October 17, 2012
Misfires are anathemas to shooters. Getting a "click" when you expect a "bang" is infuriating whether you are plinking into the riverbank or have a 7x7 elk in your crosshairs. This subject is complex enough to warrant two columns, and for the first I'll focus on rimfire ammo. For the sake of this discussion, let's omit cases when a user deliberately modified some component of the firearm's ignition hardware.
In spite of its unheralded success, rimfire priming is a primitive system with inherent weaknesses. The big ones are weatherproofing, crush protection, and vibration resistance. The only protection against liquid incursion is the outside coating of bullet lube. The ring of primer compound is protected from crushing by a thin envelope of cartridge brass. The compound itself is essentially glued into the rim cavity with the binders that hold the compound together. Too much shock or vibration can cause a piece of the compound to fall out of the cavity. If that's where the firing pin happens to land, you get a "click." Fortunately, the industry has access to better binders than it did 50 to 75 years ago, reducing the "chipped pellet" as a misfire cause.
Still, manufacturers have overcome most of these weaknesses, and today's rimfire ammo is so well made that you should look for a firearm problem first unless you know the ammo is decades old, took a bath, or is of questionable origin. In rimfire, the major gun-related factors are excessive headspace, worn chamber mouths, damaged or misaligned firing pins, and too much firing pin surface.
The Causes & The Cures
Excessive headspace leaves insufficient case support for the "rim pinch" to occur that's required for rimfire reliability. The result is a "light hit." Many inexpensive rimfire rifles control headspace with the root of the bolt handle engaging a recess in the receiver. After years of wear, this contact can grow loose, and headspace increases. A sure sign of this approaching problem is in the fired case. If the rim looks thicker after firing than before, misfires are not far away.
The chamber mouth can become deformed from dry-firing or from careless application of a steel cleaning rod. This also reduces rim support.
The rimfire priming system requires fairly precise placement of the firing pin strike on the case head. If a rifle's firing pin has lateral play due to wear or poor tolerances, it can "drift" enough to cause a misfire. Likewise, any wear/damage to the tip of the pin can change the contact area with the case from the original design criteria.
The tip's shape can affect reliability. A friend picked up an older European rimfire rifle. The firing pin tip was big and round, some 0.080 inch in diameter. There was too much surface area for the available spring tension, and replacement springs were virtually impossible to find. He removed the firing pin and beveled the edge of the tip in two places, reducing the surface area about 10 percent, and the misfires went away.
Revolvers — especially double actions — can, and will, develop excessive headspace with normal use. The cylinder recoils before the rest of the firearm, bashing into the breechface and rebounding onto the yoke or crane tube that supports it and controls headspace. As this battering shortens the tube, the cylinder's at-rest position becomes farther forward, requiring more firing pin reach.
It's normal to have enough reserve firing pin length and spring power to fire the cartridge, but this slight extra space often gets filled by bullet lubricant and unburned propellant getting under the extractor. Forgetting to clean this area can cause misfires.
Semi-automatic rimfire rifles and pistols can also misfire from too much debris and too few cleanings. The area where the bolt seats must be reasonably free of debris. An accumulation here can stop the bolt before it is fully "home," requiring the firing pin to reach farther. Check both the breech end of the barrel and the breechface on the bolt and clean if needed.
While I was in college, I bought a used Remington Nylon 66 rifle. It shot accurately enough, but was not 100 percent in function. It tended to misfire and jam occasionally. After getting it home, I pulled the action cover and extracted the barrel to find the action full of sand.
Being young and dumb, I attached a hose to the tap in my mom's bathtub, let the water run until hot, and blasted the entire action area with a jet of water. The bottom of the tub was full of coarse sand, but at least it was no longer in the rifle. The big sand grains were preventing the bolt from closing fully and caused light firing pin hits. A thorough hosing with a water-displacing spray finished the job, and the rifle never hiccupped again. And someone was watching over me — it never rusted!
Most of the things I mentioned are easy to check. Even headspace problems are heralded by a thickened rim, but you can't check everything. A dirty or damaged firing pin spring in a rifle may be hard for the end user to access, so get a pro to check it.
If I get a misfire in a rimfire gun, I check the gun first. It's a tribute to our manufacturers that they pack so much reliability and accuracy into cartridges that still cost so little to shoot.