November 05, 2012
In my last column, I wrote about misfires in rimfire ammo, and so this time I'll cover centerfire ammo, where either the firearm or the ammo, especially if the latter isn't carefully handloaded, can contribute to a misfire. Although an improvement over rimfire, centerfire priming is a complex system with more components. The fact that centerfire priming makes the case consumer-reusable introduces a lot of human factors.
I'll use the same criteria as last month: eliminating deliberate firearm alterations and obvious physical damage to the ammo.
Centerfire rifles have very robust breeching and powerful springs compared to their rimfire counterparts, so excessive headspace from typical use is seldom a cause of misfires as in rimfire. Likewise, a misfire in a semiauto pistol is usually due to an ammo issue. It is the revolver that produces the most gun-related misfires.
As a class, revolvers are unique because their headspace relations can change significantly with normal use. The cylinder starts to recoil first, before the rest of the handgun. It bounces off the breechface and slams into its support structure — the front of the frame in a single-action model or the yoke (crane) tube in a double action. This battering of the bearing surfaces starts slowly because there is little front-to-rear play in a new revolver. Over time, some play develops, and the rate of change increases. Misfires happen because the cartridge is farther forward at the time of firing than before, and the firing pin may not be able to deliver a full blow.
The cure for this looseness depends on the type and make of revolver and is beyond this discussion. What you need to know is how to check for this condition — this varies between single- and double-action revolvers.
First, make certain the firearm is unloaded — twice — then clean it thoroughly. For the SA revolver, you can simply take a shooting grip with your strong hand and use your weak hand to move the cylinder front to rear. A DA revolver needs a modified technique; before moving the cylinder, pull and hold the trigger to the rear as if you've just loosed off a shot. Then try to move the cylinder.
There is no universal spec that can be applied, so I use my ears. If the cylinder moves a little but I hear little or no sound, it's likely within normal range. If there's more movement and I can hear a noticeable "clack-clack" sound, the gun needs to see a gun doctor. Another quick check is to look at the barrel-cylinder gap. If the cylinder is dragging against the rear face of the barrel, endplay is grossly out of control. Barrels can't grow, so what moved was the cylinder.
Handloaded Ammo Causes
The apex cause of centerfire hand-loads misfiring is a "high primer," one that is not seated fully. Proper seating puts the tip of the anvil at a specified distance from the inside of the primer cup, leaving the correct amount of primer compound in between to achieve specified sensitivity. With most brands of Boxer primers, this happens when the base of the primer cup is 0.003 to 0.005 inch below flush with the case head for specified sensitivity.
High primers mean there's too much compound over the anvil tip for proper ignition. It's been compared to a car engine with a too-rich fuel/air mixture. One thing I learned over years of working with this stuff is that when a misfire happens on the first strike but the cartridge fires on the second try, there's a 98 percent chance the handloader did not seat the primer deeply enough.
You can make or buy a gauge to check seating depth, but nothing beats an "educated finger" for picking out high primers. I run a finger across every case head after the priming stage. If I load progressively, I check for high primers as I box the ammo. Remember that you never attempt to reseat a primer in a loaded cartridge. If you find one, set it aside for later disassembly and rework. Safety first, people! High primers can cause premature ignition in some firearm types, so it behooves you to take extra care to ensure proper primer seating.
We all know that a primer has to be anvil-down in the pocket to work, but there are degrees of "down." In industry jargon, a "tipped" primer is one that entered the pocket 90 degrees out of phase, and the "inverted" primer is 180 degrees out of phase. What often gets overlooked is the slightly tipped primer where the primer is only a few degrees out of square. These are hard to spot, but they can cause misfires if the slight angling causes the anvil to shift during seating.
I find the main cause of these slightly tipped primers to be the swing-arm priming system on loading presses. I'm not condemning this method — I've primed thousands of cases this way. The problem is in letting things get dirty. If enough residue from the decapping operation gets around the primer arm spring and its cutout, the arm can stop rotating before its seater punch is parallel to the case head. The primer cannot start into the pocket straight.
Straight-line priming tools can also create an angled primer when routine checks are ignored. I helped two friends who had these; one had not properly tightened the two-piece punch assembly after cleaning, and it had worked loose. The other had apparently bent the tip of the punch while trying to prime a case with a crimped primer pocket he missed in his initial case inspection. Either situation could have been avoided with a 1-minute equipment check before priming commenced.
In factory ammo, continuous improvements in loading line technology combined with computer-aided vision systems to supplement human inspection have made the traditional causes of misfires quite rare. Still, whether it's handloads or the factory stuff, I inspect every cartridge before I load it for serious purposes. Old habits are hard to break.