What is Headspace and why is it important?
Headspace is the amount of space in the chamber from the face of the bolt when it is locked in firing position to whatever stops a cartridge when it is dropped into the chamber. Actually what concerns us the most is the amount of space that is left when a loaded round is in the chamber. In an ideal world the cartridge will completely fill the chamber with no space but have no interference. This is not an ideal world so we have to have room for variations in ammo and dirt and powder fouling. We do this by having a minimum (go) and a maximum (no go) usually this range is .004 inch. We want to have no more space than a human hair so this is a job to be done with care and knowledge.
So why is this so important? Consider what happens when the cartridge is fired. Once the bolt is closed the firing pin hit’s the primer with sufficient force to crush the metal cover and ignite the explosive charge of the primer. This also pushes the loaded cartridge as far forward in the chamber as it can go and increases the space between the bolt face and the base of the cartridge to the maximum. Next the flash comes through the flash hole and ignites the powder. Pressure quickly builds and the thin brass walls of the case seal off and prevent any gasses from flowing towards the bolt. As the pressure quickly climbs it forces the head of case rearward stretching the thin sides of the case until the case head rest against the bolt face. If the distance is excessive the case head will be blown off and the full pressure of the cartridge will be loosed into the action. Remember the shooters eye is only a few inches behind this area and his hands and arms are lying parallel to the action. We are talking 50,000 lbs. plus with normal rifle loads. SCUBA tanks and welding cylinder have less than 3,000 so you get the idea.
But what if it just stretches the case but not enough to rupture? This is bad too especially for the reloader. This case is stretched thin just above the case head. When the reloader resizes he will push the shoulder back and the case will chamber again but the brass is already thin in this area just above the case head and when fired again the process repeats but maybe this time the thin area ruptures. If not this time maybe the third or fourth or sometime down the line. Also pushing that shoulder back each time usually shows up as a longer case neck needing to be trimmed and deburred.
All this shifting of the cartridge during firing also results in poor ignition of the powder and inconsistent accuracy as the bullet has a hard time getting started into the rifling straight. Misfires often result from the cartridge being pushed away from the firing pin.
But what happens if there is not enough room? Most firearms have built in safety mechanisms to prevent the gun from firing if the bolt is not fully locked in firing position. This is the scenario that insufficient headspace presents. The bolt face touches the case head before closing completely and when the trigger is pulled the shooter hears a click but it doesn’t fire. Many times a gun brought in for a new firing pin or a good cleaning when the real problem will be insufficient headspace.
So how do you measure headspace? The space to be measured is enclosed and out of sight so normal methods won’t work. Headspace gauges to the rescue. These are precision ground gauges that fit the area between the bolt face and whatever the cartridge comes to rest against. Most common are the family of rimless bottleneck cartridges. These include the 223, 308 and 3006 family of cartridges. Many times headspace gauges will work for a whole family of cartridges. The 3006 gauges will work for 2506, 270 Winchester, 8mm06, 35 Whelen, and 400 Whelen. 308 gauges handle 243 Winchester, 260 Remington, 7mm/08 and 358 Winchester. These cartridges and many others do not have a rim that projects outward past the case wall and rest on the bottleneck shoulder. The next type is the belted magnum. This type has a belt on the outside of the case just above the groove cut for the rim. This type headspaces like the rimmed case. The gauge measures only the distance from bolt face to rim. These types include 7mm Remington Mag, 300 Winchester, etc.
Next are the rimmed cases like 30-30, 303 British and 45-70. This gauge only measures rim thickness because that is what these cartridges rest upon even though some are bottleneck design. Last is the straight wall rimless that headspaces on the case mouth like 30 carbine and 45 ACP. Whatever the case type the gauges are used in the same way. First the chamber is cleaned of dirt or metal chips and then the go gauge is inserted. If the bolt will close without resistance the chamber is deep enough. Never try to force the bolt closed on the gauge because it is hardened steel and can damage chamber or bolt. Next remove the go and insert the no go gauge. This time when you try to close you should not be to close the bolt all the way. If you can, then the chamber is too deep for best performance. Next remove the no go and insert the field gauge and try to close. This gauge is usually .004 larger than the no go and if the bolt will close the gun is unsafe and should not be fired.
How can headspace problems be corrected? Too little headspace is the best problem. Usually a reamer turned by hand will remove a couple of thousandths and get us where we need to be. If more than a small adjustment the barrel should be removed and set up in the lathe to keep the reamer on line with the bore. For excessive headspace the barrel must be removed and the barrel shoulder set back towards the muzzle to allow the chamber to move closer to the bolt face. On an average barrel with 16 threads to the inch one full turn will send the chamber .0625 inch towards the bolt. Of course the barrel will need to be machined back maintain clearance and not touch the bolt face.
But what if the chamber is correct and the ammo is too short or long? Very important to understand that excessive headspace or insufficient headspace is a relationship of chamber size and ammo size. The same problems of an oversize chamber will be present when we have undersize ammo. In the last year I have seen two nice 7mm Remington Magnum rifles badly damaged when they were loaded and fired with 270 Winchester ammo. When the case expanded to fit the chamber it required more stretch that the brass could survive and the case ruptured. This amount of pressure blew out the magazine boxes and broke the floor plates turning them into projectiles that injured one shooters leg.
Headspace is critical for safety, functioning and accuracy. Make sure you measure everything several times and understand what you are measuring. This is not a “poke and hope job”.