As we saw over the last couple of days, it’s possible for a very brief period of very high intensity firing to drive a weapon to catastrophic failure. The managers of US small arms programs have identified two additional failure modes that are exacerbated by high rates of fire (and therefore, high temperatures: bolt failures and barrel failures).
Let Us Propose a Way of Viewing Malfunctions
If you were to classify failures, you might measure them by the seriousness of their effect, or by its permanence. For example, a malfunction that renders the weapon unfireable (like a broken bolt, for instance) might be a Class A malfunction . A malfunction that degrades its performance in a militarily significant way (a burnt-out barrel, causing misses) might be a Class B malfunction. A malfunction that is relatively trivial (loose flash suppressor) in its impact on combat readiness is a Class C. Then, we’ll add a numeric value for where the repair must be made. We come up with a matrix like this:
||Irreparable / Depot
Class A (total dysfunction)
Class B (serious degradation)
|Class C (mild malfunction)
The most significant, as in urgently addressable, problems, are in the upper left; the most trivial, the lower right. It might make sense to prioritize the Class A totals (in numeric priority) and the B1, irreparable serious problems.
This 2006 power-point from that year’s NDIA small arms meeting reviews a wide range of small arms program highlights, but we’re going to focus on two problems it identifies, one of them being an A1 or A2 problem (depending on whether your unit was authorized to stock the repair part or not): the failure of a rifle or carbine bolt. The next is a potential B1 problem: the shot-out barrel.
Bolt Failure in the M4
There are two common places where the bolt fails: in the web, where the bolt has had a large hole hollowed out for the carrier key, and having lugs simply shear off. The first of these is always a gun-down, non-repairable failure. Sometimes it can be detected ahead of time by carefully inspecting the bolt, under magnification.
In a grimy, operational gun these small cracks can go unseen. If one starts on one side of the web, it will soon crack through, and the asymmetrical stress is now loaded up on the other side, which is as battered and worn as the first one was when it failed — so it soon lets go, too. The bolt in the picture above would still function in the rifle — right up until the moment it didn’t:
If this malfunction happened in combat, the weapon in question would be reduced to the status of “bayonet handle” for the duration of the fight, and hardly anybody carries a bayonet any more. Big time Class A-1 failure, weapon down for the count, and you can’t fix it here. (If you are authorized individual repair parts at unit level, your organizational armorer can fix it. If not, you’re screwed, dude. This is why some savvy guys bring an illegal stash of privately purchased common failure parts on deployments).
Two other problems commonly seen on M4 bolts are sheared lugs and burnt-out gas rings. The weapon may continue firing, after a fashion, with these failures. But it’s a sick puppy and needs a trip to the gun vet, or these problems will worsen until it’s an A1 failure, too.
The failure mode of that lug is really interesting. You would think that the lugs would fail on an angle from where the forces bear on its after surface, and this one seems to have done that, at first glance. But look at the shear surfaces. The smooth part (usually where the failure started as a crack) is in the bolt pocket for the cartridge head. It’s possible that the stress that failed this bolt was the radial stress from an expanding case head, not the locking force applied to the after surface of the AR bolt.
If the first lug fails, the load which had perhaps been divided seven ways is now divided six. (We say “perhaps” because, without lapping the lugs in, there’s no guarantee you have optimum contact, and in fact, you almost certainly don’t in a factory gun, which is fine: there’s a margin in the design). So the force that sheared one lug that was one of seven bearing it is now laid on only 6 lugs… we can’t say for certainty when the next lug or lugs fail, but we can say it will be a shorter interval, in terms of round count, than it took for the first one to let go.
Hard use will damage a bolt within 3,000 to 6,000 rounds; cracks will be visible on inspection. Almost all M16/4s will show damage by 10,000 rounds. The damage may not be mission-stopping: what no one knows is how long a cracked bolt can soldier on like that.
