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Measure bore diameter without slugging?

Discussion in 'Handloading and Reloading' started by bainter1212, Feb 13, 2013.

  1. bainter1212

    bainter1212 Well-Known Member

    Hey folks,
    I've got an old Mosin that was poorly counterbored, so I cut down the barrel and re-crowned with 11 degree target crown. I want to handload for this rifle once I get it finished, and I understand that these old Mosins often have bores that measure out larger than .308.

    My question is this - is there any reason that the bore diameter can't be measured at the crown, now that it's nice and sharp? My calipers have jaws that are meant to measure "inside" , on the backside of the caliper. So far, measuring with the calipers has given me a consistent groove-to-groove reading of .3105. As always, I measured about 15 times, re-zeroing my calipers each time. A couple of times the # was different, however by far .3105 was the most consistent number that it came out to.
    Can I rely on this number? If so, should I load .310 or .311 bullets?
    Any response is appreciated, thanks a bunch guys.
  2. rcmodel

    rcmodel Member in memoriam


    Bigger is always better, regardless of what you think it measures without slugging it.

  3. wgaynor

    wgaynor Well-Known Member

    Slugging a bore is so easy, I'd do it to ensure accuracy and match that to what your calipers are currently telling you. That way, you know if your calipers were right, and you have confident in what you think is correct.

    When I slugged my Mosin's bore, is was .313

    Like RC said, bigger is better.
  4. 45lcshooter

    45lcshooter Well-Known Member

    Go with the closest whole number RC is right.
  5. bainter1212

    bainter1212 Well-Known Member

    Thanks guys. .311 it is.

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