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Gauging Bore Wear

Discussion in 'Gunsmithing and Repairs' started by JDinFbg, Jun 16, 2019.

  1. JDinFbg

    JDinFbg Member

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    Dec 15, 2018
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    Location:
    Fredericksburg, TX
    I have a US Model 1917 rifle with the original 5-grove, left hand twist barrel that has been shortened to 22". I've slugged the barrel and as near as I can tell (by carefully rotating the slug inside the jaws of my caliper) it is .311" groove diameter. I've tried both .308" and .311 diameter bullets without much success. At the muzzle the lands of the rifling look fairly worn down, and I determined that I can stick the smooth shank end of an "N" drill bit (0.302") about an inch into the muzzle. So, not a good sign if the bore diameter was supposed to be 0.300" originally and is tighter 'up stream'. I'm wondering if there is a way to gauge the bore diameter over the entire barrel length? Does anyone make a set of plug gauges of varying sizes (say, incremented up in 5/10,000ths of an inch) that could be mounted on a cleaning rod and slid down the barrel to determine what will pass all the way through, what won't, and if there are any tight spots along the barrel? I've searched the web and cannot find any such thing.
     
  2. Blue68f100

    Blue68f100 Member

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    Barrel mfg air gap their barrels to check for more consistency and accuracy. There may be a few gunsmith that might have this specialized equipment. I don't know of any. May contact a Shilen, their located in Ennis, Tx and see if they offer this service.

    You can get gauge pins in any size you want or need. Not sure if they will help you locate variations through the bore. Only useful at the ends.
     
  3. PRD1

    PRD1 Member

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    If the lands at the muzzle of your barrel appear to be worn down, I'd recommend that you first shorten the barrel about the 1" you say the 'N' drill will enter the muzzle and re-crown the barrel: the crown, which is the last part of the bore to influence the bullet, is also about the most significant in terms of accuracy in an otherwise good barrel. Also, FWIW, I've found typical drill bits to be up to .002" or so smaller than the nominal diameter, so you may be getting a misleading impression of the actual bore diameter, which, as you say, should be .300" - I'd suggest you measure the drill shank to be sure. I'm not sure what the permissible variation in wartime 1917 rifle bores was, but even a .002" larger bore should not be a cause of inaccuracy if you are using bullets of full groove diameter.
    Then, because an accurate rifle is a system of assembled parts, I recommend you check your stock to see if the bedding is correct, and free-float the barrel is it is not already in that condition.

    PRD1 - mhb - MIke
     
  4. JDinFbg

    JDinFbg Member

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    Location:
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    I have bedded the action with Devcon epoxy steel and free-floated the barrel with generous clearance. Everything seems tight and there are no stress points. The crown at the muzzle looks good. The bedding did indeed help the accuracy, but I'm still a long way from this being a 1 MOA rifle. Thus, the barrel is the last unknown. If I could confirm that the only 'lose spot' in the barrel was the last inch or so, shortening the barrel further would be an option, but I'd like some way to gauge the uniformity before I would do that.
     
  5. PRD1

    PRD1 Member

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    Barrel makers use an air gauge to find variations in bore diameter. The U.S. armories used to use a 'star gauge', which is an internal micrometer with a very long shank, and heads appropriate for measuring either bore or groove diameter, but these are not readily available and would be very expensive. Pin gauges or any other cylindrical gauge cannot readily find loose spots; only the smallest point(s) in the bore. Pushing a lead slug slowly through a barrel can sometimes identify loose places, if they are fairly prominent. But, unless your barrel has visible rings or other defects in the bore, it is unlikely that the bore diameter varies much over its length. The bore diameter of the barrel was established by reaming, and a properly designed, maintained and operated reamer does not cut much oversize: any small variations you might find in your barrel are most likely due to harder/softer spots in the steel and, possibly, the influence of chips produced by passage of the reamer.
    So, from the information you have provided (stating that the rifling appears worn down at the muzzle), I suspect that the cause of inaccuracy is that issue, and not possible variations in bore diameter. NOS 1917 barrels are still available, and Criterion makes excellent new ones, if you are unable to reach the accuracy level you want with your original barrel.

    PRD1 - mhb - MIke
     
  6. SGW Gunsmith

    SGW Gunsmith Member

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    I'm assuming you used a lead slug to push through your bore? Lead is considered a "dead metal" and will only help with determining the tight areas in your bore. The lead will not expand outward while slugging the bore. Using a caliper is difficult over what the slug diameter is providing you. What you might consider is, checking into a local manufacturer who would probably have what's called an "odd flute micrometer" in 1-inch capacity. Those micrometers have a "V" anvil opposite the measurement barrel that will easily measure the diameter on items such as 3, 5 and seven flute end mills. They could tell you right up front what the tightest area your bore has.
     
  7. Laphroaig

    Laphroaig Member

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    I bought a Model of 1917 with a .311 groove diameter that was determined by slugging the barrel as you did. Fortunately, the rifle had some serious headspace issues, and the seller exchanged it for a very nice Eddystone with a .308 bore. Its now one of my favorite rifles.

    My take on it was that some of the early manufacture 1917's were fitted with barrels originally intended to be chambered for .303 Pattern 14 Enfields. I remember doing some Google research that confirmed my suspicions.

    I'm not aware of any way a common person can gauge a bore for its entire length. Of course, you must be aware of 30 caliber muzzle gauges.

    https://www.fulton-armory.com/muzzleweargaugefor300-303bores.aspx
     
  8. fguffey

    fguffey Member

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    Common person? I made a few tools for determining if a barrel was bent. I also make tapered gages; I no longer have enough ambition to convince someone it can be done.

    F. Guffey
     
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