February 28, 2019
During a recent visit to my gunsmith, I met a fellow who had just experienced a serious shooting incident with a vintage Ruger No. 1 single-shot rifle. When he fired the last of his .308 Winchester handloads, the forearm splintered into several pieces and wisps of smoke seeped out from the rear of the receiver. Fortunately, he was wearing eye protection and his left hand and forearm only stung a little. The rifle’s breechblock was solidly jammed in place by the ruptured cartridge case.
My gunsmith was diligently trying to get the breech open to remove and inspect whatever might remain of the suspect cartridge. Within a few minutes, he had successfully forced the breechblock down without any apparent collateral damage. As you can see, the damaged case head indicates the cartridge had experienced a significant pressure excursion.
The case rim had extruded out between the breechblock and receiver and now measured about 0.040 inch thick. The rim had expanded to 0.535-inch diameter instead of the nominal 0.473 inch. A big chunk of the case head was missing in line with the barrel extractor cut, and “Winchester” and “308” were faintly visible on the case head. Flakes of brass fell out of the action as the various parts were removed.
When he’d finished, he reassembled the action and checked the headspace. Everything—except the forearm wood—was deemed to be okay. I started asking questions about the handload data and the components.
The shooter used Winchester cases, CCI 200 primers, 44.0 grains of IMR 4064, and Sierra 165-grain ProHunter bullets.
Checking a couple of load manuals, we concluded the recipe was approaching maximum but should not have caused the severe pressure event evidenced by the damaged case head.
Then I asked how the powder had been dispensed. The answer was an RCBS Uniflow powder measure. Right then and there I decided a plausible explanation would likely explain why the case ruptured.
IMR 4064 is a skinny, stick powder, so I suspected some of it probably bridged in the powder measure spout and when the measure’s handle was rotated, a few residual grains of propellant dropped into the case along with a full powder charge. In other words, one case was a bit short on propellant, whereas the next one was overcharged. After the bullets were seated, the difference would be indiscernible.
Nope. The shooter said he weighed every charge after dispensing them from the measure. That pretty much shot my scenario to pieces.
I asked if he could have inadvertently mixed propellants or gotten a heavier bullet mixed in with the correct ones. The shooter said that only the propellant container he was using was sitting on the loading bench and that he had opened a brand-new box of bullets for that loading session.
I was still scratching my head when I returned home. Fortunately, I have a cadre of industry friends who have encountered many unexplainable experiences during their careers. After relating the story to several of them, the most plausible theory was that perhaps the fellow had used the same powder measure to charge a different batch of ammo using a faster-burn-rate propellant. If he’d inadvertently left some powder in the measure, it may have gotten mixed in with the IMR 4064 he was using for the .308 Win. handloads. Keep in mind the excess pressure would have had to have been in the 90,000- to 100,000-psi range to the mangle the case in the way that it did.
I sat awhile and recalled how I empty the powder measure after loading a batch of cases. I remove it from the mount, turn it upside down, and pour the powder through a funnel into the bottle on the bench. Then I turn the measure upright with the spout pointed into the container’s mouth. Some amount of residual powder often remains on the baffle plate, so I cycle the handle several times to jar it loose and ensure it drops out of the hopper.
I could envision how one could leave some powder in the measure even after it had been “emptied.”
I got the shooter’s phone number, called him, and asked if he’d used the same measure to charge another batch of handloads prior to the .308 handloads. He said that he had just loaded some .45 ACP ammo with Unique powder prior to building the .308 Win. handloads.