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AR headspacing

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supertool73 View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote supertool73 Quote  Post ReplyReply Direct Link To This Post Topic: AR headspacing
    Posted: June/12/2018 at 15:02
So Sarge talked about changing head space on an AR in another topic and I didn't want to hijack that thread any more so I am creating a new one. 

I asked Sarge
Quote How are you adjusting headspace on an ar sarge? Are you taking off the
barrel extension and doing something to it? Or is there some other
trick? Always been curious on how and what parts can be adjusted
   

Sarge replied.
Quote Once I reface the receiver "usually" the bolt won't close on the go
gauge. So I take the chamber reamer and make a light cut until the bolt
will drop on the go gauge. Then check with the no go gauge and if it
checks out you have a very accurate AR. The head space check and chamber
cut is done with the barrel and barrel extension assembled.

So that brings up other curiosities now.  So Sarge and our other gunsmiths or machinists feel free to chime in.  

If you reface the receiver I was under the impression that wouldn't actually change the head space, that for the most part it was created by the bolt and the barrel extension.  That the bolt itself could slightly move back and forth in the carrier slightly to compensate for small differences.  Is this not correct?

Also if you machines much off the from of the receiver could that lead to the extension going into the upper to far and causing it to hang out over the loading ramp part of the upper causing a lip? 

Maybe the amounts are so small this isn't an issue, just curious and trying to wrap my head around all this stuff.  This stuff fascinates me and I always wanting to learn more.

Thanks guys. 


Edited by supertool73 - June/12/2018 at 18:28
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BeltFed View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote BeltFed Quote  Post ReplyReply Direct Link To This Post Posted: June/12/2018 at 17:20
I had a head space issue with a rifle I built. The bolt would not close and lock on a factory round. I just tried different bolts until I found one that would close on a go gauge, and wouldn't close on a no-go gauge. 

There are so many different parts manufacturers that there are bound to be tolerance issues between parts. The question here was; were the bolt lugs shorter, allowing more head space, or was the bolt face set back further, allowing more head space than the original bolt.
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Sgt. D View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Sgt. D Quote  Post ReplyReply Direct Link To This Post Posted: June/12/2018 at 21:33

 "If you reface the receiver I was under the impression that wouldn't actually change the head space, that for the most part it was created by the bolt and the barrel extension.  That the bolt itself could slightly move back and forth in the carrier slightly to compensate for small differences.  Is this not correct?"

That is correct. I too often let different adjustments cause me to mix my replies. When I face a bolt action receiver I have changed the barrel set and have to adjust the bolt relief cut and the chamber.

On many AR's I have worked on, I do true the receiver face and check the head space while I have it apart. I let my comment run together I guess because the last three I did needed the chamber work as well. 

"Also if you machines much off the from of the receiver could that lead to the extension going into the upper to far and causing it to hang out over the loading ramp part of the upper causing a lip?"

You should never need to cut that far back. I doubt that I have ever taken more than .3K off a receiver.

AR15's are much more forgiving than AR10's but some of the things that help a AR10 run well help the AR15's also. Meaning that the cleaner and more consistent the bolt locks up the more accurate the rifle will be. You want to make sure that there is little to no contact with the gas tube when the bolt carrier drives forward. You want the bolt carrier group to be very lightly lapped for the smoothest possible function. Any issues with the lockup effects gas and anything that effects gas/pressure effects velocity. The greater swing you have in velocity the larger your groups are going to be. I have consistently seen low budget AR's  shooting 1"1/2 to 2" groups out of the box go to 1/2 moa with these simple adjustments. Gunsmith simple I mean..............

Thanks for, giving me opportunity to correct my mistakes.

Salute!

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