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Now we will look into what happens if the headspacing is not correct. There are two scenarios we must consider here. The first is excessive headspacing, where there is extra space between the bolt face and the cartridge in the chamber. The second is insufficient headspacing, where there is too little space between the bolt and the cartridge.
What if there is excessive headspacing? In this scenario, when the firing pin hits the cartridge, it will move forward into the chamber before detonating. When the propellant explodes, the walls of the cartridge will expand due to heat and firmly stick to the walls of the chamber, preventing rearward motion of the cartridge. However, the thicker base of the cartridge will move backwards, because there is now a gap between it and the bolt face and this will cause the walls of the cartridge to stretch. If the stretching is too much, the walls of the cartridge could rupture and release hot gases into the action and also potentially spray case fragments out from the action of the firearm, which could be hazardous to the shooter or people standing next to him.
Well, what if there is insufficient headspacing in the chamber? In this scenario, the back of the cartridge will stick out of it and the user will not be able to close the bolt fully on the loaded cartridge. The user will not be able to properly operate the firearm if this happens. If the user were to force the bolt to close on the cartridge, this pushes the bullet tightly into the case neck and if the firearm is fired, this will cause excessive pressure to build up inside it, leading to hot gases coming out of the cartridge's primer pocket with similar results as excessive spacing. In worst case scenarios, the excess pressure could cause the action to rupture and cause damage to the gun and its user.
So, how do we determine that a firearm has proper headspacing? We can do that by using a set of gauges, called headspacing gauges. These are measuring instruments that are precisely machined to the SAAMI or CIP or military standards of a cartridge caliber. Typically, these are made of heat-treated steel and are machined to tolerances below 0.001 inches or so. They are made for various calibers by several reputable commercial companies at fairly reasonable prices. Typically, there is a "Go" gauge, a "No-go" gauge and (for military specification rifles), a "Field" gauge.
Headspacing gauges. Top is a "No-go" gauge and bottom is a "Go" gauge
The "Go" Gauge: A firearm must be able to close the bolt with no resistance at all, when a "Go" gauge is inserted into the chamber. This signifies that the firearm is able to meet the minimum length specification for the cartridge. If the bolt does not close on the "Go" gauge, this means that the firearm has insufficient headspacing and may be dangerous to fire, even if the user manages to force standard cartridges to fit in the chamber. Sometimes, the cause of this may be a dirty chamber or bolt face, because the accumulated dirt may be thick enough to prevent the bolt from closing on the gauge. However, if the firearm is clean and the bolt still does not close on the "Go" gauge, it must be taken to a competent gunsmith for adjustments.
If a firearm successfully closes on a "Go" gauge, it means that the firearm at least has sufficient headspace, but it may still have excessive headspace. That can be determined with our next gauge.
The "No-go" Gauge: A new (or overhauled) firearm must not be able to close on a "No-go" gauge. If the bolt closes successfully on a "No-go" gauge, this means the firearm has excessive headspace and there is a risk of cartridge cases rupturing inside the chamber. If the firearm is new or recently repaired, that means it should be returned to the manufacturer immediately.
A used firearm may be able to close on a "No-go" gauge, due to wear of the bolt and chamber surfaces, but this means that it should probably go to a gunsmith for repair soon and it may be possible to fire new factory ammunition in it until then, but reloaded ammunition is probably a bad idea and the firearm may malfunction on slightly out-of-spec cartridges. Here's where the test with the third gauge comes in (especially for firearms built to military specifications).
The "Field" Gauge: The bolt of any firearm, whether old or new, should not be able to close on a "Field" gauge. A bolt that closes on "no-go", but not on a "field" gauge, may be considered close to being unsafe, but may work on new cartridges (and should be sent to a gunsmith soon for possible re-headspacing). However, if the bolt closes on a "field" gauge as well, then it is definitely not safe to fire and should be sent for repairs immediately (or mounted on a wall as a decorative piece!).
Some calibers have a fourth gauge called a "Field II" gauge, for which the bolt should never lock on. This type of gauge is only used by some rifles (for example, Colt uses it to reject M-16 rifles).
It must be noted that gauges are usually manufactured to either SAAMI, CIP or military standards and therefore may have different dimensions, even for the same caliber cartridge. Therefore, it may be possible that a rifle manufactured to NATO specifications, may lock on a "No-go" gauge built to SAAMI specifications, but correctly not lock on a "No-go" gauge built to NATO specifications. This is because military weapons are generally designed to operate with wider tolerances and military ammunition cases are generally thicker than commercial ammunition and can tolerate more stretching without rupturing. Therefore, a military firearm may fail the test using SAAMI gauges, but still be deemed somewhat safe to fire per the military specification gauges. However, if it passes even with using SAAMI gauges, that means it is very likely to work correctly.
Go, No-go and Field gauges are available from many firearm supply stores and cost about $25 to $50 per gauge or about $70 to $140 per set.