Found this question in a thread that had been closed. [seen plenty of videos and write up about headspacing, but no one ever talks about how to correct it! So you determine that your headspacing is wrong. Then what????? Please dont say take it to a gunsmith, the point of this is DIY.] Well, unless you are a gunsmith or a machinist you have to take to one. So, what is excessive headspace and how is it corrected? Headspace is the distance from the case head [where the primer is] to the face of the bolt when there is a cartridge in the chamber and the bolt is closed. The cartridge shoulder is being pressed firmly against the chamber shoulder. This stops the forward movement of the cartridge and the bolt applies pressure to the case head to hold the cartridge stationary when the gun fires. This distance should be Zero but usually is not. Why should this distance be Zero? Several reasons. Number one is safety. When the gun fires, if the brass cartridge case is not held captive by Zero headspace the case move to the rear as the bullet moves forward. This means the case backs out of the chamber until it contacts the boltface. It this distance is great enough, the cartridge case, unsupported by the chamber will blow a hole in the unsupported brass. Not good when 30 to 50,000 psi is that close to your face looking for a way out of the firearms action. Number two is also safety. If the case did not blow out it definitely stretched the brass, and if it is reloaded that brass now has a thin spot and may or may not fit in your chamber.The SAMMI spec. for .308 Winchester is two thousands max between go and no go. The field specs allow eight thousands. If you want to know what the headspace is, and you should, all you need is a go gauge of the proper caliber and a set of feeler gauges from the auto parts store. The flat leaf type used to adjust valve clearance. First, remove the extractor and the ejector. If you don't do this you are wasting your time. Cut a piece of the .001 feeler gauge small enough to fit in the bolt, up against the bolt face. Insert your go gauge and close the bolt. You will have to hold the rifle slightly barrel high to keep the feeler gauge in place. If the barrel is too high the gauge will try to slide back out so its a balancing act. If all fails place a TINY drop of oil on the bolt face under the feeler gauge.This acts as an adhesive to hold the feeler gauge in place. Understanding if you use the oil it may add a thousands or two onto the measurement. If the bolt will close on the one thousandths cut a piece of two thousandths and repeat. Keep going up in size until the bolt will not close. OK. Now suppose there is nine thousandths excessive headspace. How do you get rid of it. There is only one way. The barrel must be removed and place in a lathe. Nine thousandths material must be removed from the shoulder of the barrel where it fits against the barrel extension. Yes it could be removed from the barrel extension but the heat treat on those parts is very hard. In any event, the barrel chamber needs to move toward the boltface nine thousandths to achieve Zero headspace. Having done that, depending on how much clearance the front of the bolt locking LUGS had from the rear of the barrel, before the barrel was set back, it my be necessary to remove another nine thousandths from the rear of the barrel to allow clearance for the front of the bolt locking LUGS. If, by mistake, too much was removed from the barrel shoulder and the barrel goes too far to the rear, the only fix is to use a chamber reamer of the correct caliber to deepen the chamber to the point where the go gauge will fit properly.
I have tried to write this so that the person with little or no knowledge of the subject may be able to understand it. That was me 40 years ago and all of you at some point.
Note. Head space measurement on a blueprint will be read from a point on the shoulder of the chamber as can be seen in the drawing below.
http://forums.thecmp.org/showthread.php?t=146769 Regards to all, Alan Waters