go-no go-field gauge??

Discussion in 'AR15/10 Rifles' started by dirttyt, Jun 22, 2013.

  1. dirttyt

    dirttyt Active Member

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    My question to all is: I am doin my first AR15 .223/5.56 build and am curious as to which gauge I need??
    I have read and compiled every answer you could imagine!! I am pretty sure all I need is the field gauge, and if the bolt closes on that, then it's way too loose and dangerous!!

    Although, I have also read that because AR components are built to spec, it is the one gun you don't have to worry about using a head space gauge at all!!

    I would greatly appreciate any and all input you seasoned builders have to offer, thanx!!!
     
  2. Greenleader

    Greenleader Active Member

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    It's always a good idea to check. That said, I havn't had an AR15 not headspace correctly in a very long time. A field gauge would work fine for a just to be sure check. I would be tempted to check for people in your area that might have gauges. I'm in W Wa and would be happy to let you use mine.
     

  3. dirttyt

    dirttyt Active Member

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    Thanx greenleader!! That is a real nice offer! I am in N E Ohio, and still have a couple of local gun shops to poke around at!!

    As I said, this is my first AR build, what I also need to add is, this won't be my last (I already have my stripped lower sitting here for my second build) so I am just tryin to get everyone's take on what gauge if any I would need. Thanx again for your offer and input!!
     
  4. msalm

    msalm Well-Known Member

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    If sticking with standard components (5.56), there's a very good chance you don't need to worry about it. If you want to check just to be sure take out the ejector from your bolt and install a new unloaded case in the barrel and see how much play you have. You can add a layer of tape or two and see if it will still chamber. New brass is .002 to .004 thou under a 'go' gage (on average) so take that into consideration. This is just a feel good step to see if things are 'close', and not a for sure 'it's spot on'. Again, with the specs that are out there I probably wouldn't have any hesitation in just throwing a bolt in and shooting it and seeing how the cases looked. Only way to really know is with a standard go and no-go gage. The field gage is another .004 larger than the no-go.

    If you have a depth mic, before you install your barrel slip an empty case into the chamber and use a depth mic to measure from the end of the extension to the case head. Your measurement should be around .370" + or - . If your in that ballpark things are probably good. If you're more than .007" off (to the greater than) things are not right and I'd check with gages.
     
  5. dirttyt

    dirttyt Active Member

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    Thanx msalm! That's some great sizing info to know and have. I really appreciate your input!!
     
  6. msalm

    msalm Well-Known Member

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    The math behind the depth reading is that headspace on a standard AR tenon is typically .750". I measure the bolt and barrel extension and chamber to that spec when I make AR barrels. Measurements vary from .750 to .752 on average with .750" being the norm. An easy way to measure headspace while chambering is to thread the extension on the tenon and measure from the back of it to the headspace gage. Most every barrel extension I've measured comes in at 1.121" with the exception of BAT extensions. They will measure 1.122". Take that measurement and subtract the .750-.752" and you will come up with an average difference of .370". This is the distance from the rear of the barrel extension to the back of the headspace gage for a zero headspace. You can add up to .004" from that and still be within spec, any more and you have excessive headspace. If doing this with brand new brass, you will have a slightly greater measurement depending on how undersized the new brass is.
     
  7. Greenleader

    Greenleader Active Member

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    Wouldn't it be easier to cut the chamber short and after assy use a finish reamer to set headspace?
     
  8. msalm

    msalm Well-Known Member

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    Not when you have it set up in the lathe to chamber correctly in the first place. I can measure the specific headspace of a bolt/extension combination, cut the tenon, thread, chamber and cut the chamfer all in one setup, why would I want to take it out of the lathe, torque on the extenstion, then have to ream again? Doesn't make sense to me at all. On AR's I ream to go + .002 as semi-auto's don't like tight chambers. I haven't timed myself but after a barrel is dialed in through the headstock (less than 5 minutes) I can have it done in less than an hour. Finishing up the contour, polishing, cutting and crowning takes another hour.
     
  9. dirttyt

    dirttyt Active Member

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    You guys are miles ahead of me, I am just tryin to make sure the darn thing don't blow up on me!!! ( I love tryin to soak in all the knowledge though)