Firearms are, at once, both very simple and very complex. The theory behind using a firearm is easy. You point it at the target, pull the trigger, and it sends a projectile to impact or destroy that target.
Somewhere between conception and reality are a thousand smaller variables that affect that seemingly simple turn of events. Firearm design is a complicated field of study involving mechanical engineering, chemistry, applied ballistics, and other general sciences. For the beginning to average shooter, a lot of the science behind the shooting can seem overwhelming or incomprehensible for anybody without a science degree. Fortunately, with basic understanding of a few key principles, much of firearm ballistics such as bullet weight, velocity and ballistic coefficient can be demystified.
Bullet Weight and What It Means
New shooters buying ammo commonly ask about the number of grains listed on a box of bullets. Two boxes of .30-06, for example, might seem identical other than the fact that one is marked 165 grain and the other 180 grain. Some new shooters are confused by this, and it’s commonly assumed that the grain number refers to the amount of gunpowder loaded in the cartridge.
In general nomenclature, a bullet (referring to the projectile and not the loaded cartridge which is sometimes erroneously called a bullet) is measured in weight by a unit called a grain. To give a relative idea, there are 437.5 grains in one ounce. Super light varmint rounds may weigh as little as 17 grains, while intermediate big game rounds tend to run in the 125-200 grain range and some specialty big-bore rounds may weigh 500 grains or more — heavier than a 1 oz slug from a 12 gauge shotgun!
So what does the weight of a given bullet mean? The first major effect is velocity. Consider two bowling balls; one weighing 6 pounds and the other 10. The 6 pound is lighter and easier to move. It will be easier to roll that bowling ball to a given velocity than the heavier ball. Because it has less mass, there is less inertia keeping it in place so you have to use less force to overcome it.
Once at the same speed, however, the heavier bowling ball will remain at that speed longer and will be less likely to have its path affected by obstacles and other resistance. An object in motion wants to stay in motion, and a heavier object has more momentum at the same speed than a lighter object.
Following this logic, a lighter bullet will leave the muzzle of a firearm faster than a heavier bullet when launched from the same case with the same propellant. The heavier bullet will tend to maintain the velocity it does have better than the lighter bullet and may have a flatter trajectory. Lighter bullets will also have a lower recoil in the same load than a heavier bullet. These are useful generalities to consider, but are only generalities and are not necessarily rules.
How Bullet Weight Affects the Shooter
Knowing that information, how can you better apply it to shooting and hunting?
For a new shooter considering the type of firearm they need for hunting, a little bit of knowledge about the cartridges involved can help you make an informed decision on which cartridge will suit your needs best. Lighter calibers like .223 Remington and .243 Winchester use a lighter bullet at a relatively high velocity. These cartridges will produce a low and tolerable recoil which may be desired for younger, smaller, or more inexperienced shooters who wouldn’t effectively be able to use a high recoil round. These rounds, being smaller, are typically used for varmint shooting but have also found success in the right circumstances for intermediate game such as small deer or hog.
Keeping in mind that all energy leaving the barrel will be mirrored by felt recoil, it’s important to know that a lighter and lower recoiling round will have a lower muzzle energy than larger rounds. While these lighter rounds are great for their low recoil and have a relatively flat trajectory, they may lack the oomph necessary for heavier applications. What is suitable for a woodchuck or coyote would be grossly inappropriate for a water buffalo, and the same would be true in reverse.
When considering a certain job, picking the correct cartridge and bullet to do that job is paramount. Bullet construction and design also plays a large part, but that will be covered in a later section.
So how does bullet weight and velocity affect performance?
Penetration vs. Expansion
Considering that the end objective in hunting is to kill an animal quickly and as humanely as possible, there are two major effects of the wound channel that will provide a quick incapacitating shot; the penetration and expansion of the round.
