FREEBORE, AND THE MEASURING OF IT.

Discussion in 'Main Message Board' started by golfer2b2000, Nov 11, 2018.

  1. golfer2b2000

    golfer2b2000 Silver $$ Contributor

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    I would like to know, and understand freebore on a rifle.
    I see members on here wanting to possibly purchase a rifle, and sometimes the amount of freebore on them sometimes makes their mind up on whether to purchase, or not to purchase a firearm from someone.
    I have read articles on how to measure it and one of the easiest ways I see is to just take a bullet, turn it around 180 degrees, place it in a case that will allow the bullet to be pushed into the case as the round is chambered, and measure the distance from the end of the bullet to the top of the cartridge. This distance is now the freebore?
    Is there other ways to accuratly measure this dimension?
     
  2. Dusty Stevens

    Dusty Stevens COVFEFE- Thread Derail Crew Gold $$ Contributor

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    Theres a few ways to measure it. The most reliable is by the reamer print IF the reamer maker can be trusted. The second way is by using the exact same bullet finding zero touch in one that you trust then comparing (if it has the same leade angle). Its a taking somebodys word for it thing in the end.
     
  3. LA50SHOOTER

    LA50SHOOTER Silver $$ Contributor

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    I offer this in effort to explain from a reamer print. - the dimension (U & V) shown in the below drawing.

    What I do to calculate the measurement on a new build is get the brass & bullet(s) that I intend to use. - I like to place the lower bearing surface above the cases neck/shoulder junction. - By using a pair of comparators and a set of calipers I can measure the bearing surface "with some degree accuracy, depending on the exact size of the comparator insert" - Then do the math and establish the seating depth, depending on exactly where it is desired to have the lower bearing surface sit in the case neck. - To check the free-bore then as Dusty has pointed out - find zero touch and a comparator with the dial caliper & modified case.



    free-bore.jpg
     
  4. golfer2b2000

    golfer2b2000 Silver $$ Contributor

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    Freebore seems to never be discussed among production rifles. Is it correct to say that freebore sould only really be considered in custom, or highly accurate builds?
    I live in a state that almost everyone hunts to some extent. They know good firearms, and optics. But if you were to try and talk to them about freebore, they would probably not have any clue what you are talking about.
     
  5. Dusty Stevens

    Dusty Stevens COVFEFE- Thread Derail Crew Gold $$ Contributor

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    Freebore in a factory gun is huge. They do this to keep you from jamming a bullet and overpressuring it.
     
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  6. Mike McCasland

    Mike McCasland Gold $$ Contributor

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    The reason its not discussed in factory rifles is because it's not a variable you can control, and it will change over the years as they use new/different reamers. It's basically a "you get what you get" type situation.


    The reason you see it on custom rifles is because people usually know the bullet they want to shoot (usually based on bc or availability), and they're trying to build the rifle around it. Specifically so they can touch the lands with the bullet, yet have the bullet seated above the donut/shoulder
     
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  7. golfer2b2000

    golfer2b2000 Silver $$ Contributor

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    So, If I take a plug gage, and put it into a case that I have fired, and it slides back and forth into the case mouth without a lot of drag, and then chamber it into the rifle, and allow the plug gage to touch the rifling and push the plug gage back into the case to where the bolt closes, and then measure that overall measurment and subtract it from the overall length of the case, will that give me the freebore??
     
  8. milanuk

    milanuk Gold $$ Contributor

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    Not really. You still have to account for the distance from the end of the case to the end of the chamber (where it starts the 45° angle in the print shown above), then the length of that angled portion... *then* the 'freebore' starts.
     
  9. GSPV

    GSPV A failure to plan is a plan for failure.

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    We don’t really care about freebore, per se, we care about being able to shoot the bullet that we want to shoot, with a workable jump or jam, seated a workable amount into the neck. For repeaters, mag length might come into play as well.

    A given freebore will be known to “work” with a certain bullet in a certain cartridge. For example, say, .220 in the 284 with Berger 180 Hybrids.
     
  10. Ned Ludd

    Ned Ludd Gold $$ Contributor

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    Freebore really matters most in terms of where it allows you to seat your bullet of choice. Otherwise, it's not a number that means very much, either in the grand scheme of things, or in daily reloading routines. Of course, having an actual numerical value from a reamer print is psychologically satisfying, but it is not essential. Using a tool such as the Hornady OAL gauge and making dummy rounds is so easy that many people won't go to the effort of trying to accurately "measure" the actual freebore. As outlined above, most of the simpler methods for measurement are indirect, anyhow.

    The reason many would like to know their freebore value is so they can estimate where a particular bullet will be positioned in the case neck at some specified seating depth off/into the lands, both in terms of seating depth and how far down in the case the bullet will be. Seating depth optimum is best evaluated by how the rifle groups, not some measurement that may or may not be accurate. Determining where a partcular bullet's boattail/bearing surface junction is positioned in relation to the case neck/shoulder junction at a given seating depth is also a consideration. However, you have a fairly generous margin for that; placing the boattail/bearing surface junction somewhere above the case neck/shoulder junction, but not much more than about halfway out the neck is what most aim for. Frankly, visual inspection by comparison to a naked bullet is more than sufficient; again, an accurate freebore "measurement" is not necessary.
     
    Last edited: Nov 24, 2019
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  11. RedfootRanch

    RedfootRanch Silver $$ Contributor

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    Ned, for clarification, you are referring to the boattail/bearing surface junction here. Correct?
     
  12. Ned Ludd

    Ned Ludd Gold $$ Contributor

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    Yes - I accidentally left out the term, "junction". I corrected the post to reflect that.
     
  13. RedfootRanch

    RedfootRanch Silver $$ Contributor

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    Thank you, sir. Just making sure...
     
  14. K22

    K22

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    The dimension that has meaning to me is the amount seating distance of the bullet to the lands. This is easily measured with a simple tool like the Frankfort Arsenal tool or you can make you own.

    Once that distance is known for that bullet / rifle combo the decision then becomes the over all length of the cartridge you are going to use. For me, the cartridge length has to satisfy three criteria: 1.) it has to fit the magazine, 2.) it has to permit enough bullet depth in the neck to provide adequate bullet tension and help reduce run out usually a depth consisting of a minimum of one bullet diameter excluding the boat tail portion, and 3.) it has to be approximately no closer that .010" to the lands to prevent jamming allowing for variations in ogives in the bullets I use.

    After those three criteria are satisfied I then can experiment with varying the length if necessary to refine group size. Surprisingly enough I discovered with a few rifles that they perform better with more than less jump.

    All my Remington 700's and 7's with factory barrels have a lot of free bore. My 700 varmint is a 1/4 moa rifle with tailored reloads that has a considerable amount of jump because the large free bore.
     

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