Open Tip Bullets

A week or so ago, there was an interesting article over at Murdoc's site concerning a Army JAG who issued an order banning snipers from using open tip rifle bullets over concern that they are similar to expanding hollow point bullets. Blackfive also posted about this issue. The open tip bullets are used because they are more accurate than regular full metal jacketed military bullets and, in fact, are not expanding bullets at all.

Murdoc and Blackfive have pretty good discussions of the legality of open tip and hollow point bullets and the degree to which the JAG stepped out of bounds. Specifically, it appears that JAGs are not even authorized to issue these types of orders in the first place. They advise commanders who then issue orders based on that advice.

After reading these articles, I found myself asking a variety of questions. Why are open tip bullets more accurate? Since they have an open tip, why don't they expand the way hollow points do? Knowing nothing about bullet design/aerodynamics, I spent some time over the last week looking around for some information on-line. I did not find a lot, so I figured I'd draw some pictures and post what I have been able to find.

[If you click on the graphic it will open in a new window. I recommend doing this so you don't have to keep scrolling back up to refer to it during the following discussion.]

The graphic shows four 7.62 mm/.308 Winchester bullets currently available. (I should thank Gary for his 7.62 mm (7.62 x 51 mm) Ammunition guide, since a lot of my data comes from that page.)

The three bullets on the left are used in U.S. military rounds, while the one on the right is a Winchester hunting bullet. Each of these bullets could be incorporated into rounds chambered for military 7.62 mm or civilian .308 Winchester — which differ only in the cartridge.

When incorporated into cartridges, the high accuracy rounds (M118 and M118LR) are slightly longer than the standard 7.62 mm rounds that the military uses (2.83 inches vs 2.80 inches). I saw some indication that these longer rounds would not be compatible with belt-fed machine guns, but you wouldn't need highly accurate bullets in those weapons anyway.

As you can see, all four bullets have roughly similar noses — utilizing tangential ogive shapes. I did not find perfectly detailed diagrams of the bullets, some are traced from small pictures found on-line, so they may not have exactly the same nose shape, but they are very similar. The tangential ogive, or "Spitzer", shape is very common for bullet noses, rocket nosecones and the like. All four bullets also have boattail bases. The boattail design reduces drag by tapering the tail of the bullet.

As far as material, all four have lead alloy cores with harder metal jackets. The military bullets have copper-plated steel jackets while the hunting bullet has a copper alloy jacket. The hunting round does not need to penetrate light armor and is designed to expand inside the game, so a softer jacket is called for.

Construction methods for the four bullets are also similar. Jacketed bullets are generally manufactured by swaging (forcing room temperature material into molds under high pressure) rather than casting (pouring molten material into molds). The bullet core is shaped from thick lead alloy wire through a series of swaging steps — each one resulting in a shape closer to the desired finished step. The jacket is formed in a similar series of steps, but starts as a disk of material that is formed into an intermediate shape that looks like the cap of a pen, and from there formed into close to the final jacket shape. When the core and jacket are almost at their final shapes, the core is placed in the jacket and a final pressing (or series of pressing), forces the parts together and into the final shape.

The two bullets on the left are referred to as full metal jacket. In this design, the core is placed in the jacket through an opening in the base, and then the swaging closes the jacket around the core, leaving a small area of exposed core in the base of the bullet. The jacket covers the entire tip, nose and body of the bullet. The two bullets on the right are referred to as open tip bullets and have the opening in the jacket at the tip — leaving very small opening at the tip (like the M118LR), or filling the opening with an aerodynamic polymer cap (like the .308 Winchester shown).

The first bullet on the left is the bullet from the U.S. military M59 round, which is the U.S. designation for the older standard 7.62 mm round. The newer round is the M80, but I could not find a diagram of the M80 to sketch and the two bullets are very similar. The M59 is a much smaller/lighter bullet than the others, which allows for more propellant in the cartridge and less inertia to overcome. As expected, this round has a higher muzzle velocity than the more accurate, heavier rounds (2,750 fps vs 2,650 fps). The other advantage of the lighter bullet is that it allows the infantryman to carry more ammunition.

Also notice the cannelure around the body of the bullet. This indentation allows for a tighter seal between the bullet and the cartridge. This is important for the infantryman, who does not have the luxury of taking careful care of his ammunition. The tighter seal provided by the cannelure reduces the chance of the bullet loosening within the cartridge as it is roughly handled and keeps moisture from getting into the propellant. The cost of the cannelure is aerodynamic performance, which is why the high accuracy rounds make do without it.

The other three rounds share a similar shape to the M59, but are less stubby and lack the cannelure. Both features help reduce drag and turbulence — improving both range and accuracy.

The next bullet to the right of the M59 is the bullet from the U.S. military M118 round. I chose to include this bullet because it represents the intermediate step between M59 and the more radically different open tip bullets. The M118 shares the materials and full metal jacket construction with the M59. The difference is the M118’s length and lack of cannelure. What is not shown here, however, is that the M118 — as with all high accuracy rounds — is manufactured with more stringent tolerances and quality controls. Even very small imperfections in the surface of the bullet or distribution of weight will affect accuracy over the range that snipers work.

