Barrel throating pros & cons

Discussion in 'Gunsmithing' started by jrw1976, Aug 15, 2010.

  1. jrw1976

    jrw1976 Well-Known Member

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    I would like to know what the process for barrel throating is? I would like to know if throating a barrel for a specific bullet warrents the time effort and expensive of doing it? Also, if I was to throat for a specific bullet how will it effect the accuracy of a different bullet configuration if I decide to try something different down the road? Oh by the way I am thinking about building a custom gun soon in 7mm rem mag and I am just trying to get my ducks in a row before start the build with all my decissions made first.
    Thanks for the info in advance, JRW
     
  2. NesikaChad

    NesikaChad Well-Known Member

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    In my shop I try to stick to what I call the "girlfriend or wife" doctrine.

    What that means is only one particular hard cylindrical object ever enters the hole. . . I don't care to stack reamer upon reamer as in my head it just invites the potential for more TIR (total indicated runout) If a cst wants a "X" cartridge with a "Y" bullet and I don't have a reamer made for the COAL I order one up.

    That being said I know of many who chase chambers with a throating reamer and the guns shoot just fine. They shoot quite well in fact.

    It's a quirky thing for me.


    Essentially the primary purpose for doing this is typically so that a chamber can accept heavier (longer) bullets without having a portion of the case swallowing up the bullet in order for the thing to chamber. If your gun was chambered/throated for 40 grain bullets and you suddenly decide you want to shoot an 80 grain Sierra your probably going to discover that a good portion of the bullet is going to be squished down inside the neck. Throating the chamber will allow you to seat the bullet out further to take advantage of case capacity.


    It can also be used to breath a little more life in a barrel that's destined for the old folks home. My best friend David Karcher's National Championship win in 2002 at Perry was done on a barrel with over 7500 rounds through it. Every time it opens up at the 600, we just chase it with a reamer and it comes back. (for awhile) He's since decided to have a new tube installed. When the throat starts to look like a dry lake bed you just chase it out a little and then seat your bullets out a little further. In some cases you'll find that you don't even have to do that. More freebore doesn't always make the gun fussy about seating depth.

    Hope this helped.

    C
     

  3. jrw1976

    jrw1976 Well-Known Member

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    thank you very much that helps greatly.
     
  4. B23

    B23 Well-Known Member

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    Chad, or anyone for that matter, would you please explain for the ignorant like myself what actually happens or is taking place when the freebore portion is being created. The names of the tools/reamers or whatever you use would help me understand it better too.

    I, sometimes, think I understand it but then I'll read something else and it sends my brain and thought process in another direction.

    I often joke that I am mostly ignorant not stupid. Help educate make me and I will no longer be ignorant. If I still don't get it after being educated, well, then I'm just stupid. :)
     
  5. BobbyL

    BobbyL Well-Known Member

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    Most reamers have the freebore already in front of the neck. Some gunsmiths order the reamers without freebore then use a throating reamer to put the throat in it. Most freebores are 1 deg 30 min. So you are cutting taper on the lands in the rifle to allow the bullets to be seated out to length. When i set one up i use a case and seat the bullet so the pressure ring is just starting to enter the neck. This gives us the best accuracy and allows for the most FPS by using more powder. The more room in the case the less the pressure will build with a specific charge. This has to be done carefully or you wont have enough bullet left in the neck to shoot it.
     
  6. B23

    B23 Well-Known Member

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    When the freebore is cut into the bore part of the tube is that first part of the bore's rifling cut completely out to were it is now a smooth bore or is the rifling only cut away enough as to not come in contact with the bullet until the bullet is up in the bore far enough to touch the rifling just past the freebore part of the tube???

    Freebore and exactly how it is created/cut is something I have always wondered about so I appreciate any and all explanations. I have zero interest in building my own guns but I would, at the least, like to be able to know what the heck they're talking about. :)
     
  7. jrw1976

    jrw1976 Well-Known Member

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    Ok if I am understanding this correctly, throating a barrel lengthens out the area between the edge of the case neck and the leading edge of the rifling. If this is true is this space tapered into the rifling and if so can you get different angled reamers or is this space square to the rifling. I am also curious what the correct seating depth of a bullet in the cartridge. I am assuming that idealy you would want the base edge of the bullet no deeper or no shallower than the neck shoulder junction (correct?). JRW
     
  8. BobbyL

    BobbyL Well-Known Member

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    Its 1 deg 30 mins or 1.5 degrees. I have seen them higher towards the 3 degrees but the 1 30 is standard. Yes you want the gas ring or the boattail bearing surface junction right at or up in the neck of the case above the neck shoulder intersect.
     
