scope expansion

Discussion in 'Competition Forum (All Calibers)' started by Alex Wheeler, Oct 29, 2016.

  1. Alex Wheeler

    Alex Wheeler Gold $$ Contributor

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    @carlsbad I think you could do this math. I have a 1.250 barrel thats shot out I use for bedding. I stuck a torch inside the bore for a few seconds. Infared thermometer was used to measure the temp on the outside of the barrel. Starting temp was 65, after heating 103. The OD grew. Easily measured. Question, how much??? Curious how my results match up to the math.
     
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  2. gunsandgunsmithing

    gunsandgunsmithing The best tuners and wind flags on the market Gold $$ Contributor

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    11 pages and going strong.
     
  3. tom

    tom Gold $$ Contributor

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    No caliper laying around?
     
  4. Stan Taylor

    Stan Taylor

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    That's funny Alex I don't care who you are what you're talking about is interesting to say the least.PS Alex I tell you what I did do when cyro first started rifled a barrel the bore was green at .309 had it cyro treated it opened upafter I turned it an warped when I turned but it came back the same size when it came back from cyro keep up the good work.
     
    Last edited: Jan 10, 2018
  5. Ledd Slinger

    Ledd Slinger Gold $$ Contributor

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    To add to the confusion...
    There's no way the bore will have an even temperature from lands to crown during a string of fire. Will always be hotter at the lands. Nor will it heat and cool at the same rate due to varying steel thickness in the contour. So now you're dealing with different rates of expansion and contraction on the length of the barrel.
    How are you gonna predict and calculate that?

    802.jpg
     
    Last edited: Jan 10, 2018
  6. fyrewall

    fyrewall

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    Holes or cavities in materials show the same thermal expansion or contraction as the surrounding materials - like the hole will get bigger or smaller when it is either heated or cooled just like the surrounding material.
     
  7. Ledd Slinger

    Ledd Slinger Gold $$ Contributor

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    I know a lot of 1K BR shooters who 'run' their string are pretty consistent with the amount of time it takes them to complete a string of fire. Guys like Tom have a nice consistent rythm as they fire the string so I think that whatever the heating and expansion may be on the barrel, this could be considered somewhat of a 'constant' in the equation.

    So now when you guys are firing during your sighter period, do you always do the same amount of sighters and do you time them in accordance with when the record shots commence? I would think a person should develop a method of load testing that can be repeated in a relay. For example, say 4 sighter shots are fired regardless of POI exactly 30 seconds apart and the first sighter shot is fired at EXACTLY 2 minutes and 30 seconds prior to the record sting of fire. This would also give you 30 seconds before the record string from the last sighter. Then you fire the record which is always about the same rate of fire. Correct me if I'm wrong, but I think this would help to somewhat control metal expansion rates. The key would be to test loads in this manner so the tune might be a little more repeatable in competition.

    Don't know if this is a regular practice by some (I've never seen anyone with a stop watch on the line), just brainstorming with the current subject in mind.
     
    Last edited: Jan 10, 2018
  8. gunsandgunsmithing

    gunsandgunsmithing The best tuners and wind flags on the market Gold $$ Contributor

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    What about the growth of the bore as 60,000+psi ramps up and down?

    I've always wondered how a barrels dimensions can be so crucial when the barrel expands and the bullets obturates to fit it.
     
  9. carlsbad

    carlsbad Details matter. Silver $$ Contributor

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    Alex,

    coefficients of thermal expansion for 4140 is 6.8E-6 in/in - deg F. 400 series stainless is 6.0

    4140: So you heated up 39 deg x 1.250" x 6.8e-6 = .00033" Less than .001"

    for SS multiply by 6/6.8= .00029"
     
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  10. Macropod

    Macropod

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    The only common thing I can think of that expands with cooling is water turning to ice. I guess all I have learnt about thermal expansion in metals hasn't changed much. Darned if I know why mercury rises in a thermometer when it gets warmer. It must be the glass shrinking, forcing the mercury up the scale...
     
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  11. Alex Wheeler

    Alex Wheeler Gold $$ Contributor

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    Well it measured .0009" larger. I did put the torch inside the bore. The outside measured 103, the inside was warmer no doubt.
     
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  12. Ringostar

    Ringostar

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    I have a new excuse when I have a bad shot. My barrel shrunk when it warmed up! Or is it that only happens to bridges? This turned into an entertaining thread for a couple of pages.
     
