AR headspace question

HappyJack · Oct 1, 2023

I have a M&P 15 sport and had wanted an AR in 7.62x39 for a long time. Found a deal on an AR Stoner upper and PSA lower. Took it out and ran a box of brass case (15) ammo through it and it did well. I read that steel case ammo would break the extractor. Then I found a good deal on a advertised super strong Faxton bolt. I switched it out and took one of the fired empties and chambered it. Let the bolt slam forward. I pushed the rear pin and the upper slide clear and up and the rear of the BCG was flush with the back of the upper receiver. Now the package with the Faxton bolt says to have the head space checked. Is this good enough or should I take it to a Smith to have them check it? Plenty of smiths around I just don't know how good or not they are and if I am just spending money for something that I already checked.

Thanks in advance for any help with this.

s&wchad · Oct 1, 2023

This should help...
Midway - How to Check Headspace on an AR-15

WR Moore · Oct 1, 2023

First and foremost, steel case ammo doesn't break the bolt, it excessively loads only the extractor.

Now, a couple of things about the linked article.

1. The NO-GO gauges DO NOT SHOW AN UNSAFE CONDITION! You use a GO gauge to make sure the chamber/bolt will accept a standard cartridge. The NO-GO gauge is a few thousandths longer to make sure the headspace will still be acceptable after the parts wear in. A FIELD gauge is used to show that the head space is now excessive.

2. If checking an assembled rifle, the test is done using the BCG. The end of the carrier should be flush with the rear of the upper with the GO gauge and protruding with the NO-GO gauge (bolt won't lock up). You shouldn't need/use more than gentle thumb pressure to seat the BCG.

To do this, clean the chamber, remove the ejector and spring from the bolt and insert the gauge under the extractor. GENTLY move the BCG forward and see if the results are as in the paragraph above. If there's no extractor and ejector, gently place the gauge in the chamber. A magnet might help with that.

HappyJack · Oct 1, 2023

WR Moore said:
First and foremost, steel case ammo doesn't break the bolt, it excessively loads only the extractor.

Thank you for the reply. I did state in my OP that the reason for the bolt purchase was breakage of the AR Stoner extractor. The AR Stoner has bare metal and tool marks where a stock 5.56 extractor was cut down and is very thin.
Thus the problem with breakage with steel cases. The new purchase has stronger heavier new not cut down and stronger material including the extractor. I just did not take the time to write all the information out above but you can see I did indicate the extractor breakage was the problem I am looking to avoid from the post of others who bought the same upper.

Now, a couple of things about the linked article.

1. The NO-GO gauges DO NOT SHOW AN UNSAFE CONDITION! You use a GO gauge to make sure the chamber/bolt will accept a standard cartridge. The NO-GO gauge is a few thousandths longer to make sure the headspace will still be acceptable after the parts wear in. A FIELD gauge is used to show that the head space is now excessive.

2. If checking an assembled rifle, the test is done using the BCG. The end of the carrier should be flush with the rear of the upper with the GO gauge and protruding with the NO-GO gauge (bolt won't lock up). You shouldn't need/use more than gentle thumb pressure to seat the BCG.

To do this, clean the chamber, remove the ejector and spring from the bolt and insert the gauge under the extractor. GENTLY move the BCG forward and see if the results are as in the paragraph above. If there's no extractor and ejector, gently place the gauge in the chamber. A magnet might help with that.

Does not a case then act the same as a go gauge? Are not the results the same with a empty case as with the go gauge?

WR Moore · Oct 1, 2023

NO, AN EMPTY CASE IS NOT THE SAME AS A GAUGE! The case is made to different specifications than the chamber. The gauge is made to ensure that your chamber meets specifications for a chamber. Therefore, you can't use one to measure the other. You only know that your chamber will accept that particular cartridge case.

There's a machining/manufacturing principle here regarding tolerances from the nominal dimensions on the drawing. Holes (chambers) will not be less than the nominal size, but can be a specified tolerance over size. Things that go into holes (ammunition in this instance) cannot exceed the nominal size but can be a specified tolerance under size. The gauges make sure that your chamber is within tolerances for a chamber.

Example: once upon a time a maker of single action revolvers had an issue with cylinder/base pin fit to the cylinders. The pin & hole had a nominal dimension of 1/4 inch/0.250 in. The pins would be slightly undersize. Well, in the process of losing a few thousandths of pin diameter and gaining a few thousandths of hole in the cylinder, excessive clearance resulted. This caused a new industry making oversize reamers for the frames & cylinders and oversize base pins to bring tolerances back into reasonable fit.

AR headspace question

HappyJack

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s&wchad

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WR Moore

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HappyJack

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WR Moore

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