Understanding Ballistics: What Happens When You Pull the Trigger
Ballistics is the science of projectiles in motion. For gun owners, understanding ballistics helps you choose the right ammunition, zero your sights effectively, predict bullet behavior at distance, and understand why certain loads perform better for specific purposes. This guide breaks the science into plain language.
By Dwight Ringdahl — GunExpos.com
The Three Phases of Ballistics
Ballistics is divided into three phases, each covering a different part of the bullet's journey:
- Internal ballistics — What happens inside the gun (from trigger pull to muzzle exit)
- External ballistics — What happens during flight (from muzzle to target)
- Terminal ballistics — What happens at impact (how the bullet performs when it hits something)
Internal Ballistics: Inside the Gun
The Firing Sequence
When you pull the trigger, a remarkably fast chain of events occurs in milliseconds:
- Trigger releases the firing mechanism — The striker or hammer strikes the primer on the cartridge base
- Primer ignites — The small impact-sensitive charge in the primer produces a flash of hot gas and particles
- Powder ignites — The primer flash ignites the propellant powder inside the cartridge case. Modern smokeless powder burns extremely rapidly, producing expanding gas at pressures of 15,000–65,000 PSI depending on the cartridge.
- Bullet accelerates — The expanding gas pushes the bullet forward through the barrel. The bullet engages the rifling grooves cut into the barrel's interior, which impart spin.
- Bullet exits the muzzle — The bullet leaves the barrel at its maximum velocity, followed by a blast of gas, unburned powder particles, and noise.
The entire process — from primer strike to bullet exit — takes approximately 1–2 milliseconds.
Barrel Length and Velocity
Longer barrels generally produce higher velocities because the expanding gas has more time to accelerate the bullet. However, there's a point of diminishing returns where the gas pressure drops below what's needed to continue accelerating the bullet, and friction begins to slow it.
Practical examples:
- A 9mm round fired from a 4-inch barrel (typical handgun): ~1,150 fps
- The same 9mm round from a 16-inch barrel (pistol-caliber carbine): ~1,400 fps
- A 5.56 NATO from a 10.5-inch barrel (short AR): ~2,700 fps
- The same 5.56 from a 20-inch barrel (full-length AR): ~3,200 fps
This is why the same cartridge performs differently in different firearms and why barrel length is an important specification.
Rifling and Spin
The spiral grooves cut into a barrel's interior (rifling) serve one critical purpose: they spin the bullet. A spinning bullet is gyroscopically stabilized, meaning it maintains its nose-forward orientation during flight rather than tumbling. This is the same principle that keeps a football spiraling point-first when thrown correctly.
Twist rate describes how many inches of barrel length it takes for the rifling to complete one full rotation:
- 1:7 twist — One rotation per 7 inches. Fast twist, stabilizes heavier/longer bullets.
- 1:9 twist — One rotation per 9 inches. Medium twist, good for 55–62 grain bullets in 5.56.
- 1:12 twist — One rotation per 12 inches. Slow twist, best for lighter bullets.
Using bullets that are too heavy for your barrel's twist rate results in poor accuracy because the bullet isn't spinning fast enough to stabilize.
External Ballistics: In Flight
Gravity: The Bullet Drops Immediately
The instant a bullet leaves the barrel, gravity begins pulling it downward at 32.2 feet per second squared — the same rate as every other object on Earth, regardless of how fast the bullet is moving horizontally. A bullet dropped from your hand and a bullet fired horizontally from the same height hit the ground at the same time (in a vacuum).
This is why all firearms must be sighted-in at a specific distance. Your sights are angled slightly upward relative to the bore so the bullet's arcing trajectory crosses your line of sight at the desired zeroing distance.
Bullet Drop at Distance
The faster a bullet travels, the less time gravity has to act on it before it reaches the target, and therefore the less it drops. This is why high-velocity cartridges are described as "flat-shooting."
Approximate bullet drop comparison (zeroed at 100 yards):
| Cartridge | 200 yards | 300 yards | 400 yards | 500 yards |
|---|---|---|---|---|
| .223 Rem (55gr) | -2.0" | -8.5" | -20" | -38" |
| 6.5 Creedmoor (140gr) | -1.8" | -7.5" | -17" | -30" |
| .308 Win (168gr) | -2.5" | -10" | -24" | -44" |
| .30-06 (165gr) | -2.3" | -9.5" | -22" | -41" |
| .300 Win Mag (180gr) | -1.7" | -7.0" | -16" | -28" |
This is why the 6.5 Creedmoor and .300 Win Mag are popular for long-range shooting — they drop less at distance than competing cartridges.
Wind Deflection
Wind pushes bullets sideways during flight. The amount of deflection depends on wind speed, bullet velocity, bullet weight, and — most importantly — the bullet's ballistic coefficient (BC).
Ballistic coefficient is a measure of how well a bullet resists air drag. A higher BC means the bullet cuts through the air more efficiently, retaining velocity and resisting wind deflection better.
