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Thread: Need help with chamber reamer specs

  1. #1
    Senior Member  
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    Need help with chamber reamer specs

    I am having a rifle made in 6.5 TCU, and didn't like the available reamer specs, so I decided to have a custom reamer made. However, I am ignorant! I don't really know what to specify. And it's not like I can just put together a cartridge and ship it to PTG for them to copy, because all I can make is a pre-fireformed example.

    Take for example, this drawing of .223 Ackley:

    Tell me if I'm reading that print right. It looks like the step-down starts at 1.768 from the bolt face and ends at 1.7807. Next is the freebore (although for some reason the word "LEAD" is there) ending at 1.886 from the bolt face. Next is the leade (although for some reason the word "THROAT" is there) which ends at 2.029 from the bolt face. In that area the lands ramp up from zero to max height. Is that correct? And I suppose that dimension to the far right, .2170, is the diameter across the lands? (Which has nothing to do with the reamer, but is a barrel spec.)

    It seems different people use the words "leade", "freebore" and "throat" differently. Very confusing.

    Now, I loaded a cartridge, using a Nosler 120BT with the base at the bottom of the neck. Then I took a caliper set to .262, just under bore diameter, and moved that back on the ogive until it stopped. I then measured from that point back to the neck of the case. That gives me the freebore, sorta. However, fireforming will move the neck back, right? I don't know how much. Also, there has to be some clearance between the end of the case neck and the step-down to the freebore, and I don't know what that dimension normally is. Finally one normally wants a bit of clearance between the ogive and the rifling, say .020. But I don't know if my going with .262 is a good way to find the end of the ogive either.

    Also wondering about neck diameter. Looks like the loaded round is .286 at the front of the neck and maybe .288 at the back (hard to say for sure there). I read somewhere that 2 to 3 thousandths over loaded diameter is good. Use .290?

    Any advice on this will be appreciated!
    Last edited by Paul47; August 3rd, 2010 at 07:30 PM.

  2. #2
    Senior Member  
    Join Date
    NW Ohio
    I can't help you on your specific details, except to say that the drawings for the reamer are MORE LIKELY TO BE CORRECT THAN YOU ARE.

    Also, it is a 6.5 TC/U, with specifications set by Mr. Uglade.

    If you change those specifications you will NO LONGER have a 6.5 TC/U! You will need to rename it to another name as you can't use someone else's cartridge name with changed specifications.

    Most of the times, the designer designs the ROUGH OUTLINE and lets the reamer maker make the fine decisions, they (the reamer people) know WAY more about it that the average kitchen table designer does.


  3. #3
    Senior Member  
    Join Date
    Washington State
    Consider what your desired end result is. You could be looking for more accuracy--a slightly tighter (match) chamber and throat could be cut that requires neck turning. Maybe a custom chamber can be cut where you can specify how far you want the bullet off the lands in for a seated cartridge.

    Contact the reamer manufacturer first, and talk with them about your desired end result. Chances are, they might have a reamer in stock already.
    Hiding in plain sight....

  4. #4
    Senior Member  
    Join Date
    London, Ont.
    "...didn't like the available reamer specs..." Change the reamer and you change the cartridge.
    4-D Products will rent you the proper reamer. Rent the headspace guages too.

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