And accordingly for 18.5 & 18.6 bores.

    The tubes for 18.7 mm bores are so marked as showing in the pic.


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


    And Neil,

    You are absolutely correct on this, the three ball mic. we use gives much more tolerance then ind. air gauge.

    Besides, each barrel is different in performance, pattern wise. 0.01 mm difference in bore dia. will not make us miss the bird.
     
  17. TOOLMAKER 251

    TOOLMAKER 251 Active Member

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    Neil, you lost me on your figuring of a metric dimension. If 18.4mm = .724408 and each .1 tenth of a mm=.003937 where does the .722-.726 come into play? are you using the word precision as a word for tolerance?
     
  18. Neil Winston

    Neil Winston Well-Known Member

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    No, Toolmaker, I'm not, though it ends up that way. I'm using "precision" in the sense of "how many decimal places are we talking about."

    The single digit after the decimal point means we can't go any farther than that in saying what the number is. In this case, we conclude that the number 18.4 stands for all the numbers between 18.35 and 18.45, but the probability of any one is not different from any other.

    Thus, 18.4 does not mean 0.724408, it means what I wrote. The number that would mean 0.724408 inches is 18.400 mm. Well, that's not quite true either. That 0.3737 conversion is not exact, and when we are way down at the end of a long string of decimals, that makes a difference too.

    So you take 1/25.4 and multiply by 18.400 then you get 0.724409.

    But aren't I cheating? If I say three decimal places of precision, how can I use 0.03937008, a number expressed to far greater precision than any of them, so far? It's because it's not a measurement, it's a conversion number based on an exact number and so is valid to as many places as we want.

    Neil
     
  19. TOOLMAKER 251

    TOOLMAKER 251 Active Member

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    Neil, I agree and disagree, You made a typo on the 0.3737 it's .03937 Whats not known is Perazzi manufacturing tolerances so the figure 18.4= .7244 and can never be .722-.726 unless a tolerance would allow this. Miketmx and myself posted our barrels are marked 18.4 but are actually .728 so Perazzi has a bore size limit of at least .1 mil. or is out of tolerance. My measurements were taken with a Brown&Sharpe Intrimik which will reads in .0001 increments and the Baker gauge shows .728 also.
     
  20. SMITH47

    SMITH47 Member

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    Thank you Steve for your reference to 18.4 & 18.7

    That is what i have read other places and is the origin of my confusion since my barrel says 18.6 and i find out there are 18.5 ?

    1)Will any generation 4 tube fit it any 18.4, 18.5, 18.6, 18.7 barrel ?

    2)If we agree skeet choke is .004, do if pick a tube that is .004 smaller than my 18.6 barrel in order to now have a skeet choked barrel ??

    no wonder i didn't understand, this is complicated for me !!

    ernie
     
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