Chamber concentricity

Discussion in 'Gunsmithing' started by toklat04, Aug 1, 2008.

  1. toklat04

    toklat04 Member

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    I have fire formed a few cases in my new 300 rum sendero and i am measureing .002 neck run out on my cases. is this an exceptable amount of runout or should I talk to Rem about the chamber
    Thanks Toklat
     
  2. boomtube

    boomtube Well-Known Member

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    I would at least talk to them. But, it's likely they will tell you that .002" is within factory tolerance.

    Is that run-out measured after firing or after resizing?
     

  3. NesikaChad

    NesikaChad Well-Known Member

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    The ONLY proper way to measure chamber runout is to pull the barrel from the action, indicate it in a lathe through the spindle bore, datum off the bore centerline, and then measuring how far off the chamber is from there along a few different spots, the shoulder area, mid case, and then the web region. If your stylus will reach, do the neck too. Keep in mind that the bearing cassettes in the lathe need to be nice and tight as well otherwise you'll just tolerance stack the dimensions and it won't tell you anything anyways.

    (This is why I advocate chambering lathes with tapered gamut type spindle bearings ='s zero runout in theory)

    Also understand that it is next to impossible (meaning, it can't be done) to indicate a barrel to .00000" If a guy gets it to .0005" he's doing real well. Some will argue this by stating that they shove gauge pins in the bore and then go off of them and then its easy to get the zero runout.

    Perhaps. But you still have to have a clearance between the pins and the bore. You have to otherwise it'd take a hammer or press to get them in there. So, there's still a tolerance stack, however small it may be. That and I don't care to shove a 62-65 rockwell gauge pin in a precision machined/lapped surface that bullets travel down. I know me and I'd screw the bore up. When I indicate a barrel, the process is rather "kinky" so that I index the bore exactly where the bullet enters the barrel and where it exits. What happens in between I can't control anyways so I don't care.

    I go right off the bore and indicate the "bumps" in the barrel. When I get to or under .0005" I stop and start making chips. Hasn't failed me yet.

    I'd bet you'll find that using this method, factory barrels are out by even more than .002" in a great many instances. In fact, that'd be a good one.

    Your cartridges, fire formed or not, are not qualified gauges or "castings" of the chamber so it won't tell you much.

    Sorry.

    Good luck.

    ~NC

    .
     
  4. davewilson

    davewilson Well-Known Member

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    last week i was watching a smith thread the end of a barreled action for a brake.it was a production gun, don't even know which make. what was painfully obvious was the unbelievable amount of runout the chamber area had right next to the recoil lug. the outside of the barrel was bouncing up and down more than a 1/16 of an inch. i would say close to .090 if i had to guess. he said this was a very common occurance with production rifles.

    we're talking about chamber to barrel bore runout, i think the original question was the runout of the neck and case body. sorry if we're off subject.
     
  5. James Jones

    James Jones Well-Known Member

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    I personaly use range rods to dial in a barrel and I understand that their will be a little slack that won't be read , I've never had a problem with it though.

    I have also seen a gunsmith chamber is barrels about the same way Chad does only he just indicates on where the bullet enters tha barrel (throat)and in the web area and reads off the flats. His theory is that it doesen't matter if the barrel makes a 45 degree run , if the bullet doesen't get strated strait it'll never shoot strait , he doesn't concern himself with any barrel curve , all hes worried about is making sure that the bullet hits the same spot each time (bench rest) and that can't happebn unless it gets strarted strait.

    I rechambered a savage 308 heavy barrel once to 300 WSM ,I set the barrel up and indicated on the old chamber and just foir kicks I inserted a range rod in the bore on the chamber end and was amazed how bad it was out probably .020+" , so i reindicated the barrel off the bore then bored the old chamber out to true then rechambered.
     
  6. NesikaChad

    NesikaChad Well-Known Member

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    James is on to something. . .

    This is a subject I don't often talk about, but what the hell.

    James, your right. Dead nuts right. What goes on between point A and point B don't matter.

    BUT

    The path from A to B does. (if that makes any sense)

    There's accuracy in how the barrel is clocked. Sometimes its significant. I noticed this (sorta) shortly after I started chambering barrels. Glen Harrison (my old boss and former owner of Nesika) would have long discussions (arguments-playful) over if it really mattered and how much.

    The "theory" was/is that this may very well be the distinction between good barrels and great ones. Just how the silly things wind up clocked on the action during final assembly.

    Swooping 12 to 12 seems to produce the best accuracy.

    Just an FYI.