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Thread: Micrometer recommendation for barrel slugging?

  1. #1
    Boolit Buddy
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    Micrometer recommendation for barrel slugging?

    Hey Everyone,

    I am going to slug the barrels on some of my guns and think that I have a reasonable understanding of the process except one detail.

    The micrometers that I have used back in the olden days would not work for measuring the grooves left by the rifling lands.

    Can you all recommend an appropriate tool? Link?

    This is a tool that will see very limited use in my shop so I don't want to spend a fortune.

    Thanks.

  2. #2
    Boolit Grand Master



    M-Tecs's Avatar
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    For even number lands and groove a standard mic works. For odd numbers you need a v-mic of a specific angle. S&W uses a 5 groove so I use a cheap 108 degree $94.00 SHARS 0.2-1" 108 Degree 5 Flutes V-Anvil Micrometer Graduation 0.001” Ebay sells them but can't do ebay links here.

    Some wrap the bullet with shim stock and use a caliper or standard mic for odd grooves.
    2nd Amendment of the U.S. Constitution. - "A well regulated Militia, being necessary to the security of a free State, the right of the people to keep and bear Arms, shall not be infringed."

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  3. #3
    Boolit Buddy
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    OK.... I am reading some conflicting info.

    Somewhere I read that I needed to measure the bullet both where it contacts both lands and grooves. I can't see how you could do that with a standard micrometer. Measuring the grooves left by the lands would be impossible as far as I can see.

    Then I saw something that led me to think that I only need one measurement and that is the portion of the bullet the expands into the grooves.

    If the latter is correct then I see how to use a common micrometer.

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  5. #5
    Boolit Grand Master



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    The issue may be terminology. The measurement that is of most importance to bullet casters is the diameter of the grooves in the barrel. Then land diameter is the bore diameter.
    http://mcgowenbarrel.com/bore-groove/

    Example is a standard 30 cal. barrel. The standard groove diameter is .308" with a standard bore diameter of .300". When determining proper bullet size the .308" diameter is what you are verifying and trying to match. On the bullet that will be the mirror image of the actual barrel. On the bullet you are measuring the largest diameters.

    On bullets that are boreriders the bore diameter is also of importance. Unless ordering a boreriding bullet mold most people never check this. For the average person pin gauges or small hole gauges can be used to measure the actual bore of the barrel.

    Also the bullet is normally slightly larger than the groove diameter of the barrel. The bullet should NOT be expanding into the groves rather the lands are forcing an impression into the bullet. Commonly referred to as rifling grooves in the bullet.
    Last edited by M-Tecs; 04-05-2020 at 02:23 PM.
    2nd Amendment of the U.S. Constitution. - "A well regulated Militia, being necessary to the security of a free State, the right of the people to keep and bear Arms, shall not be infringed."

    "Before you argue with someone, ask yourself, is that person even mentally mature enough to grasp the concept of different perspectives? Because if not, there’s absolutely no point."
    – Amber Veal

    "The Highest form of ignorance is when your reject something you don't know anything about".
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  6. #6
    Boolit Grand Master

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    One trick that works for measuring the bore dia ( small) is to use a small set of thread wires in the grooves from the measurement. and subtract the dia x 2.

  7. #7
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    Quote Originally Posted by PNW_Steve View Post
    Would this be a good choice?

  8. #8
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    I wouldn't for a couple reasons. First it's digital. I started using micrometers as a machine operator a couple years after I got out of high school and used a plain Starrett 1" mic. I used it steadily for about 12 years (daily) until I bought a Mitutoyo, again a 1" "standard mic. I used these to measure machined parts down to .0005" (learning to use a mic with a vernier type scale is simple and the "feel" is soon established). In all the time I used precision measuring tools I haven't found any digital type to be as reliable as a plain "analog" mics., and I like to see the hash marks lining uo rather than some numbers that "randomly" show up on a readout. (life long machinist/mechanic). If I were to recommend mics I'd say the ones sold by Lyman are as good as needed for reloading and will last many, many years if cared for properly. (I got a "Lyman" 1" mic after my good tools were stolen and it is accurate and working quite well after about 15 years use for reloading).

    There are lots of hints to measuring an odd groove slug, but I have used two. First is using a sheet of shim stock (.002" is about as thin as can be managed easily) and wrap the slug and measure. Subtract twice the thickness of the shim for the slug diameter (if using .002 shim stock, subtract .004" from the measurement). I've been using mics for quite a while and I developed a good feel so I am able to measure odd grooved slug directly with good accuracy; I can slowly close the spindle on the slug while I am turning it and I can get an accurate measurement by feeling the slug as it is being turned in the mic. I think if one learns to use a mic good enough to get no more than .001" variations in their measurements, that's good for all reloading measuring chores.

    Jes an old phart's opinion/experience...
    My Anchor is holding fast!

  9. #9
    Boolit Master
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    mdi has answered this question about as well as anybody could. If you are serious enough to actually measure down to half a thousandth, ignore all of the digital stuff out there. Ignore the dial calipers... they are useful, but not for this purpose. Get a good old fashioned micrometer and learn how to use it... anything else is just an unacceptable short cut for those too lazy to use proper tools.

    I have found that for most hobbyists, the inexpensive imported micrometers will measure as well as most casual users will be able to use them. Yes, I have Starrett and Mitutoyo mics, but I also keep a Chinese mic marked Lyman in my box for measuring bullets and other small parts at the range.

    While we're on the subject of measuring tools, a dial micrometer has its place... measuring case length, etc comes to mind. I bought a couple by Mitutoyo that stay in my shop as well as a Lyman-marked Chinese one that goes out to the range in my box. Are you starting to see a pattern here?

