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Thread: Measuring headspace/chamber/throat/leade dimensions

  1. #1

    Question Measuring headspace/chamber/throat/leade dimensions

    Hi, I've been searching the Internet for information on the actual measurement of dimensions for headspace, the chamber diameter and length, the throat length, the leade angle, and so on, but everything seems to amount to: use a go/no-go gauge set, and you're done!

    That's not what I'm looking for. That amounts to a dummy check. I want to measure, with 1/1000" precision, the actual numerical dimensions for a given barrel and bolt combination, before firing a round. As far as I can tell, cerrosafe castings are the only way to do that without firing a round, but I see mentions of it shrinking as it cools. I've found more directly useful information at this page, but the interpretation of the casts appears to be subjective--not an objective set of measurements.

    How would you exactly measure these parameters, before you ever loaded a round into the firearm, before you ever sat down at your reloading bench to make up ammunition?

  2. #2
    Boolit Master
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    I'm very interested in your questions. I'm wanting to have a custom reamer made so it's the opposite direction.

    To really measure a casting and get down to the degree of leade angle I think you would need a comparator. Not a common piece of equipment to run into outside of work.

  3. #3
    Boolit Master
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    I think you would need to do a pound cast. Fill empty case with lead place in chamber drop in price of soft lead down barrel then with protected rod down barrel and pound it, will take shape of all your asking for. This has been discussed here just have to look for it.

  4. #4
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    A lot of us will use a small charge of powder and cream of wheat as a filler to fireform brass. From that you can pretty much tell where you are since you are most likely to know the caliber of the rifle to start with, then we would seat a round long in a fireformed case with no crimp, chamber it and close the bolt, pull the handle up and use a cleaning rod to gently shove it out, set your seating die with this dummy, remove the dummy and turn the seater down a quarter to half a turn. This will measure your max COA and set the boolit back .020" to .040" from the leade ins of the rifling. You could then start working up loads using published data.

    Admitted this is a much simpler way of doing it, but it works great. All those numbers are only important to the brain. What the dummy will allow you to do is the only part important to the rifle.
    Last edited by DougGuy; 01-05-2021 at 01:45 PM.
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  5. #5
    Boolit Grand Master

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    with math and a good gauge ball off the correct diameter it can be measure with depth mics. A Cad program can give these dimensions off the ball also. In shops vees and dovetails are measured from balls or pins.

  6. #6
    Boolit Master uscra112's Avatar
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    The only tool that I know which will satisfy the O.P.'s desire for .001 precision is a Zeiss scanning CMM. There are contract inspection firms that might do it for him for a few hundred $$. If it weren't that I've been retired for 11 years I'd have some contacts. He might try one of the Zeiss tech centers: https://www.zeiss.com/metrology/serv...locations.html

    We'd been reverse engineering solid objects into 3D CAD models for several years before I retired.

    Caution: Even then there was a wry saying "This thing tells us things we didn't know we didn't want to know."
    Last edited by uscra112; 01-03-2021 at 06:36 PM.
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  7. #7
    The OP was looking for 1 ten thousands on an inch of precision.

  8. #8
    Boolit Master
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    A comparator will do this easily. I use one at work on 20x .001 accurracy is easily achievable.

  9. #9
    Boolit Master uscra112's Avatar
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    Quote Originally Posted by B R Shooter View Post
    The OP was looking for 1 ten thousands on an inch of precision.
    Easily done. Our ordinary machines typically specified "uncertainty" of about 2.5 microns per meter. (Uncertainty is the European way of expressing both precision and accuracy in one number.) Over the space of a rifle chamber it could be better than that.

    Scanning the chamber and throat would be trivial. The real trick would be teaching the machine to scan the leade by scanning up each rifling groove and land individually, so as to get angle and concentricity to the chamber and bore. Unless you start with a CAD model, in which case the post-processor would do it for you.
    Last edited by uscra112; 01-03-2021 at 07:33 PM.
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Abbreviations used in Reloading

BP Bronze Point IMR Improved Military Rifle PTD Pointed
BR Bench Rest M Magnum RN Round Nose
BT Boat Tail PL Power-Lokt SP Soft Point
C Compressed Charge PR Primer SPCL Soft Point "Core-Lokt"
HP Hollow Point PSPCL Pointed Soft Point "Core Lokt" C.O.L. Cartridge Overall Length
PSP Pointed Soft Point Spz Spitzer Point SBT Spitzer Boat Tail
LRN Lead Round Nose LWC Lead Wad Cutter LSWC Lead Semi Wad Cutter
GC Gas Check