JP Post Mortem

By The JP Engineering Team

This fall, the JP engineering team had a bit of a mystery on their hands. At a national-level match, not one, but two of our aluminum JPBC-1s go down hard within minutes of each other.

Any business that manufactures products has to deal with broken parts. But when your business is firearms, you take breakage seriously. During live fire, it can be just as dangerous as careless handling of the weapon.

So, when JP gets broken parts back from our end users, it becomes priority #1 for the engineering team. How did this happen, and what can we learn from it? Our inspection processes consists of diagnosing failures from reloading errors all the way to doing accuracy and stress workups.

The process only ends when the problem is locked down, and second guessing conclusions is mandatory. This often pulls us away from new designs and developments, but it's worthwhile. Troubleshooting failed parts helps us engineer them for greater reliability, durability and safety.

The BC-1s in question belonged to a pair of shooters we know well. They shoot similar, pre-built JP rifles and were both using the same reloaded ammo at the match. Having two instances of failure like this rare, but it offers an invaluable chance for direct comparison.

Already, we suspected the problem was ammo-related. Still, the question that occurred to everyone was whether this had something to do with the aluminum bolt carriers. Would this have happened with steel carriers and what did that mean for the aluminum BC-1?

Was this a case rupture or a fatigue failure? That is, was the problem a reload recipe that was running too hot or improperly loaded? Or was it a problem with the BC-1 failing due to material defects developed over time?

We needed more information, but in the meantime, we recommended the shooters refrain from using that load until we were sure. Both guys were good enough to answer all our questions and send in the carriers, their spent casings, and some unfired rounds of their reload.

The circumstances of the failure gradually took shape.

The Carriers

Thankfully, both the BC-1s had failed without injury or even much damage to the rest of the guns. With a quick carrier group swap at the event, both shooters were able to finish out the match.

After visually inspecting the carriers, they had all the telltale signs of a case rupture. This causes a rupture along the thin rib of material connecting the bearing surfaces to the central hammer cocking pad.

This design "weakness" of the carrier is actually a feature of the original Stoner design. It's intentional fail-point allows for excess gasses and debris to exit out the bottom the carrier through the magazine well in the case of a catastrophic case failure.

Preventing failure entirely is impossible. The best you can do is manage and direct it.

At first, fatigue failure had been a concern, but we laregely rule that out now. The BC-1 is a new design, but we'd never seen failure like this from the previous generation of aluminum carriers or any of our beta testers. With such a low round count, a carrier problem was looking less and less likely.

The Cases

Next, we turned our attention to the ammunition. Because this was a reload and not factory ammo, the cases would tell the tail. Here's what we found.

The cases had failed in the chamber and ruptured at the extractor where the case is not supported by the bolt assembly. Additionally, the bullets were still in the cases, indicating that they had never left the barrel. A fatigue failure of the carrier would not have affected case pressure when the bolt was locked, so the rounds should have fired.

Looking to the reload recipe, John Paul could see a problem right away. This particular load mixed a powder that was too fast for the bullet choice. While the shooters had used a similar reload for years without (obvious) issue, this combo was clearly too much.

In the end, we were satisfied that the BC-1s were not the problem. While a steel carrier would probably have endured longer, the pressures of this particular load were just too grueling. Failure was inevitable.

The big takeaway here is that reloading is an art and a science with hazards all its own. It is very easy to create loads that are workable but on the edge of safety.

A cartridge is really just a tiny explosive. Both the component parts—brass, powder, bullet and primer—and the assembly process must be just right to create rounds that can be fired at all, let alone accurately.

So far, we only have half the story. What caused this round to fail?

Next month, we'll pick back up for a Part 2 as John Paul weighs in on this curious case. We'll delve deeper into how neck tension and press choice might have played a role and what lessons reloaders can learn.


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