As a cartridge fires, and the bullet is forced out of the casing, it hits the barrel’s rifling; at this point there’s a certain amount of impact. Gas pressure is pressing on the rear of the bullet, there’s friction between the sides of the bullet and the bore (hole through the center of the barrel), the bullet expands to fill the bore. This process of expansion creates a seal between bullet and bore, preventing gases from flowing around the bullet as it travels down the bore. This avoids “flame cutting,” i.e. hot, abrasive powder gases and unburned powder scoring the sides of the bullet, which would lead to lower velocities and degraded accuracy. This process, of the bullet expanding to seal the bore, is called obturation, i.e. “the act of closing or stopping up, or the state or condition of being obstructed or closed.”
Imagine striking the base of a bullet with a hammer. Obviously the base would expand to be wider than the rest of the bullet. However, when we fire a gun and the bullet travels down the bore, since base expansion is constrained by the encircling barrel, the base expands to fill the bore, providing a good gas seal, but no further. Bullet expansion begins at the base, then travels up along more (but not all) of the bullet by the time it exits the barrel.
So, bullet obturation is a good thing. Question: At what pressure level do bullets obturate? According to Dennis Marshall, consulting ballistic metallurgist for Laser-Cast which sells bare lead bullets, “The most significant portion of obturation (that of permanent bullet deformation) begins as the chamber pressure approaches the bullet alloy’s yield strength as measured in pounds per square inch. For lead alloys, yield strength has been defined by scientific research as 480 times the Brinell Hardness Number (BHN). For example, a bullet with a BHN of 22 will have a yield strength of 10,600 psi.”
From Barnes Bullets came this comment on jacketed bullet obturation: “This is not a simple answer and there are many variables. For instance how thick is the jacket? Or does it consist of multiple thicknesses? Are they using pure lead or some alloy? What is the alloy made of? Is it heat treated? Some of our all copper bullets are annealed to soften them, even though they are all 99.95% pure copper, they can be different hardness depending on how many times they have been worked or smashed into form. So there really isn’t a good answer. A few years ago I worked with an engineer and we came to the conclusion that many of our pistol bullets required about 25,000 psi to obturate. At that time we believed our rifle bullets to require about 40,000 psi to obturate. However after much discussion we are reluctant to suggest a figure because of all the variables. No work has been done in regard to jacketed bullets.”
Bottom line: Bullets are designed to obturate well within the pressures generated by the cartridge into which they’re intended to be loaded. Even by pistol cartridge standards, which tend to be much lower-pressure affairs than rifle cartridges, the pressures required to produce bullet obturation are easily obtainable. Therefore, even when handloading low-pressure (in the overall scheme of things) handgun ammunition, we don’t have to worry about bullet obturation. It will happen, using anything that might be described as a “normal” load. When handloading bullets that are predictably much harder, for instance all-copper projectiles, we may have to load hotter to obtain sufficient obturation, but by that point we’re past the point of “basic” and into more advanced handloading.
4 Comments
Tom S
Do different bullet shapes obdurate differently? One bullet maker claims that rebated boat tails bullets obdurate faster and seal better. I don’t see how this can be true. I usually think of obduration as a function of bullet hardness (yield pressure) and chamber pressure.
ChrisK
I had to do a lot of shooting to finally arrive at a suitable non-lead hand load recipe for my rifle. During the course of all that shooting I believe I saw more rapid degradation of precision (commonly called “accuracy”) as the barrel got hot and expanded than I used to see using jacketed lead bullets. I had never before heard of bullet obturation but now wonder if solid copper bullets simply don’t obturate as readily as jacketed lead bullets do.
Human Manual
Flat based bullets obturate the fastest. Boat tail bullets generally do not. They are dependent upon being a press fit in the rifling for obturation.
Monolithic solid (one piece bullet with no separate lead core) bullets are the most dependent on being a press fit. As the bore expands from heating up, expect some degradation in accuracy. A series of chronograph readings at different barrel temperatures would show if the degradation is due to velocity change caused by incomplete obturation. If the bullet speed drops, then it is likely due to incomplete bore obturation.
JBR
There are some good thoughts and conclusions in this article. The expansion and therefore proper obturation of a bullet indeed depends on many factors and bullet base shape does indeed play a critical role as much as the meaterial and construction do too. All these factors are connected and unfortunately in the end we can’t have the cake and eat it at the same time. What is interesting today is when talking about jacketed rifle bullets we do see in general more boat tail bullets offered then flat base although in my opinion the majority of shooters actually does not even need the benefits of a boat tail bullet. The BT bullet really shines in reducing the backdrag of a slow flying bullet, when you need to reach out quite far but is actually a waste when punching paper up to three, four, five hundred meters or hunting at the usual closer and intermediate ranges. in fact a BT bullet suffers from more muzzle disturbance compared to a FB causing it to potentially exit the muzzle with a greater yaw. And yes it is true, powder gases tend to pry a BT pullet somewhat off the bore and the BT even has this tendency of guiding gases easier around compared to the FB. This is why a BT bullet should be as close to or ideally exactly or a tiny bit over the groove diameter, otherwise you will get substantial amounts of blow-by, causing bad accuracy and increased barrel wear. The best obturating jacketed bullet is actually an open base (exposed lead core) contruction with a softer and not too thick jacket material and a softer lead core. Gas pressure nicely upsets these to perfectly fill out the bore. As many rifle makers in the late 1880ies, early 1890ies have discovered this type of bullet actually works best with slightly undersized bullets compared to the bore. You get good obturation and less unwanted jacket smear. The moment the shooting world switched to the BT Spitzers they realized quickly the bullets now needed to be ideally at groove diameter in order to perform well in the bore. These old style flatbase open base bullets have unfortunately fallen out of fashion because the bullet makers standardized on the “one size fits all principle” of the modern spitzer (or even more so HP) BT bullet. Examples of where we would need more of this old style bullets and modern FMJBT usually suck, are the Steyr M95 in 8x50R, the 6,5mm Carcano and the 8mm Rast & Gasser revolver just to name a few prominent examples.
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