Penetration is a measure of how deep the bullet goes into its target or how much media it will travel through. Expansion is a measure of how much the diameter of the bullet expands inside its target. For example, a soft point 30 caliber bullet, measuring just under a third of an inch, may compress upon entering the target and end up as an object near a half inch in diameter. In general, a lighter and faster bullet will tend to expand more violently and penetrate less, while a heavier bullet will be less prone to expansion and have more penetrating capability. Without changing bullet construction or powder charge, there’s rarely a way to increase both penetration and expansion — to some degree you wouldn’t want to, as too much of either can have negative consequences such as severe over penetration or destruction of meat and hide. Therefore, a bullet must be picked to perform best at its specific task. A needle sized hole that goes completely through the vital organs of an animal without disrupting enough tissue may lead to a slow kill and an unrecoverable animal. On the other hand, a bullet that leaves a softball sized wound a couple of inches deep but fails to penetrate to any vital organs will lead to a gruesome wound that will fail to stop an animal immediately and may only cause death hours or days later, if at all.
Different Classes of Bullet
In general, lighter centerfire rounds (such as the .22-250, .223, .243 etc.) fire bullets weighing between 40-100 grains which tend to be moving at significant speeds. They are known for less penetration than bigger, heavier rounds and may also fragment and expand more violently than the bigger bullets and will lend themselves more to smaller game animals. These light rounds can create surprisingly large wound channels and cause severe damage from fragmentation, but the bullets capable of doing so in these lighter calibers lack the penetration and overall energy to be efficient for use in larger game. They may be used on smaller hog and deer, but would not commonly be used on much larger game. They are often used for coyotes, ground hogs and other varmints, and hogs and deer where animals tend to be smaller and the average shot tends to be short. They are also commonly used by youth shooters who use lighter guns and would be hindered by the recoil a larger round would cause in the youth sized gun.
Perhaps the most popular cartridges for deer and hog are the more intermediate cartridges shooting bullets somewhere between 120 and 200 grains. Cartridges in this range include the venerable .30-30, .308, .30-06, and .270. For some, this also includes military cartridges of the world including 7.62X39mm, 7mm Mauser, 7.62X54R, and many others that have made their way into civilian hands over the decades as collector’s pieces at low prices. Although these rounds have a benefit in being commonly available and relatively inexpensive, that’s not the only reason they are popular. Bullets of this weight are a great compromise that does everything well. Bullets of this weight are heavy enough that they will reliably penetrate very deeply and stay together well enough to ensure a consistent wound channel, but are also capable of being pushed fast enough for hollow point and polymer tip style bullets to reliably expand to a good size to ensure maximum lethality. More about expanding bullet types will be covered in the next section.
Cartridges at the low end of this intermediate scale are often used as brush guns for hunting deer, hog, and even black bear inside relatively close ranges of 150-200 yards. More powerful rounds are very versatile and can be used for everything from varminting to big game. Cartridges at the higher end of this range, including the .30-06 and .300 Winchester Magnum are suitable for taking even large game such as elk and moose. Due to their heavy weight, they will retain their energy better than lighter bullets and are capable of delivering high energy and humane shots to more appreciable distances, making them a good choice for big game even in open areas where shots may be long.
Finally, in a range typically deviating from 300 to 500 grains, you may find super heavy specialty bullets. These bullets fill a relatively rare niche, and tend to come in two flavors; super long, profiled bullets (for effective long range shooting) and very fat, round or squared bullets (for devastating wound channels and penetration in relatively short ranges). Bullets of the first category include the .338 Lapua Magnum, .408 Cheytac, .416 Barrett, and even the monster .50 BMG with bullets weighing as much as 700 grains. Many of these cartridges are designated or used for military purposes, but their nature also makes them suited for extreme long range target shooting and even hunting use by civilians. Bullets of the second category may include the .45-70 Government, .375 H&H and .458 Lott. They may also include a new generation of cartridges designed to be loaded into the AR-15 for hard-hitting power, such as the .458 Socom, .450 Bushmaster, and .50 Beowulf. These cartridges are designed for extraordinary close range power in the regular-sized AR 15 platform. They are very popular with close range hog hunters who want to have the power to stop even the largest charging hogs and are willing to sacrifice the low cost per round and long range capabilities. The .50 Beowulf is notable in that it was designed to penetrate windshields of cars with minimal deflection and even put rounds into the engine blocks of cars at military check points. It now enjoys popularity in the civilian hunting world for very similar reasons.