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  13. Ledd Slinger

    Ledd Slinger Gold $$ Contributor

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    All I know is I'm pissed off because I couldn't get the plastic lid back on the store bought lasagna I cooked this evening. The damn aluminum tray grew after I cooked the lasagna in the oven...I thought only rifle barrels expanded with heat?!
     
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  14. Ringostar

    Ringostar

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    Did the pan grow, or the lid shrink? Would you also get pissed if someone told you that you didn't really cook the lasagna, vs just heating it up?
     
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  15. Rich S

    Rich S Silver $$ Contributor

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    Matt
    Alot of variables for sure. I will say that if you have a piece of steel with a hole in it (pipe, bearing, coupling, etc...) the ID will grow as its heated. If evenly heated the formula Jerry stated applied to the ID is correct. On a 6mm bore there would be about 0.0001" increase in the ID for a 100 degree temp rise. I use to work in an assembly area where we installed bearings on a shaft with an interference fit. We soaked the shaft in liquid nitrogen to decrease the OD and heated a bearing in an oven to increase the ID. You could drop them together with no external force.
     
  16. fyrewall

    fyrewall

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    Help me out on this:

    I found the coefficient of thermal expansion to be:

    12.7 * 10**-6 for 4140 type steel
    5.5 * 10** -6 for 400 series stainless

    12.7 - 12 = .7, 39 deg. * .0000007 * 1.250 =
    *****

    I sort of approached the situation as:

    coefficient of expansion for 4140 steel = 12.7 * 10**-6 ::: this is inches of expansion per 1 degree of temperature (real tiny)

    the barrel is a cylinder having a uniform radius, thus the circumference is 2 * pi * radius or 3.142 * 1.250 = 3.928

    the circumference a linear value is subjected to heat and the heat increased by 39 deg. F

    the linear expansion would increase the circumference of the circular barrel

    thus the circumference increase would be 3.928 * .0000127 * 39 = .0019

    the diameter increase would be circumference / pi, or .0019/3.142 = .0006 (6, 10 thousand difference) real tiny; the diameter for 400 series stainless have a coefficient of expansion of .0000057 would be more tiny.

    Alex's value was .0009, only .0003 more than my calculated value, Alex did not specify what type barrel steel was tested. The coefficient of expansion for 400 series stainless is .0000057 inches per degree (smaller than real tiny)

    If I was charged with testing something like this I would start with a cool barrel at 50-60 degrees F and toast it real good with a hair dryer to get it up to 120 degrees then immediately infra red measure it.
     
  17. carlsbad

    carlsbad Details matter. Silver $$ Contributor

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    fyrewall, You have a unit problem. You used the value for coefficient of thermal expansion for deg C and Alex's temps are in deg F. So your number is high by a factor of 9/5. --Jerry
     
  18. Rich S

    Rich S Silver $$ Contributor

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    Probably didnt make a difference but keep in mind IR does not work well on all materials or surface finishes. Particularly glass and highly polished surfaces can give bad readings. If you have these materials slap on a piece of electric tape and read on the tape.
     
  19. carlsbad

    carlsbad Details matter. Silver $$ Contributor

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  20. fyrewall

    fyrewall

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    Jerry,

    Yes, I stand cheerfully corrected.

    so

    3.927 * .0000068 * 39 = .0001, coefficient of expansion for 4140 .0000068 inches per degree, increasing the circumference

    I used 6.8 * 10**-6 from your diameter calculation, my thinking is the barrel is round and the circumference would be affected.

    .0001 / 3.142(pi) = .000033 inches diameter - this is only a 1.25 inch diameter barrel having a small 39 F temp change

    Alex came up with .0009 by internally heating the barrel, possibly the out side measured infa red temp was much lower than the torched innards of the barrel. I would like to see the process repeated heating the whole works from the outside to a temperature difference like near 212 degrees F from -15 degrees F.

    In that case:

    3.927 * .0000068 * 227 = .006 circumference ; .006 / 3.142 = .002 inches diameter :: this is big enough so I can see daylight through my caliper.

    Who knows, some type of national emergency might occur where President Trump is forced to select some gun guy to make an incredibly difficult shot requiring minute calculations to hit some tiny target.
     
    Last edited: Jan 11, 2018
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