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go-no go-field gauge??

 
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  #1  
Old 06-22-2013, 08:27 PM
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Join Date: Nov 2012
Posts: 39
go-no go-field gauge??

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!!!
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  #2  
Old 06-23-2013, 02:50 AM
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Join Date: Jun 2013
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Re: go-no go-field gauge??

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.
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  #3  
Old 06-23-2013, 08:24 AM
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Re: go-no go-field gauge??

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!!
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  #4  
Old 06-24-2013, 10:04 AM
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Location: SW Wisconsin
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Re: go-no go-field gauge??

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.
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  #5  
Old 06-24-2013, 02:38 PM
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Re: go-no go-field gauge??

Thanx msalm! That's some great sizing info to know and have. I really appreciate your input!!
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  #6  
Old 06-25-2013, 12:38 PM
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Location: SW Wisconsin
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Re: go-no go-field gauge??

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.
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  #7  
Old 06-25-2013, 01:00 PM
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Join Date: Jun 2013
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Re: go-no go-field gauge??

Quote:
Originally Posted by msalm View Post
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.
Wouldn't it be easier to cut the chamber short and after assy use a finish reamer to set headspace?
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