- Low BC (0.200–0.300): Light, short bullets. Deflect easily in wind. Typical of .22 LR and light varmint bullets.
- Medium BC (0.300–0.450): Standard hunting and defensive bullets. Moderate wind resistance.
- High BC (0.450–0.650+): Long, heavy, streamlined bullets designed for long-range precision. 6.5 Creedmoor and .300 Win Mag excel here.
Wind deflection comparison (10 mph crosswind):
| Cartridge | 300 yards | 500 yards |
|---|---|---|
| .223 (55gr, BC .243) | 6.5" | 19" |
| 6.5 CM (140gr, BC .585) | 3.5" | 10" |
| .308 (168gr, BC .462) | 4.8" | 14" |
| .300 WM (180gr, BC .507) | 4.0" | 11.5" |
This illustrates why the 6.5 Creedmoor's high-BC bullets have made it the dominant precision cartridge — it deflects roughly half as much as .223 in the same wind.
Other External Factors
- Altitude — Higher altitude means thinner air, which means less drag. Bullets shoot flatter and drift less at elevation.
- Temperature — Hotter air is thinner (less dense), slightly reducing drag. Cold air is denser, increasing drag.
- Humidity — Counterintuitively, humid air is slightly less dense than dry air (water vapor is lighter than nitrogen and oxygen). The effect is minimal.
Terminal Ballistics: At Impact
Terminal ballistics is what happens when the bullet hits the target. This is where ammunition selection matters most for self-defense and hunting.
How Bullets Wound
Bullets cause tissue damage through two mechanisms:
- Permanent cavity — The actual hole the bullet cuts through tissue. Larger bullets and expanding bullets create larger permanent cavities.
- Temporary cavity — The stretching of tissue around the bullet's path caused by the transfer of kinetic energy. In elastic tissue (muscle), the temporary cavity collapses. In inelastic tissue (liver, brain), the temporary cavity causes additional damage.
Bullet Behavior on Impact
Full Metal Jacket (FMJ) — Penetrates deeply in a straight line without expanding. The bullet punches a narrow channel equal to its diameter. FMJ is designed for target practice and military use (Hague Convention prohibits expanding ammunition in warfare). For self-defense, FMJ over-penetrates (passes through the target and continues traveling) and creates a minimal wound channel.
Hollow Point (JHP) — Designed to expand (mushroom) upon impact. The hollow cavity in the bullet's nose fills with tissue and hydraulic pressure, forcing the bullet to open up to 1.5–2x its original diameter. This creates a larger wound channel, transfers energy more efficiently, and reduces over-penetration. Hollow points are the standard for self-defense and law enforcement.
Soft Point (SP) — A partially jacketed bullet with an exposed lead tip. Expands upon impact but more gradually than hollow points. Popular for hunting because it provides controlled expansion with deep penetration — ideal for reaching vital organs through muscle and bone.
Bonded — The jacket is chemically bonded to the lead core, preventing jacket separation during penetration through barriers (auto glass, drywall, heavy clothing). Bonded bullets maintain weight and expand more consistently. Premium self-defense loads (Speer Gold Dot, Federal HST) use bonded construction.
Frangible — Designed to break apart upon impact with hard surfaces. Used for training in close-quarters environments and shoot houses. Reduces ricochet risk. Some frangible loads are marketed for home defense to reduce wall penetration.
The FBI Penetration Standard
The FBI's Wound Ballistics Workshop established that effective defensive ammunition must penetrate 12 to 18 inches in calibrated 10% ballistic gelatin. This standard accounts for variations in body size, clothing, and intermediate barriers.
- Under 12 inches: May not reach vital organs, especially through heavy clothing or at oblique angles
- 12–18 inches: The ideal range — sufficient to reach vitals while minimizing exit wound and over-penetration
- Over 18 inches: Excessive penetration increases risk to bystanders behind the target
Modern premium defensive ammunition from Federal (HST), Speer (Gold Dot), Hornady (Critical Duty), and Winchester (Ranger T) is specifically engineered to meet this standard.
Why This Matters for You
For Self-Defense
Use quality hollow point ammunition that meets the FBI penetration standard. FMJ is for the range, not your carry gun or home defense setup. See our self-defense ammo guide.
For Hunting
Match your bullet construction to your game. Soft points and bonded bullets for deer-sized game, heavy-for-caliber bonded or monolithic bullets for elk and larger game.
For Long-Range Shooting
Choose cartridges and bullets with high ballistic coefficients for less drop and less wind drift. 6.5 Creedmoor's dominance in precision shooting is a direct result of its outstanding BC values. See our rifle caliber comparison.
For Practice
Use affordable FMJ ammunition at the range. Match the bullet weight of your practice ammo to your carry/defense ammo so the point of impact and recoil impulse are similar.
Find ammunition for every purpose at your next gun show — bulk pricing with no shipping costs. Browse our dealer directory for local shops with expert advice on ammunition selection.