Folks, I thought it might be a good idea to make one sticky about headspace that people can refer back to as needed, rather than having short posts strung all through the forum. So let’s start with what Headspace is and why it is important.
Having the correct headspace is a HUGE Safety Concern in all firearms and especially in center fire rifle cartridges because the pressures involved are much higher than other firearms. Both the M1 Garand and the M14 operate at 55,000 pounds of pressure at the breech. Nothing to sneeze at or mess around with and this is serious business.
MINIMUM HEADSPACE: This is the amount of room necessary to ensure the longest cartridge manufactured to correct specifications will fit and will have enough room to expand to seal the chamber. There has to be enough room for the cartridge case to expand so there isn’t too much pressure on the rifle. I personally fixed a DCM M1 Garand that had exceptionally short headspace. (Don’t know how it got out of the Arsenal that way, but they never finish reamed the chamber.) When fired, that rifle cracked stocks within two clips of ammo and the owner said it “kicked like a mule.” I’m sure it did. Fortunately, when he told me about it I got him to STOP shooting the rifle until I could correctly cut the minimum headspace. Had he continued to fire that rifle that way, the super high chamber pressures generated because the cartridge case didn’t have enough room to expand would have at least indented his receiver and bolt, besides continuing to damage stocks. It would also have caused the bolt and receiver to crack, chip or break had he continued to fire them and his rifle would have wound up as a pile of junk. You don’t want a chunk of metal from the receiver or bolt coming back into your eyes, even if you wear shooting glasses.
MAXIMUM HEADSPACE: This is the amount of room necessary so the shortest cartridge manufactured to correct specifications will fit and will have enough room to expand to seal the chamber without expanding so far that the cartridge case will expand until it ruptures or a piece of the cartridge case breaks off. When that happens, you are going to get a good deal of nasty and hot gas coming back into the action and towards your eyes. God Help you if you aren't wearing shooting glasses, as you could very likely suffer permanent eye damage. Much of the gas coming back normally blows the magazine base out the bottom of the magazine well and cracks and splinters the stock along the sides of the magazine well at least. That usually isn't life threatening, but you may wind up with some good sized chunks of stock in your supporting hand/arm. I've seen up close one real, G.I. M14 blow up and while it didn't come close to killing the Marine, it would have taken out his eyes had he not been wearing shooting glasses.
Military rifles often allow more of a minimum to maximum headspace length to take care of the variance of Mil Spec ammo and because the firearms will be fired in bad weather, will fill up with powder residue when fired a lot and/or where sand, mud and other “junk” can get into the chamber. The headspace is also more generous because military rifles are fired faster and with more rounds fired faster. That means the barrel will heat up and thus “close in” the headspace when the barrel gets hot. The heat expansion will the actually cause headspace to close up a bit when the barrel really heats up. So, there has to be a little more headspace room than in civilian rifles. EDITED TO ADD: Military spec. cartridge cases also have thicker brass to take the strain of larger headspace dimensions as well.
OK, so how is headspace measured? Well, that’s different for different types of cartridges, so let’s just stick to the M1 and M14 rifle because this thread is going to get long enough as it is. Grin. Headspace in these rifles is defined as the space or distance between the face of the bolt and to a point in the barrel chamber for the angled shoulder for the cartridge case. That point is called the datum point (or sometimes Datum line) in the chamber. Correct Headspace overall length and the datum point for these cartridges are specified in government specifications and/or The American Sporting Arms and Ammunition Manufacturers' Institute; hereafter mentioned by the common acronym of SAAMI and pronounced “sammy.” (A little more on this later.) The government arsenals had some very precise special adjustable gauges to check headspace, but even they usually used Headspace Gages that are precision ground to a precise length. Almost every civilian gunsmith or armorer uses individual Headspace Gages to inspect or chamber a rifle. So let’s go over the most common ones next.
The “ GO “ Gage: This is the shortest Headspace Gage and is used to check and ensure the chamber has the MINIMUM headspace necessary for reliable and safe operation for even the longest cartridge case that is still inside manufacturing specifications.
The “ NO GO “ Gage: This is the most widely MISUNDERSTOOD Headspace Gage and causes the most confusion to a whole lot of folks. I can’t tell you how many times I’ve heard or seen incorrect things said or written about this Gage and sometimes by people who SHOULD know better. This Gage is used by Arsenals or gunsmiths/armorers when they cut the Headspace in the chamber of a rifle along with the GO Gage. You have to cut the Headspace deep enough so the GO Gage will fit without showing additional friction on the bolt. That gives you the Minimum Required Headspace. You then use the NO GO Gage to ensure you don’t cut too much Headspace when you first chamber a rifle. Headspace may increase as much as one or two thousandths of an inch ( .001” to .002” ) during the entire life of the barrel until you shoot the lands out. SO you have to have a NO GO Gage to show you where to stop cutting and ensure there will not be too much Headspace throughout the entire life of the barrel. (IOW, when you stop cutting the headspace - there will still be a few thousandths of an inch before you hit the Maximum Headspace length.) The NO GO Gage is NOT the maximum gage and it is NOT the Gage that tells you the Headspace is too much or too deep. If you don’t cut Headspace, you really don’t even need this gage to check for safe Minimum and Maximum Headspace.
The “ Field Reject “ Gage: OK, THIS Headspace gage is the gage you use to check for Maximum Safe Headspace even when you get the shortest cartridge made to specifications. If the bolt closes on this gage without additional friction than before you had the Field Reject Gage in the chamber, then the barrel is UNSAFE and no one should fire the rifle until the barrel is pulled and a new one is installed. If one wishes to have the two necessary Gages to check Headspace, you need this Field Reject Gage and the GO Gage for any individual caliber.
There are going to be additional posts containing additional information. So please stay tuned. Grin.