  9. jrw1976

    jrw1976 Well-Known Member

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    Perfect it is starting to make sense to me now. Thank you, JRW
     
  10. Crowe284

    Crowe284 Well-Known Member

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    This is a very old thread I know. I am going down this road of having a barrel throated for a bullet. Can somebody explain to me what the most common way to set up a dummy round is? I have heard 15 things from 15 different people.

    The biggest thing that I am hung up on is whether or not to factor jump to the lands AND still keep bearing surface above neck shoulder junction? If so, how do you know where to start if you haven't developed a load and know how much the bullet will like to jump?? I have looked at a lot of data on people's loads all over the Internet, but in a gun that has not even been built yet, i would assume that it is just a little better than a wild ass quess...!

    I will be throating for 180 gr hybrid (7saum). Everything I have read leads me to believe that I will find an accuracy node for seating depth somewhere between .003 and .025 off lands. So.....would it be smart to measure the OAL with boat tail bearing surface junction at neck shoulder junction then add .025 thou for jump and another .005 to stay above neck shoulder junction???

    If someone has done this with the 7saum on a long action using the 180 hybrid, let me know what you did.

    Any gunsmiths out there, steer me down the right road please!
     
  11. J E Custom

    J E Custom Well-Known Member

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    +1

    I am also one of those that don't like to throat a chamber after it has been chambered. If you "Must"
    throat, Buy a reamer with the added throat to it. This way there is no possibility of getting anything off the bore centerline. If everything is aligned perfectly and the throating reamer if held in perfect alignment everything should be ok, but the risk are greater when throating later.

    Another thing is that when throating for a specific bullet there is a risk of it not shooting well and then you may not find 'any' bullet that will shoot well.

    Normally when going to a heavier bullet less powder is needed anyway so I personally don't see any advantage in throating because it changes other things (Mag lengths, pressure and in many cases it can degrade accuracy if the barrel doesn't like the bullet.

    There are many ways to improve accuracy and velocity without throating.

    This is just my opinion.

    J E CUSTOM
     
  12. specweldtom

    specweldtom Well-Known Member

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    I have used throating reamers so that longer, heavier bullets or blunter ogive bullets, in hot loads, could be seated well off the rifling, usually long range rifles with Sierra mkhp bullets.
    After I got a Hawkeye borescope, I stopped using separate throating reamers. Every throat done with a throating reamer showed circular tool marks (weasel words for rings). The rifles shot great so I never knew it until I started 'scoping them. OK, so if the rifles shoot, what's the problem? Simple, now I know the rings are there, and can't make myself do it anymore.
    I agree with Nesika Chad and JE Custom. Just order a reamer with the desired throat ground into it.
    I do not know why the throating reamers made rings, but I talked with two top reamer grinders and they made several recommendations to try to stop them. None worked. I have since theorized that the throating reamer, even though piloted, doesn't have the stiffness, torsional rigidity, or stability of a chambering reamer. May not clear chips away as well either....Just my best guess.

    Goof hunting, Tom
     
  13. specweldtom

    specweldtom Well-Known Member

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    B23, freebore is a bullet diameter cylindrical section of the chamber immediately in front of the case mouth. Throat (or leade) is a tapered section in front of the freebore that eases the bullet into the rifling. Many chambers don't have freebore, so the throat starts right in front of the case mouth. Some well-known freebored chambers are the Weatherby calibers... 0.162" for the .224 Wby Mag and up to 0.756" for the .378 and .460 Wby Mags. THAT'S NOT A MISSPRINT. 3/4" of freebore! Some not-so well-known freebored chambers are the Remington Ultra Mag calibers.
    Speaking of the Weatherbys, Their factory ammo cannot be used in custom short or no freebore chambers. I saw a .30 x .378 custom no freebore rifle that bent the bottom metal and blew the floorplate open with factory 180gr ammo. Two other custom .30 x .378 rifles with factory dimension chambers shot the exact same ammo uneventfully. One of those rifles shot 5 handloads into 0.093" at 100 yds. The other shot 5 handloads into 0.237" at 100 yds. Both were heavy loaded, the first, I don't remember, The second (mine) was a 190gr Sierra mkhp, that I hunted with.
    To quote a LRH member from years ago: "I do not fear freebore".
    In fact on the boomers, I fear not having it.
    I know the benchresters shoot no-jump loads, but they aren't loading over 100grs of powder either.

    I get long-winded, but old guys do that.

    Tom
     
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  14. J E Custom

    J E Custom Well-Known Member

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    Here is a drawing that may help to understand all of the parts of a chamber and there purpose.

    Some confuse the names and the differences and this may help. Note these dimensions change from cartridge to cartridge but the different parts are still called the same even with the dimensional changes.

    https://www.google.com/search?q=Cha...quYAQ#imgrc=YoWoMK0C652NKM:&spf=1500924649489

    J E CUSTOM