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Posted: 2/5/2011 6:59:51 PM EDT
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If I take a new barrel, and a new bolt, and a new upper and assemble them, what are the odds that my headspace will be in a safe range?
I was looking at this barrel: 20 inch pencil barrel This upper: BCM Upper This BCG: BCM Bolt Carrier Group Would also use BCM .625 diameter FSB, charging handle. Would get the FSB pinned by ADCO. |
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The head space is set when the barrel is made. The barrel extension on the barrel at the factory sets the head space. As long as it's a quality barrel. it's good to go. Headspace is the distance, in a rimless case, from the bolt face to a point on the chamber shoulder called the datum line, typically halfway up the shoulder. In other words headspace is a combination of the bolt and the chamber, not just the chamber. It is always a good idea to check headspace. |
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The head space is set when the barrel is made. The barrel extension on the barrel at the factory sets the head space. As long as it's a quality barrel. it's good to go. Headspace is the distance, in a rimless case, from the bolt face to a point on the chamber shoulder called the datum line, typically halfway up the shoulder. In other words headspace is a combination of the bolt and the chamber, not just the chamber. It is always a good idea to check headspace. As long as the bolt is undamaged and the buffer spring is strong enough to slam the bolt all the way forward, the bolt in the AR has nothing to do with head spacing because the barrel extension that stops the bolt in place is attached to the barrel permanently at the factory. You simply can not change the head space in an AR. You can check it all you want, but there is nothing you can do to change it besides replacing the barrel. |
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The head space is set when the barrel is made. The barrel extension on the barrel at the factory sets the head space. As long as it's a quality barrel. it's good to go. Headspace is the distance, in a rimless case, from the bolt face to a point on the chamber shoulder called the datum line, typically halfway up the shoulder. In other words headspace is a combination of the bolt and the chamber, not just the chamber. It is always a good idea to check headspace. As long as the bolt is undamaged and the buffer spring is strong enough to slam the bolt all the way forward, the bolt in the AR has nothing to do with head spacing because the barrel extension that stops the bolt in place is attached to the barrel permanently at the factory. You simply can not change the head space in an AR. You can check it all you want, but there is nothing you can do to change it besides replacing the barrel. True, you can't change it; however that doesn't mean that it is always GTG. Tolerance stacking *could* result in excessive headspace. It's extremely rare, but possible. Extremely unlikely with new parts and a barrel chambered by a competent smith, but with a used bolt or barrel extension, the possibility begins to increase. |
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Sure you can change the headspace. If it is tight or short you can ream it. Not only that but a different bolt can change the HS. The buffer spring is not a part of HS, nor the upper receiver Machined parts are made to be within tolerances, most of the time everything works as it should. However, the bolt and chamber can both be within specs but if one is at the far end of the plus and the other is at the minus end excessive headspace can result. |
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I imagine you probably selected the dez arms barrel for the. 625 profile. But it seems you might spend the same amount buying a complete BCM 20" upper, or possibly save a little, once you factor in the pin job at Adco. I wouldn't worry too much about headspacing problems from a BCM setup. I probably would be concerned from many other companies though, including a $160 barrel from dez.
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Sure you can change the headspace. If it is tight or short you can ream it. Not only that but a different bolt can change the HS. The buffer spring is not a part of HS, nor the upper receiver Machined parts are made to be within tolerances, most of the time everything works as it should. However, the bolt and chamber can both be within specs but if one is at the far end of the plus and the other is at the minus end excessive headspace can result. Interesting. What are those tolerances? If they're both within spec but can be a problem, what kind of numbers are we looking at? More data or citations might be helpful. |
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I imagine you probably selected the dez arms barrel for the. 625 profile. But it seems you might spend the same amount buying a complete BCM 20" upper, or possibly save a little, once you factor in the pin job at Adco. I wouldn't worry too much about headspacing problems from a BCM setup. I probably would be concerned from many other companies though, including a $160 barrel from dez. BCM uppers aren't .625" profile. They're also chrome lined, which I'm of two minds about. There are no mid-weight stainless barrels outside of Sabre Defence, and I can't actually find the barrel I want in stock anywhere. |
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