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Thread: Measuring your bore

  1. #1
    Boolit Master

    Join Date
    Mar 2011
    Utah (Salt Lake Area)

    Measuring your bore

    Hi all, I was slugging my .270 bore to see what it was. I have a micrometer. The gun is a model 70 from the late 60's. The grooves are easy to measure even though it looks to be a five grooved barrel. My question is: how do you measure the actual bore dimension? I need to find the difference between the lands and the grooves so I can paper patch some boolits.The slug is so small I can't use my mic. I'm thinking maybe I will need to buy some pin gauges. How do you all do it? I have a dial caliper but that seems a little coarse. Anyway, thanks for any feedback, Gary

  2. #2
    Boolit Master

    Join Date
    Mar 2005
    Bloomfield, Nebraska
    I have a miike with interchangable heads if you have a GOOD set of dial calipers and keep them clean and learn how to read and set them you can come to a half of a thou.

  3. #3
    Boolit Master 303Guy's Avatar
    Join Date
    Oct 2008
    New Zealand
    Well, truthfully, if you are wanting to paper patch then the bore and groove diameter is not all that important. It's the throat you need to measure. Likewise the bore at the throat and for that a rod of some sort that only just enters will give an indication. The boolit core should be a shade larger than bore and the patched diameter large enough to fit the throat snugly and touch the leade.

    I have measured a 303 Brit using a slug and micrometer and it quite tricky being a five groove. Worse is the softness of the slug. Best to go to a machine shop that has a set of pin gauges. They likey won't even charge you. One could measure the groove diameter using a turned brass rod with ridges and a hole gauge.
    Rest In Peace My Son (01/06/1986 - 14/01/2014)

    ''Assume everything that moves is a human before identifying as otherwise''

  4. #4
    Boolit Master
    Join Date
    Mar 2010
    Hello, GaryN. When I was setting up for P.P. the .22 Hornet & .222Rem. I chucked up lead G.G. bullets in bench lathe collet & turned to various dia.'s I figured I was about there when pushed into muzzle showed very light pressure marks from rifling..but not cutting into surface.

  5. #5
    Boolit Master uscra112's Avatar
    Join Date
    Jun 2006
    Switzerland of Ohio
    To get groove dimension, wrap a very thin feeler gage around the slug, measure over that, then subtract twice the thickness of the feeler gage. To get bore dimension, it's best to use pin gages.
    So long, and thanks for all the fish.

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