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Armorer's Corner - Headspace
Why should I care about my rifle's headspace?
When a rifle is fired, the pressure generated inside the chamber rises to tens of thousands of pounds per inch. This pressure generated by the primer and burning powder can do serious harm to you or people around you if something goes wrong and the hot, high pressure gases escape through a breech in the brass cartridge case. Excessive headspace will cause the brass to stretch more than it should and increases the likelihood of a case failure. Insufficient headspace may contribute to slam fires, light strikes on primers, misfires and more wear on parts due to the additional force needed to chamber the rounds.
What is headspace?
Headspace is defined as the space or distance between the face of the bolt and a specific predetermined point in or at the chamber. For bottlenecked rimless cartridges like the 30-06, headspace is measured from a point on the chamber's shoulder to the bolt face. There are other types of cartridge designs that require headspace to be measured in very different ways, some of these are known as rimmed, belted and straight wall rimless. We won’t cover these here.
Understanding what headspace is and knowing how to properly measure this distance is fundamental, necessary and critical information for anyone interested in changing or altering any of the parts that affect this aspect of a firearm.
There are a number of companies that manufacture headspace gauges, the two that are most commonly encountered are Forster and Clymer. Both Forster and Clymer make fine gauges but we have found that there are differences between the two companies’ gauges that make the Clymer gauges best for use with the M1.
The M1 was not designed as a match rifle with minimum tolerances; it was a semi automatic battle rifle that had to function in dirty, wet, sandy unpleasant conditions on the battlefield.
The headspace that the original manufacturers of the M1 considered correct can be determined by checking new or nearly new rifles that we have here at CMP. With that information we have determined that Springfield Armory and the other manufacturers of the M1 used gauges that were very close to the Clymer dimensions (closer than the Forster) and therefore we use, and recommend using only the Clymer gauges.
The three gauges you will encounter are the “GO”, “NO GO” and “FIELD”. CMP only uses “GO” and “NO GO” gauges but I will describe all three.
The “GO” gauge - is most commonly used when installing a new barrel and reaming the chamber to size. The bolt should fully close on the “GO” gauge, if it fully closes you can be sure you have enough room in the chamber to prevent the cartridge from being crushed during chambering. The “GO” gauge can also be thought of as a minimum safe headspace gauge and the rifle's bolt must be able to fully close with it in the chamber.
The “NO GO” gauge - is used to make sure a firearm does not have excessive headspace. The bolt should NOT fully close on the “NO GO” gauge, if the bolt cannot be closed on the “NO GO” gauge then you know your rifle does not have headspace that is excessive. The “NO GO” gauge can be thought of as a maximum headspace gauge and should not be able to fit in the rifle's chamber with the bolt fully closed. If the bolt DOES close on the “NO GO” gauge, it does not necessarily mean that the rifle is unsafe; it does however show that a further check with the “FIELD” gauge would be necessary to determine if it is safe to shoot.
The “FIELD” gauge - is used to check absolute maximum headspace. If the bolt closes fully on the “FIELD” gauge the rifle IS NOT to be fired and should be considered unsafe to shoot. CMP does not use this gauge because rifles that pass the “FIELD” check but fail the “NO GO” are approaching the point where they will be unsafe to shoot. Our standard for maximum headspace is the “NO GO” gauge to ensure our customers will be able to shoot safely for many years.
Using the headspace gauges:
In order to obtain a true measurement of the rifle's headspace there are several things that must be done before inserting the gauges into the chamber:
- The stock, trigger group, op rod, op rod spring, and follower rod must be removed from the rifle.
- Because the M1 rifle has a spring loaded ejector on the face of the bolt, one of
two things must be done to prevent the ejector from affecting your proper reading of the headspace. The bolt must be disassembled and the ejector
removed, or clearance notches must be made on the headspace gauges so there will be no contact between the headspace gauges and the ejector. See figure 3 (click for larger image)
- The chamber and the bolt face must be clean and free of anything that would
cause a false measurement to be received, this would include dirt, heavy fouling and rust. When replacing a rifle's barrel, the process of reaming the barrel's chamber to the proper dimensions will produce metal shavings and possibly burrs or other debris which must be removed.
- Manually move the bolt by itself through its cycle of movement in the receiver. Make note of any binding as you move the bolt forward and rotate it fully into battery. You should be able to easily close the bolt on an empty chamber. If there is some very minor friction as you rotate the bolt fully into battery you need to be aware of it so it does not cause you to receive a false headspace reading. If the friction or resistance is more than “very minor” have your rifle checked out by an experienced gunsmith.
When the rifle has been properly prepared and the gauges notched to clear the ejector, the following procedure should be followed:
- With the bolt tilted up but still in the receiver, place the “NO GO” headspace
gauge on the face of the bolt with the extractor hooked around the rim of the gauge and the ejector notch lined up with the ejector. See figure 1 (click for larger image)
- Lower the bolt into its normal position in the receiver and slide it forward,
rotate it gently as far into battery as it will go with light pressure on the right
bolt lug. The bolt should not be able to rotate fully into battery and there should be a gap beneath the right lug of at least the thickness of a couple of business cards. See figure 2 (click for larger image)
- If the rifle passes the “NO GO” gauge, remove the gauge from the rifle.
Insert the “GO” gauge. The bolt should be able to fully close on the “GO” gauge.
- If your rifle passes both of these checks then your rifles headspace is within
the safe operating range as demonstrated by proper use of the gauges.
SAFETY WARNING: This information is provided as a base of information and cannot take into consideration every possible condition or defect that may be encountered. Correct and safe headspace is governed by the bolt, barrel and receiver in a rifle and is determined by qualified people using quality headspace gauges. Bolts varied in length during their years of production. If for any reason you wish to replace the bolt in an M1 rifle, the bolt should be checked to ensure it is serviceable and safe headspace verified. Swapping bolts without these checks may result in a dangerous condition.