These behemoth specialty rounds are not suited to beginner use, and are not even particularly well suited to standard shooting and hunting uses. The massive energy produced creates equally massive recoil for the shooter, and loaded cartridges for this ammunition can cost anywhere from $1 a round to $6 a round for specialty hunting ammunition. They are not pleasant to shoot repeatedly, nor are they conducive to standard target practice. For those who have a special need for deep penetration and high muzzle energy, they will deliver what few other rounds are capable of delivering.
Bullet Velocity, Ballistic Coefficient, and Long Range Shooting
Bullets in flight are fast and can be sent with high precision, but they are not lasers or beams. They are susceptible to being acted on from outside forces, including gravity and wind resistance. In this case, not all bullets are created equal. It’s usually considered common knowledge that different bullets will drop at different rates and that some will lob in a steeper trajectory than others. However, this is often wrongly (or at the least, over-simplisticly) attributed solely to the weight of the bullet. Regardless of weight, two bullets dropped in a vacuum will fall at the same speed. Although real world performance is different than performance in a vacuum, the difference in falling speed is more attributable to bullet velocity, inertia, and how the construction of the bullet allows it to maintain its velocity while being acted on by outside forces.
The faster a bullet moves, the quicker its flight path in which it can be acted on by gravity. Therefore, a faster bullet with a shorter flight time will reach the target much faster than a slower bullet and will drop less from its initial point of aim compared to the slower one. Additionally, a longer bullet will (to a certain degree) be more efficient than a bullet of the same weight which is shorter and more round. Long profile bullets, such as the .338 Winchester Magnum, will be more efficient at resisting wind and drag than a rounder bullet of a similar weight, such as one from the .375 H&H Magnum. Although both shoot bullets in the 200-250 grain range, the .338 Win Mag will retain its energy better over distance. The bullet from the .375 starts off faster (roughly 3,100 FPS vs. 2,900 FPS with a 200 grain bullet), but the .338 Win Mag will lose that velocity slower and have a flatter trajectory, requiring less of a hold over on target.
A good technical quantification of this is called the ballistic coefficient. So what is ballistic coefficient you may ask? The ballistic coefficient is a measure of the ability of a body in flight to overcome air resistance and maintain its velocity. A generally correct way of saying it would be that a higher ballistic coefficient can equate to a lower drag. It is inversely correlated with negative acceleration, meaning that a bullet with a high ballistic coefficient will slow down more slowly than a bullet with lower ballistic coefficient.
Taking the example above, a .338 Win Mag with a ballistic coefficient of .515 will lose just over 700 feet per second (roughly 25% of its initial velocity) and drop 42.3 inches at 500 yards. It also retains 2025 of its initial 4089 ft-lbs of energy, just a fraction below 50%, at 500 yards. Conversely, a .375 H&H Magnum with a ballistic coefficient of .430 loses almost 1,000 feet per second (33%) of its initial muzzle velocity and drops 43.6 inches at 500 yards. This drop, while only 1.3 inches more than the .338 Win Mag, will become much more pronounced at ranges past 500 yards as its kinetic energy is quickly lost and the bullet slows down considerably. At this distance, has dropped from an initial kinetic energy of 4636 ft-lbs to 2027, a loss of 56% compared to the .338’s 50% at this same distance.
Let’s take a look at another common comparison: The 5.56X45mm and 7.62X39mm cartridges. These cartridges, most commonly fired by the AR-15 and AK-47, are a hot debate in the shooting community. The 123 grain 7.62mm bullet, at over twice the weight and 75% of the initial velocity of the 55 grain 5.56 round, is often considered to be a harder hitting round in close ranges. However, it drops significantly with a trajectory often referred to as a “rainbow”. The 5.56 round, with significantly less energy at its entire effective range than the 7.62X39, is a favorite of people who prefer its high velocities and relatively flat trajectory at longer ranges. User Skeptical on the survivalistboards forums put together this reference chart to help visualize the difference in flight paths between the two rounds.
