When a gun goes off, the expanding gasses created by the burning gunpowder pressurize the chamber and force the bullet down the barrel. There’s all sorts of interesting science behind what the maximum pressure of the ammunition can be before the chamber ruptures, and what the proper pressure is for the best and most consistent grouping. But that all assumes a “normal” pressure wave. What happens if your ammunition isn’t actually conforming to that normal curve? And is a secondary spike even possible? . . .
Let me start by talking about measuring those spikes in the first place.
The standard method for measuring the pressure in a firearm’s chamber is by direct observation. A hole is drilled into the chamber, and a small sensor is placed in the hole. Historically the sensor was made of either copper or lead, but more common today is a piezoelectric sensor that translates pressure into electrical current.
The problem with direct observation is that it’s expensive and you have to sacrifice a barrel to the process. However, Charlie Sisk at Sisk Rifles has a pretty smart way of doing pressure testing on the cheap. Instead of directly measuring the pressure in the barrel, he simply measures the expansion of the metal surrounding the chamber (since the metal deforms slightly as the gun is pressurized) using some very accurate sensors and computers. Through years of testing, he has perfected the formula for determining chamber pressure based on metal expansion.
This is a “normal” pressure curve. The pressure in the chamber increases as the powder burns, then decreases as the bullet moves down the barrel and increases the available volume. Eventually the bullet leaves the barrel and the pressure drops off.
However, Charlie started to notice that sometimes when he was working up a load he’d get a secondary spike. One that didn’t make any sense.
You expect the first spike from the initial gunpowder burn, but where the hell did that second pressure spike come from? It didn’t make sense.
Naturally, Charlie kept testing. As a result, he can create a secondary pressure spike whenever he wants just by varying the load parameters. The ability to repeat the results on demand indicate that this isn’t some kind of instrumentation fluke. There really is a secondary spike going on in some loads. And not just in handloads — Charlie has seen this same pattern in some commercial ammunition, too.
“So what?” I hear you say. “That’s interesting, but what does that have to do with me?” Well, turns out that secondary spikes might have some safety implications.
Charlie was playing around with a new, full-length barrel (26 inches, I believe) in his gunsmithing shop and creating some secondary spikes in order to figure out what was going on. After the seventh round, the shot sounded… different. When he looked, it turns out that the front five inches or so of his barrel had fallen off.
He repeated the same test with three more barrels, and all three were cleanly chopped off at 21.6 inches exactly. Secondary pressure spikes caused spontaneous barrel shortening. Or, put another way, they blasted off the front of his barrel.
The cause of these spikes is still unknown. But that doesn’t mean there isn’t a theory. The best of those is that the water vapor in the atmosphere of the barrel forms a “vapor cone” around the bullet, much like it does around aircraft breaking the sound barrier. That vapor cone, as the theory postulates, creates increased drag and therefore a pressure spike. Again, it’s still just a theory.
Charlie is continuing to do his testing, and others have picked up on the issue, too. There’s a paper on the subject currently being written by a couple of other internal ballistics boffins, so we should have some more hard evidence of what’s going on shortly. Until then, its just another fascinating mystery.