    Back to the OP's question, I also go with mdi's advice that for an odd number of grooves, a thin shim wrapped around, or two shims placed one on either side of the bullet provide the simplest technique. Just don't forget to subtract the two thicknesses of shim from your measurement. The number you get is the groove diameter and I generally like my bullet to be a thousandth or so larger than that number.

    Regards,
    Froggie
    "It aint easy being green!"

  10. #10
    Boolit Buddy
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    Thanks for the recommendations.

    I don't know yet how many lands and grooves that I am dealing with. The only mention of odd numbers that I have been in Smith and Wesson guns.

    Initially I am going to slug my new 9mm's. A Walther PPS M2 and a PSA 9mm AR (16").

    Then my Colt 1911's and then my S&W 586 and my 5.56 AR.

    The 5.56 is going to be the last one that I am considering casting for.

    I will look around for a standard micrometer. I have a couple of digital calipers and have not had any issue.

    I have not used a standard micrometer in 35-40 years but if I can see it, I can figure it out.

    Thank you again.

    S.

  11. #11
    Boolit Grand Master



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    Digital micrometers are widely used and they have there place. The issue with the one the OP asked about is it is digital only. Ones that have both digital and conventional readout campability would be the way I go if I wanted a digital. Here is and example. https://www.amazon.com/Holite-Microm...%2C171&sr=8-10

    I don't have clue as to quality. It appears to be metric on the conventional readout so I would not recommend this one. Just used it as an example.

    Micrometers definitely are more accurate than dial calipers. For the average user with limited experience + or - .0005" is realistic accuracy with a mic. With a quality dial caliper it's more like + or - .001". After you develop a good technique and feel you can do better.
    Last edited by M-Tecs; 04-06-2020 at 06:26 PM.
    2nd Amendment of the U.S. Constitution. - "A well regulated Militia, being necessary to the security of a free State, the right of the people to keep and bear Arms, shall not be infringed."

    "Before you argue with someone, ask yourself, is that person even mentally mature enough to grasp the concept of different perspectives? Because if not, there’s absolutely no point."
    – Amber Veal

    "The Highest form of ignorance is when your reject something you don't know anything about".
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  12. #12
    Boolit Master
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    https://www.amazon.com/Mitutoyo-193-...1&sr=8-21&th=1

    This is a mechanical digital as opposed to an electronic digital. The digital part reads to the nearest thousandth, but the barrel graduations reads to the nearest ten-thousandth. This is the type I purchased in 1975, maybe the same model.
    Spell check doesn't work in Chrome, so if something is spelled wrong, it's just a typo that I missed.

  13. #13
    Boolit Buddy
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    I don't understand why anyone would measure the bore. I size cast bullets to fit my chamber throat. Even then I find which diameter slip fits into a fired case from that chamber and voila, I get good groups using published load data. Seems every diameter that fits the throat also magically fits the bore. Please explain how all the extra trouble pays any dividends.

    Thanks, Dinny
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  14. #14
    Boolit Master
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    Quote Originally Posted by Dinny View Post
    I don't understand why anyone would measure the bore. I size cast bullets to fit my chamber throat. Even then I find which diameter slip fits into a fired case from that chamber and voila, I get good groups using published load data. Seems every diameter that fits the throat also magically fits the bore. Please explain how all the extra trouble pays any dividends.

    Thanks, Dinny
    not every gun has a throat it sometimes tapers right from case to rifling. found that out when trying to slug and size to my throat lol

  15. #15
    Boolit Grand Master

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    You will need the bore dia if your loading traditional Paper Patched bullets since they are bore riding. BPCR PP bullets are loaded only 1/7"-3/16" into case and set in the bore when chambered.

  16. #16
    Boolit Buddy
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    I bought one of these. $24 and it measures to .0001"

    https://www.amazon.com/gp/product/B0...?ie=UTF8&psc=1

  17. #17
    Boolit Grand Master
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    Buy a set of pin gages .... https://www.amazon.com/HFS-Class-Ste...rd_i=401593011
    For minus set tolerance: +0.0000" / -0.0002"
    For plus set tolerance: +0.0002" / -0.0000"
    Measures the bore ID, land to land

    Then take a caliper and measure the ID in the bore, groove to groove

    Fast and accurate
    Regards
    John

  18. #18
    Boolit Master
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    GONRA sez - listen to everybody - slug the bore and use a really nice DIGITAL micrometer for Groove diameters.
    Also, listen to John Boy and get a set of Pin Gages for Bore diameters.

  19. #19
    Boolit Grand Master

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    Ive been looking at caliper style dros for on a lathe tail stock. 6" version of while they read ( resolution) to .0005 the acuracy over the range is +/- .0015. On the longer units it even more. My mitutoyo digital mikes are .00005.

  20. #20
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    I use and highly recommend a MITUTOYO 193-201 0-1" .001" DIGITAL OUTSIDE MICROMETER WITH CARBIDE TIPS. $48 on fleabay in the wooden case.

    This is not the electronic digital, it is mechanical digital, you can see the numbers much like an odometer in a car speedometer, you can guess how many tenths between digits, and it has the .0001" scale around the thimble as well. It is a friction thimble which repeats the closing torque consistently. Been using an ebayed one of these for years and years, I mic my Zminus pin gages to check it's calibration and it's been rock solid.

    Let me recommend a stand to clamp the mic in so you have both hands free, worth their weight in gold!!
    Got a .22 .30 .32 .357 .38 .40 .41 .44 .45 .480 or .500 S&W cylinder that needs throat reaming? 9mm, 10mm/40S&W, 45 ACP pistol barrel that won't "plunk" your handloads? Shoot me a PM! Also on Facebook: https://www.facebook.com/Cylinderhone-756429174391912/

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