Now, we finally come to the bullet from the newer M118LR round, which is simply the Sierra MatchKing 175 grain .308 Winchester bullet which the military combines with a 7.62 mm cartridge. Why is this open tip bullet more accurate than its full metal jacket counterparts? If you can believe it, I could find very little information on-line explaining the source of this accuracy. However, there was enough to make some guesses. If anyone seeing this has better information, please let me know.

There are two basic potential sources for aerodynamic improvements: better design and better manufacturing.

The design of the M118LR is fundamentally very similar to the full metal jacket M118 bullet. The opening in the tip of the M118LR bullet is very small. The aerodynamics for such an opening on the leading edge like this can be simplified as a flat surface since there would be very little, if any, flow of air into/out of the open tip of the bullet. The opening is very small, the air is being forced straight in and the air has no other opening through which to escape. So, in terms of design, the tip of the M118 and M118LR should be aerodynamically very similar. The bases of the two bullets are also very similar, although it is possible that the M118’s cup base with exposed lead creates additional drag and/or uneven airflow over the base.

Another possible design advantage for the open tip bullet is the distribution of weight. The center of gravity of the bullet is further back than the M118 bullet. The center of gravity, in conjunction with the rotation speed of the bullet, can affect the bullet’s angle with regards to its trajectory. Ideally, a bullet points tangentially to its trajectory at all points. To do this, the bullet needs to rotate its nose downward over the course of its trajectory. The weighting and spin of the bullet both contribute to whether a bullet flies nose-up, nose-down or in-line with its trajectory. The argument against this center of gravity theory is that there is no reason to believe that the M118 would not have been designed and tested to achieve the same effect.

There is another issue associated with the distribution of weight that might be helpful to the bullet’s performance. Due to the empty space at the tip o the M118LR, this bullet has a higher moment of inertia — its mass is distributed further from the axis of rotation. A higher moment of inertia should make the bullet retain its spin better. The bullet will be spinning faster out at long distances and spin = stability = accuracy.

Interestingly, it seems the bullet is actually less accurate if the open tip is fully closed in the manufacturing process. It’s possible that the open tip is doing something to stabilize the bullet’s flight, although I could find no explanation on-line why this would be so. One possible explanation is that interfering with the flow of air over the nose of the bullet could act similarly to the dimples on a golf ball – by interfering with the laminar flow, golf ball dimples actually allow a golf ball to fly further (see this page for better explanation). It is also possible that closing off an air cavity of that size could create problems. It’s not hard to imagine that the shockwave (from being fired) could deform the relatively thin walls of such an enclosed cavity.

More likely, though, this is a manufacturing issue, rather than a design feature. I would imagine that it is virtually impossible to press the edges of this opening fully closed without incorporating aerodynamically unsound surface defects and/or weighting defects.

In fact, it is possible that the ability to reduce manufacturing defects is one of the reasons why the open tip design provides better performance. Notice that in the M118LR, there is no lead core at the tip of the bullet. In a full metal jacket bullet, the core is pressed into the tip of the jacket under high pressure, making it hard to ensure that the jacket retains a consistent thickness. Consistent thickness is important because surface imperfections are most influential on aerodynamics at the tip and uneven weighting will cause the bullet to wobble slightly as it spins (and bullet wobble is the enemy of accuracy). Under the open tip manufacturing process, the core is pressed into the jacket at the rear, where the aerodynamics is less important and where the lack of a sharp point makes it easier to control the mating of jacket and core.

So now that I’ve discussed why the open tip might provide better performance, it probably makes sense to make a few comments about the difference between the open tip M118LR, which is not an expanding bullet, and the .308 Winchester Supreme Ballistic Silvertip, (which, for brevity, I will refer to as “SBS”) which is an expanding bullet, designed to kill large game. (For those not “in the know” big game bullets are designed to expand for the same reason police use expanding bullets — maximum stopping power and reduced chance of the bullet going all the way through the target and hitting something behind it.)

There are a few key distinctions between the SBS and the M118LR. The most important distinction with regards to the SBS’s ability to expand within the target is the shape of the lead core. Even without the polymer insert, the hole in the leading edge of the lead core would result in expansion of the bullet. The air pocket in the core would expand under the pressure of encountering the target, pushing the edges outward. As the core expands outward, the continuing forces of the bullet moving through the target continues to flatten the bullet. The polymer insert helps this process by acting as a wedge pushing the core apart from the inside. As discussed before, the softer copper jacket — as opposed to military steel jacket — helps this process as well.

The flat leading edge of the core in the M118LR drastically reduces the propensity of the core to expand outwards when contacting the target. Although it is not necessarily obvious merely by looking at the diagrams that the open tip will not expand, it is clear (at least to me) that the mechanics at the two tips might be very different. Frankly, to my untrained eye it appears that the M118LR tip is more likely to collapse downward than expand when contacting a target.

One other observation I have about the open tip bullets is that it seems they would be less useful against lightly armored targets, such as body armor. There’s still a lot of kinetic energy in them, so they are probably effective (especially at shorter ranges) but I find it hard to believe that an open tip bullet could penetrate all of the same materials that a full metal jacket design could (considering its solid steel tip).

Again, please feel free to weigh in if you have any additional insight into the dynamics of these bullets.