Bullets are bullets, right? Well, not really. There are a wide array of variables in bullet shape, weight, materials, and construction. Each of these variables can have a dramatic impact on the performance, and safety, of any given load. In other words, loading recipe data for one 147-grain, .308 bullet may be incorrect, and even unsafe, for another projectile of equal caliber and weight.
Let’s take a closer look at some of the variables and what you need to know about each.
Long ago, the universe of bullet materials could be summed up in one word: lead. Now, we can choose all lead, lead alloys, copper-jacketed lead, plated lead, coated lead, all-copper, compressed powder frangible, and even polymer and copper blend bullets. You’ll even find ammo with made up metallic names that sound like the stuff that powers the Starship Enterprise.
Even among bullets with the same overall weight, performance can vary widely, often as a result of friction as a bullet travels down the bore. As a simple example, it’s easier to push a soft lead bullet down the barrel than a hard copper jacketed one.
Jacketed bullets have a comparatively hard copper or copper alloy exterior. Usually, lead interior cores are stuffed into this hard jacket during the bullet construction process. The intent behind this design is to have a bullet that’s as heavy as possible for its size because the bulk of the interior is made of dense lead. The copper jacket adds strength and allows the bullet to be forced down the barrel at higher velocities. Under higher pressure and velocity, lead bullets would melt and deform. Since the jacket is tougher, more force is required to drive a jacketed bullet through the rifling, so, generally speaking, you’ll see larger powder charges stated in load recipes than for a comparable weight lead bullet.
As lead is softer and squishier than copper, it’s easier to push through rifling in the barrel. Most load recipes for same-weight bullets will be about 10% less than those for copper-jacketed projectiles. The 10% figure is a rule of thumb, so always check specific load data rather than relying on that number – it’s just a guideline.
A note on barrel leading…
Lead bullets (usually made by adding other metals like tin or antimony if different proportions) can, in certain cases, leave excessive lead residue in gun barrels. Whether a lead bullet will cause “leading” in the barrel isn’t so simple as being a result of a bullet being too soft. In reality, whether a bullet in a given load will lead the barrel or not is usually a function of pressure and bullet hardness. In other words, using a harder bullet, in some situations, can cause more leading than the use of a softer one. Here’s why.
A lead bullet is designed to expand or obturate slightly under pressure from the hot gas blast behind it. The idea is that the lead base expands into the rifling of the barrel, preventing hot gas from slipping by. This seal helps create maximum efficiency of a given load and powder charge. If a bullet is too soft for the pressure level of a given charge, the base will melt and leave a mess in the barrel. If the bullet is too hard, the base won’t expand and seal the bore, so hot gasses blow by the bullet, melting the sides and leaving excessive leading. You want to match the hardness of your projectile to the anticipated pressure of the load.
On the outside, plated bullets look like jacketed ones, but there’s an important difference. A plated bullet is manufactured by chemically and electronically applying a “coat” of copper jacketing material to the outside of a lead projectile. As a result, the “jacket” is much thinner and softer than that of a true jacketed bullet. Plated bullets won’t handle the same pressure and velocity of jacketed counterparts, although we’re starting to see some new plated designs that have thicker plating.
Regarding performance, a plated bullet performs more like a lead one of identical shape and size and usually requires a powder charge similar to a lead bullet. A rule of thumb is that many plated bullets use a charge weight about 10% less than that of an identical jacketed bullet. Don’t rely on that math – it’s just for informational purposes. Always check with the bullet manufacturer to find the correct charge.
It’s important to recognize that bullet weight isn’t everything. That means that you can’t necessarily use the same powder charge for different bullets of the same caliber and weight – you also have to consider the shape of the bullet.
The shape defines how much of the bullet contacts the rifling as it travels down the bore. The more surface area contact, the more force is required to move the bullet through the rifling. If one bullet shape is harder to push through the bore, there’s going to be a higher pressure buildup behind it.
To illustrate this point, think about pushing two objects through a cannon – a football and a giant #10 can of lima beans – you know, those giant ones you find at places like Sams Club. Assuming that the widest part of the football and lima bean can are identical in diameter, it’s clear that there is much more surface area contact between the lima beans and tube than there is with the football. As the football is sharply tapered at both ends, only the very widest section in the center of the ball will contact the barrel walls. Since the lima bean can is a cylinder, the entire can will create friction against the inside of the National Vegetable League cannon barrel. Even if the football and ballistic vegetables weighed exactly the same, it’s going to take more pressure to launch the beans at the same velocity.
It’s exactly the same with bullets. Round or flat nose bullets are more cylindrical in shape, so much more of the length of the bullet will scrape against the rifling. On the other hand, an aerodynamic ballistic tip boat tail bullet only has a small section in the middle that contacts the rifling so it will operate with less pressure.
You’ll even find some bullet designs with driving bands. Rather than a smooth surface that contacts rifling, some bullets have grooves cut in the bodies. These grooves reduce surface area contact, which minimizes friction. The idea is to create more velocity with a given powder charge or allow a larger powder charge without an increase in pressure.
Summing it up
All bullets are not the same, even if they are the same caliber and weight. The hardness (read friction) of the bullet and the amount of surface area contact can both have big impacts on the pressure level of any given load. It’s yet another reason to stick to published loading information exactly – don’t make assumptions.