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Joined:
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AZ, USA
Posted: 9/13/2025 3:26:51 PM EDT
[Last Edit: ARedWhiteBlue][Edited]
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I've got a 5.56 upper I built some years ago and have since put the original bolt in another upper (don't remember which). I tried a couple BCG combos I have around (extractor and ejector removed) and none of them will close on a GO Gauge. It will close easily on a resized case though). I also cleaned and inspected it for obstructions/debris. From what I've read, it seems like that because it will close on a case, it's probably safe to shoot, but not sure. Should I still have concerns if I shoot it and brass looks good? Is there an increased risk of a slam fire or it somehow firing out of battery? I also don't know the manufacturer (might be BCA, but it isn't stamped with a maker mark, just caliber and twist). I can hunt around to see if I can figure it out, but if I don't have to take the time to do that, I'd rather not. |
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Originally Posted By ARedWhiteBlue: I've got a 5.56 upper I built some years ago and have since put the original bolt in another upper (don't remember which). I tried a couple BCG combos I have around (extractor and ejector removed) and none of them will close on a GO Gauge. It will close easily on a resized case though). I also cleaned and inspected it for obstructions/debris. From what I've read, it seems like that because it will close on a case, it's probably safe to shoot, but not sure. Should I still have concerns if I shoot it and brass looks good? Is there an increased risk of a slam fire or it somehow firing out of battery? I also don't know the manufacturer (might be BCA, but it isn't stamped with a maker mark, just caliber and twist). I can hunt around to see if I can figure it out, but if I don't have to take the time to do that, I'd rather not. The reason there is a GO gage is because the chamber space cannot be too small. The smaller the space the higher the pressures will be for the same powder charge. A bolt will resize a case, so that is not a safe way to test. Risk of slam firing, or firing malfunction, no. But there is a serious risk of excessive pressure, especially if the chamber crimps the the bullet. |
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The previous reply is spot on. It is NOT safe to assume that if the BCG closes on a case under the influence of the recoil spring that your headspace is safe. The system is designed to overcome dirt, crud, and fouling, so the spring has plenty of "oomph" to make a case conform to the available space. How are you using those "BCG combos"? Ideally you'll separate the upper from the lower, then manually - and gently, with light finger pressure - push the carrier forward until it stops. If the headspace is safe, it WILL close on a GO gauge. The carrier is irrelevant except for helping you align the bolt with the chamber. Headspace has two parts: the depth of the chamber, and the combination of the depth of the bolt face and the length of the lugs. From what you've posted, your chamber is not compatible with the bolts you've tried. That doesn't mean that it's bad, just that you need a bolt that provides the cartridge/gauge with a little extra length. |
Joined:
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AZ, USA
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All of this makes sense. I just took two different new bolts from different manufacturers and swapped carriers around. Yep, light pressure with no lower. I don't think the carrier matters for headspace, but I wanted to eliminate unknown variables. I guess I could try one more bolt, but at a certain point, maybe it's just too tight - perhaps the old combo just had a bolt at the very opposite end of the specs so it barely made it. What I was hoping for is that it closing on the case meant it sat in the zone between "safe margin for error in manufacturing so we don't get sued" and actually being short enough to be functionally reliable (referring to the gauge). Even still, if I get a round that is just slightly out of spec AND with a little too much powder, the "tolerance stacking" could pop it. Where we occasionally get an extra hot random round that causes a malfunction or busted case with a typical chamber, this one would actually go unsafely over pressure because of all the stacking factors. Not worth the risk. Thanks for the help. |
Joined:
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AZ, USA
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An update...I found an email thread from some years ago with Bear Creek Arsenal. I brought it up with them and they said they use a match chamber that won't work with regular gauges, but that they are tested at the factory. My question though, how can they test it without my bolt? I'm sure it is within spec with their bolts (assuming they have a more narrow spec that leaves room for tolerance stacking). So while they say its ok to shoot, I still don't know that it is worth the risk. |
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Originally Posted By GHPorter: The previous reply is spot on. It is NOT safe to assume that if the BCG closes on a case under the influence of the recoil spring that your headspace is safe. The system is designed to overcome dirt, crud, and fouling, so the spring has plenty of "oomph" to make a case conform to the available space. How are you using those "BCG combos"? Ideally you'll separate the upper from the lower, then manually - and gently, with light finger pressure - push the carrier forward until it stops. If the headspace is safe, it WILL close on a GO gauge. The carrier is irrelevant except for helping you align the bolt with the chamber. Headspace has two parts: the depth of the chamber, and the combination of the depth of the bolt face and the length of the lugs. From what you've posted, your chamber is not compatible with the bolts you've tried. That doesn't mean that it's bad, just that you need a bolt that provides the cartridge/gauge with a little extra length. Several times over the years I've had head space issues. I generally test with the bolt only, not carrier, and the go gauge. I drop the GO gauge in, insert the bolt and spin/rock it with my fingers holding the tail of the bolt. I've had some pretty tight chambers where you almost think it's too short. When that happens I grab two or three other AR15's and try different bolts till I get a looser combination. Then I take the "tight" bolt and check it in the donor rifle/carbine to make sure I'm not just moving the problem elsewhere. Guns are funny things. |
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