Now look at the burned-out gas ring on a carbine bolt:
There should be three small gaps in the rings, and they should never be aligned, instead, always, staggered. This is a safety-of-operation item: these gas-check rings keep the combustion gases in the internal cylinder of the bolt carrier. It also produces hard-to-diagnose failures to eject, extract, and/or feed: you should always inspect and, if necessary, rearrange or replace, the gas-check rings any time you have the sort of malfunctions that might be due to weak strokes and short-stroking.
The estimated life of a carbine barrel closely tracks that of the bolt; from 4,000 to 6,000 rounds if used hard, 10,000 plus rounds if gently treated. The problem with barrel life is that it’s had to know when you’ve reached it. One way to judge it is empirical: your shot groups get larger and larger over time, because the throat erosion that is the primary cause of accuracy degradation is a progressive thing.
At first, given good aim, it’s not much of a factor, as the weapon has an accuracy reserve at most combat ranges. But soon the normal shot-to-shot dispersion and the increasing size of the shot group mean that even perfect aim is frequently unable to hit the target.
Now, here’s the kicker: the normal tool we use to measure erosion, the taper erosion gage, doesn’t work reliably. According to the presentation, it’s right six times out of ten, but the other four times it can fail either way, identifying a good barrel as a reject needing replacement (false positive) or failing to identify a bad one (false negative). The first error wastes a fortune scrapping viable barrels, and the second may send a soldier into combat with a weapon that will make him miss his enemy.
A tool this inaccurate is worse than no tool at all. We would do better to measure throat erosion by chucking the rifle in a machine rest and measuring the size of a five-round shot group, than to rely on the meretricious promise of that solid-seeming gage.
In addition to the throat-erosion problem, there is a secondary gas-port erosion problem. This manifests as many different symptoms: cycling problems, failures to feed and eject, changes in weapon cyclic rate, and degraded accuracy.
What ties these problems together?
Ever single one of them is caused or exacerbated by heavy use, especially at cyclic rates. The weapon will last much longer if it is treated with care and allowed to cool between shot strings. If it’s fired as if you’re faced with a human-wave attack from the 3rd Shock Horde (or if you’re faced with such an attack and need to fire it that way) it is ripe for any of a number of interesting and troublesome failure modalities.
So What’s the Answer?
In the maintenance world, you can replace “on condition,” by inspecting things to see if they’re still serviceable, and rejecting them when they fail inspection, or fail in service, or “on schedule,” replacing them on time. We all use these concepts every day: we replace our light bulbs when they burn out, but we change the oil in our car every 5,000 miles. That makes a certain sense; there’s no great consequences to letting a bulb go our, but if your motor oil fails to lubricate you’re looking at a big repair bill.
The Army’s approach to weapons historically has been to maintain them “on condition,” with operator, organizational, and depot-level Preventative Maintenance Checks and Services that are outlined in the maintenance Technical Manuals. But since those inspections don’t work, at least insofar as they want them to ID their failing parts with high accuracy, they’re trying to move to maintenance on schedule.
The proxy they’ll use for wear on the weapons will be round count, and the Army’s plan is to make a round counter a component of every weapon.
We wrote about this before, last April (looking at this same presentation, actually, but from a different angle):
The trouble is, of course, that logging rounds is a great deal of work. But if the whole Army could do it, we’d get a lot more information about how long small arms and their components are good for, and we could begin to schedule inspections and overhauls more intelligently. Too many inspections waste money, and some percentage of overhauls go and rebuild guns that don’t need it, while some other percentage of guns that need overhaul, based on their condition, don’t get picked up. (Army ordnance experts think that both of these numbers, the false positives and the false negatives, are about 40%).
You don’t have to wait for the Army to beat this problem; while automated round counters are in the future for most of us, some of them are coming online; and there’s really nothing wrong with the old-school approach of logging every round with a pencil and notebook. (That works fine for personal weapons. For issue ones, that may get swapped around a lot, it’s not so good).
Any and every weapon can be made to fail. The better its career is logged, the more likely the career will be long; the more operators (in the “users” sense, not “ninjas”) understand it, the more they will rely upon it. The better it’s understood, the more it can be improved.
And that all starts with a #2 pencil….