montana Wrote:Unless you purchase a barrel and bolt already head spaced together, you will need to use a go and no gauge to verify head space with the bolt you intend to use.
Nismo,
I think differently and would instead cautiously fire the first round from the hip while briefly looking away (obviously pointing in a safe direction). I do that with all new barrels anyway. You can even wear gloves and hold it away from your body if concerned.
With AR's, headspace is fixed by the gunsmith who chambered that barrel. He/she would have used gauges and a bolt. Obviously not the same bolt you have but it will be pretty close. The most likely scenario is that it will be fine. The other two scenarios are too much headspace, or not enough headspace.
Too much headspace
With too much space the case expands and ruptures. When that happens it will split about halfway along the case separating it into two halves. There will be a louder than normal bang, more smoke out the action than normal, and it will leave a circular trench mark in the chamber. The front half of the case might need some fiddling with a pick to get it out. Or it might just come out with an oversized cleaning brush. At that point you take it back for a refund - providing you used factory rounds for the firing. If you use handloads for the first firing they could refuse, as it could be your fault in how you headspaced the case in the first place, and/or how much powder you loaded.
If the case ruptures from too much headspace don't forget the the gun is still in battery. The case is designed to take some of the pressure, while the bolt lugs are designed to take the rest. The case will absorb some of the pressure prior to rupturing and the bolt lugs will do their thing. The likelihood it will be a catastrophic event that hurts you is remote, especially if you fire the first round from the hip.
Not enough headspace
If this, then the bolt won't close on factory ammunition and you take it back for a refund. Even if your chamber is slightly less than SAAMI and it fires it is perfectly safe. If the bolt does not close on a factory round then it can't fire and that is safe too.
GO/NO GO gauges cost money so you will need to weigh up the need, and part of that is wondering how often you will use them, and the gun. If you have calipers you can figure out the chamber dimensions by measuring the spent cases.
One last point, this is a different scenario to handloading too much powder into a case and rupturing the gun from overpressure. If in the unlikely event your new chamber has too much headspace the factory cartridge will not be overpressured. The rearward force on the case as it splits will represent overpressure on the lugs (only), but a lot of that pressure will have been absorbed in the split first. It's not like you are pulling the trigger on a grenade. I have experienced two case separations from too much headspace; it was handloader error (me), but it is the same scenario you are concerned about. Although there was smoke and a bit of surprise at the time the same gun with two separations still fired normally after.