Bolt, Barrel, Headspace...again...

[shannerdrake]

Hello All, newer to the forum, but a big digester of info. I’m working a Grendel build now and have a 20in AA barrel and AIM Surplus BCG (thanks to a forum member recommendation) on the way.

I have read pages and pages and pages on headspace and the necessity, process, work arounds, gauge rentals, etc. I’m honestly more confused than when I started.

AA customer support said their bolts and barrels were checked and didn’t need headspacing checked. However, I ended up with the AIM bolt. I was going to rent the gauges from reamerrental.com but have read that those are Pacific and problematic for various reasons.

I was going to go the live round/scotch tape or unfired brass/tape route, but have been unable to find brass or ammo (yes I’m building a gun and have no ammo for it). I thoroughly believe that sometime in the next year that ammo will be available.

I’m looking for an honest assessment of my headspace situation with the AIM bolt and AA barrel. Do I need to be concerned? When I built 556s back in college I worked at a range and test fired first round after hours with one hand behind plexiglass then checked my brass to make sure it didn’t split. Not saying that was the best approach.

[Klem]

Sh,

I would skip the GO/NO GO gauges, put a round in it, and fire it from the hip.

AR headspacing is fixed and checked by the gunsmiths that screw barrel extensions onto barrel blanks when they are chambered. This is a design feature of the AR, so that repairs can be more easily performed in the field by lesser-trained armourers. When we buy barrels fitted with an extension they will have already been checked. Some shooters like yourself go the extra step and use GO/NO GO gauges to be sure, but if you are that untrusting of the brand then maybe you should buy a more reputable barrel. If however you are just trying to be thorough then good luck to you.

Bottom line, you don't need the gauges because there is no imperative to check new AR barrels.

The firing from the hip is something I do for the first round of every new barrel, regardless. Then you collect the spent case and that tells you all you need to know about the chamber's dimensions. I would spend money on a Hornady headspace gauge instead. That is a much more useful gauge which I am constantly using for reloading

Back to your situation - there are three scenarios that will occur with firing a live round.

1. The bolt won't close which means too little headspace and if the bolt won't close on a factory round then you are perfectly safe. It can't fire and you take it back for a make-good.
   
2. The bolt closes and it fires the first round and is fine.
   
3. The bolt closes and it fires the first round, but there is too much headspace.
   

In scenario 3 you discover this by measuring the stretch of a fired case. Worse case scenario is it stretches so far the case splits around the middle, maybe into two halves. Then you take it back for a make-good. You are quite safe because as long as the gun goes into battery on a factory round then it does what it's designed to do, which is safely contain most of the high pressure. Where the brass splits (if it splits) there will be a bit of pressure leakage back towards the firer, but not much - a bit of smoke and a bit louder than normal - but still quite safe.

99.99999% of the time everything will be fine and therefore you will never need a set of GO/NO GO gauges. For that .00001% you will still be fine, which affords you the opportunity to save your money.

[Wheelhorse]

Hello All, newer to the forum, but a big digester of info. I’m working a Grendel build now and have a 20in AA barrel and AIM Surplus BCG (thanks to a forum member recommendation) on the way.

I have read pages and pages and pages on headspace and the necessity, process, work arounds, gauge rentals, etc. I’m honestly more confused than when I started.

AA customer support said their bolts and barrels were checked and didn’t need headspacing checked. However, I ended up with the AIM bolt. I was going to rent the gauges from reamerrental.com but have read that those are Pacific and problematic for various reasons.

I was going to go the live round/scotch tape or unfired brass/tape route, but have been unable to find brass or ammo (yes I’m building a gun and have no ammo for it). I thoroughly believe that sometime in the next year that ammo will be available.

I’m looking for an honest assessment of my headspace situation with the AIM bolt and AA barrel. Do I need to be concerned? When I built 556s back in college I worked at a range and test fired first round after hours with one hand behind plexiglass then checked my brass to make sure it didn’t split. Not saying that was the best approach.

Why do you think you have a problem?

[shannerdrake]

What an amazing and excellent response! Thank you so much. I’m not in the slightest a paranoid person and I see my risk approaching near absolute zero if the bolt locks on the round even if the headspace is excessive.

Appreciate you taking time to respond.

BTW I’m a hunter and am active on the hunttalk.com forums and am happy to extend support to my new 6.5 community over on that forum. I have the same username.

[Chance-dg]

Klem, That explaination of ARs and headspacing of barrel and bolts is the most straightforward and simplified I have read. Thanks

[montana]

Sh,

I would skip the GO/NO GO gauges, put a round in it, and fire it from the hip.

AR headspacing is fixed and checked by the gunsmiths that screw barrel extensions onto barrel blanks when they are chambered. This is a design feature of the AR, so that repairs can be more easily performed in the field by lesser-trained armourers. When we buy barrels fitted with an extension they will have already been checked. Some shooters like yourself go the extra step and use GO/NO GO gauges to be sure, but if you are that untrusting of the brand then maybe you should buy a more reputable barrel. If however you are just trying to be thorough then good luck to you.

Bottom line, you don't need the gauges because there is no imperative to check new AR barrels.

The firing from the hip is something I do for the first round of every new barrel, regardless. Then you collect the spent case and that tells you all you need to know about the chamber's dimensions. I would spend money on a Hornady headspace gauge instead. That is a much more useful gauge which I am constantly using for reloading

Back to your situation - there are three scenarios that will occur with firing a live round.

1. The bolt won't close which means too little headspace and if the bolt won't close on a factory round then you are perfectly safe. It can't fire and you take it back for a make-good.
   
2. The bolt closes and it fires the first round and is fine.
   
3. The bolt closes and it fires the first round, but there is too much headspace.
   

In scenario 3 you discover this by measuring the stretch of a fired case. Worse case scenario is it stretches so far the case splits around the middle, maybe into two halves. Then you take it back for a make-good. You are quite safe because as long as the gun goes into battery on a factory round then it does what it's designed to do, which is safely contain most of the high pressure. Where the brass splits (if it splits) there will be a bit of pressure leakage back towards the firer, but not much - a bit of smoke and a bit louder than normal - but still quite safe.

99.99999% of the time everything will be fine and therefore you will never need a set of GO/NO GO gauges. For that .00001% you will still be fine, which affords you the opportunity to save your money.

I had a factory barrel in 5.56 that I used for awhile. The cases would stick every once and awhile which was irritating. I checked the head space with a 5.56 gauge and it did not pass the go gauge. The 223 go gauge barely passed with a selected smaller AR bolt I had. The barrel was marked 5.56, but barely passed the 223 head space gauge. It can be rare for chambers to be out of spec but it does happen.

[Red*Lion]

I have only used a field gauge on AR's. As long as that does not chamber, all good.

[montana]

I have only used a field gauge on AR's. As long as that does not chamber, all good.

I would wager there are more tight chambers found than over sized. When barrel manufacturers re-sharpen chamber reamers, "they will some times re-sharpen them too many times" creating an undersized chamber. AR bolts also vary in size, which is why it is important to chamber gauge with the bolt that is being used. The short chamber barrel I have was not a cheep barrel from a manufacturer of questionable reputation. This barrel failed with an extension head space gauge" which is precisely ground to spec, taking the differences in bolt size out of the equation. The barrel did barely pass with a smaller sized bolt I had in inventory..Klem's method can work fine, but sometimes gauges are the best tools to prevent any future problems, if one has access to them. The field gauge will determine if the chamber is too long and is un-safe. The no go gauge is a good gauge to have if one wishes to get the most life out of their brass for reloading....