Zeroed to hit the point of aim exactly at 200 yards, the difference in trajectory becomes clear. The 7.62X39 round has a distinct arch to its trajectory, raising more than twice as high at 100 yards and falling 85 inches below the point of aim at 500 yards. The 5.56 or .223 Remington round raises less on its path to zero, requiring less of a hold under at closer ranges, and only drops 44 inches at the same 500 yards. According to gundata.org, the standard 123 grain military round has a ballistic coefficient of roughly .225 and, when zeroed at 100 meters, will drop as much as 1,000 inches at 1,000 yards. Gundata lists the .223 as having a ballistic coefficient as high as .257, and dropping only 750 inches at 1,000 yards, meaning 25% less hold over than the 7.62X39.
In this case, it’s relatively clear that the ballistic coefficient of the .223/5.56 is higher than the 7.62X39mm and contributes to its low drop, but the higher velocity of the .223 round cannot be overlooked. For a more pure example of the effects of ballistic coefficient, let’s consider two very similar cartridges: the 7.62X39mm, which we looked at, and the 6.5 Grendel. The 6.5 Grendel, otherwise called a 6.5X39mm, is actually a 7.62X39mm cartridge with a 6.5mm diameter bullet. Both bullets weigh about the same (123 grains for a standard FMJ load). Both cartridges push the bullet to between 2300 and 2400 feet per second in a rifle with a 16″ barrel, although guns chambered in 6.5 Grendel often have longer barrels to push the bullet to 2600 feet per second, giving the round a slight edge. The big difference, however, is the ballistic coefficient. Because the bullets are the same weight, and the 6.5mm bullet is smaller in diameter, the extra mass must be incorporated through a longer bullet design. This long profile gives the 6.5mm bullet a ballistic coefficient of well over .500 with 120-125 grain bullets. With a 200 yard zero, this high ballistic coefficient provides a drop of only 54 inches at 500 yards, a significant difference from the 86 inches of the 7.62mm bullet of the same weight pushed at the same speed. From this comparison, it’s easy to see that both velocity and ballistic coefficient are important in considering the long range capabilities of a cartridge.
We can also compare the data of the 6.5 grendel to that of the .223 caliber cartridge. Although it may seem to be a disappointment in comparison to the .223 bullet, which drops only 45 inches at 500 yards, it has a muzzle energy of 2025 ft-lbs at this distance compared to the paltry 268 ft-lbs the .223 will provide. The .223 Remington initially offers a blazing 3130 feet per second and 1196 ft-lbs, at 500 yards it drops to 1480 feet per second (a 52% reduction) and the mentioned 268 ft-lbs of energy (a 77% reduction). The heavier 6.5 Grendel, initially leaving the barrel at 2580 feet per second and 1818 ft-lbs of energy, retains 1796 feet per second velocity (a 30% velocity reduction) and 881 ft-lbs of energy (a reduction of ~50%). Thanks to a ballistic coefficient over twice that of the .223 Remington, the 6.5 Grendel retains its energy far better. Its trajectory also shines at long distances. Although the .223 Remington drops 750 inches at 1000 yards according to Gundata, this chart from 68ics shows that a 6.5mm 123 grain bullet leaving the barrel at 2575 will drop barely half of that, at just 387 inches below point of aim. The performance of this round is obviously superior to the .223 or 7.62X39mm at long ranges, due to its very high ballistic coefficient.
If your head isn’t spinning yet from part 1, in part 2 we will cover more about bullet construction in terms of building materials and design to retain energy and create lethal wounds for humane hunting and specialty bullet types for other purposes. If your head is spinning, just remember that these types of studies are only as complicated as you want to make them. You don’t need to be a physics major to understand basic concepts about what your bullets do in flight and to make good choices based on that. Let me know what you think about your favorite bullets in the comments below.
Header Image from user Guy Miner at the Nosler Forums. All other pictures as attributed
“A needle sized hole that goes completely through the vital organs of an animal without disrupting enough tissue may lead to a slow kill and an unrecoverable animal. On the other hand, a bullet that leaves a softball sized wound a couple of inches deep but fails to penetrate to any vital organs will lead to a gruesome wound that will fail to stop an animal immediately and may only cause death hours or days later, if at all.”
This is a great paragraph. It really sums up why the topic of this article is so important.