Every part on every gun has a life span. U.S. Army Ordnance established minimum life spans based on the minimum number of factory loaded cartridges (U.S. Army Ordnance contract ammunition only) fired before the part would fail. This then became the minimum standard for each manufacturer of that particular part. No one, especially not U.S. Army Ordnance, logged the number of cartridges the average issue weapon subsequently fired. Instead, the weapons were inspected periodically or fixed when something stopped working.
The practice of buying a carbine and shooting it without first inspecting it is unfortunately not uncommon. Even more rare is for a new owner to take the carbine to a qualified armorer or gunsmith that knows M1 carbines, and have them inspect it before you shoot it. Cost and time are usually the reason this doesn't happen, but consider the cost and time should a firearm have a problem you discover post mortem, or worse. Most gunsmith safety inspections cost $40-$60 and take only a day or two, depending on the gunsmith's workload.
This page is meant to address the more common issues related to the commercially manufactured carbines. It is by no means all inclusive. The issues addressed on this page potentially can happen with any semi-automatic military rifle and are not specific to any one manufacturer. If an issue is specific to a commercial M1 Carbine manufacturer, the issue is covered on the web page for that particular manufacturer.
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For an out of battery discharge to occur the firing pin must be capable of striking the cartridge primer before the bolt rotates and locks. Carbines manufactured to the specifications set forth by the U.S. Army Office of Ordnance have safety mechanisms built into their design to prevent this from happening.
The GI firing pin floats freely and relies on two safety mechanisms to keep it from striking a primer prematurely before the bolt rotates and locks. One of the two safety mechanisms is the cut in the rear of the receiver bridge, which engaged the firing pin's rear tang, holding the firing pin at the rear of the bolt until the bolt moved almost completely into the fully locked position.
The receiver above and the receiver at the top in the below photo, have been cut improperly, allowing the firing pin to move forward before the bolt has turned and locked. Even with this improper cut, the GI specifications provide for a second safety mechanism should this one fail.
The second safety mechanism to keep the GI type firing pin from striking the primer before the bolt has locked in place was the design of the rear of the bolt and face of the hammer. Bolts manufactured by the original contractors for U.S. Ordnance (GI M1 carbines) had specifications as to the hardening of the front of the bolt, and even stronger hardening of the rear of the bolt. Below, you can see why. Commercially manufactured bolts not hardened to Ordnance specifications are fairly common. Measuring the hardness of a carbine bolt is not practical. Instead, visually inspect the bolt and hammer. If either are defaced to the point they could allow the hammer to strike the firing pin before the bolt has rotated into position, replace them.
A side note. Universal redesigned their bolts sometime during the mid 1970's, placing the firing pin completely inside the bolt. The firing pin was held in the rearward position, away from the primer, by a spring around the front of the firing pin. This eliminated the need for the precise cut in the receiver bridge by eliminating the firing pin tang and utilizing a more reliable means of hold the firing pin away from the primer. However, they didn't harden the rear of the bolt to GI specifications. Had they done so, this was one of the changes in design that might have been an actual improvement over the GI specs. If you own a Universal with the later bolt, make sure to inspect it for damage on a regular basis.
Out of battery discharges are preventable. The GI carbine specifications provided two safety mechanisms to prevent the firing pin from striking the primer prematurely. Some of the commercially manufactured receivers were machined improperly, eliminating one of the two safety designs. Some of the commercially manufactured bolts were not properly hardened, which could lead to damage that would circumvent the second safety design. To counter these defects in manufacture, if they exist, it is recommended you do the following.
M1 carbine headspace is measured from the front of the bolt face when the bolt is fully closed (A) to the "chamber heading seat" (B). For the .30 caliber carbine cartridge, the chamber heading seat is that point in the barrel where the front of the cartridge casing is stopped by the change in the diameter of the barrel, which is also referred to as the "Datum Line" (B). There are two organizations that have determined two different measurements for what the distance between points (A) and (B) should safely be, each based on their own criteria. Both need to be understood as one applies specifically to U.S. GI carbines using military ammo and the other applies to any carbine using commercial ammo.
U.S. Army Office of Ordnance specifications were developed for the U.S. GI M1 carbines firing military .30 caliber carbine cartridges. Ordnance personnel used Ordnance supplied headspace gauges manufactured to measure to Ordnance specifications. These gauges are rare and not for use on commercially manufactured carbines. The vast majority of gunsmiths do not own U.S. Ordnance .30 caliber carbine headspace gauges and they are not commercially available.
"The Sporting Arms and Ammunition Manufacturers' Institute (SAAMI) is an association of the nation's leading manufacturers of firearms, ammunition and components. SAAMI was founded in 1926 at the request of the federal government and tasked with...", amongst other things, "Creating and publishing industry standards for safety, interchangeability, reliability. and quality." It is not necessary to purchase the tools used by machinists and manufacturers. It is far less expensive and easier for gunsmiths and gun owners to purchased commercially manufactured headspace gauges (by caliber) that have been manufactured to the specifications set by SAAMI.
Headspace is measured using three gauges. Each gauge is placed inside the carbine chamber and checked with the bolt used in that specific carbine. The bolt should be disassembled during gauging.
Someone who owns only one carbine is likely not interested in expending $75 for these gauges. Understandable. Keep in mind headspace is something that can change over time, so the gauges should be used more than once. Also, if the gauge is undamaged, they tend to retain their value. SAAMI approved headspace gauges are available at most major firearms supply stores (available online at Brownell's, Cabela's, Midway, and others).
For further details regarding headspace and why you should have it checked, have a read of the following, keeping in mind the .30 caliber carbine round is a rimless cartridge and the location of the cartridge "Datum Line". Measuring Headspace at Surplusrifle.com
If it makes you feel any better, this is not an issue specific to commercially manufactured carbines alone. The more used and/or abused the original GI carbines, M1 Garands, and other military surplus weapons are, the more likely they will also have improper headspace. You would too after two or three wars.
It's really worth the money to get your carbine safety inspected by a qualified gunsmith. Remember the risk when your son, daughter, or loved one picks up that carbine.
Almost all M1 carbines, M1 Garands, M14's, and other rifles using the free floating firing pin will allow slight contact between the firing pin and primer before the bolt locks completely. The force with which the firing pin normally strikes the primer is insufficient to cause the cartridge to fire.
Two conditions common to slam fires are improper headspace or use of reloads that have an improper case length and/or a primer that is not fully seated. As the bolt closes the firing pin strikes the protruding primer with enough force to discharge the round. For a full discussion of slam fires with illustrations, visit the website of Springfield Armory and download their manual for their M1 Garand (flash activated link is at the bottom of their page at center).