Ballistics is the study of a projectile in motion. Shooters split it into three phases. Each one is controlled by different things, so it helps to know which phase a problem lives in.

When the firing pin strikes the primer, it ignites the powder. The burning powder produces gas, pressure climbs fast, and that pressure pushes the bullet down the bore. All of this takes a few thousandths of a second.

Barrel length and velocity

Longer barrels give the gas more time to push, so velocity generally goes up with length. Published velocities come from test barrels, so a short-barreled rifle or a compact pistol will often run slower than the box says. That's one reason to measure your own.

Twist rate and stability

Rifling spins the bullet so it flies point-first. Twist rate is written as one turn in a number of inches, such as 1:8. A lower number means faster spin. Longer, heavier bullets for a caliber need faster twist to stabilize, which is why a 1:7 or 1:8 AR-15 handles heavy 77-grain bullets better than an older 1:12 barrel. An unstable bullet can tumble and print sideways holes in paper.

Consistency matters more than raw speed

For accuracy at distance, how much velocity varies from shot to shot matters as much as the average. Match ammunition is loaded to keep that variation small, so bullets land at the same height at long range.

Once the bullet leaves the muzzle, nothing pushes it forward anymore. Two forces take over: gravity pulls it down and air resistance slows it. Everything about trajectory comes from those two.

Ballistic coefficient

Ballistic coefficient (BC) measures how well a bullet cuts through air compared with a standard shape. Higher BC means it holds velocity longer, drops less, and drifts less in wind. Long, pointed, boat-tail bullets have the highest BCs.

You'll see BC listed as G1 or G7. They use different reference shapes. G7 models modern long-range bullets more accurately, and its numbers are lower. Never compare a G1 figure to a G7 figure. Enter whichever your ballistic calculator asks for.

Speed of sound

At roughly 1,100 feet per second, near sea level, a bullet crosses the speed of sound. Rifle bullets that slow through that point at long range can become less stable and less predictable. Subsonic loads, like heavy .300 Blackout, stay below it the whole way for quieter suppressed shooting, at the cost of a steeper drop.

Your scope sits above the bore and is angled slightly down toward it, so the bullet rises to cross your line of sight, peaks, and comes back down through it at your zero distance. Beyond that, drop increases quickly as the bullet slows.

MOA and MIL, made simple

Scopes adjust in angles, not inches, so the size of a correction grows with distance. Minute of angle (MOA) scopes usually click in quarter-MOA steps. MIL scopes usually click in tenths of a MIL. Either works. What matters is that your turrets and reticle use the same system.

Wind drift is usually the biggest source of misses at distance. Gravity is predictable, but wind changes speed and direction, and it can blow differently at the target than where you're standing.

Value by direction

Imagine a clock face with the target at 12 o'clock:

Why drift grows so fast

Drift doesn't increase evenly with distance. The bullet is slowing down, so each extra hundred yards takes longer to cross and gives the wind more time to work. A crosswind that barely shows at 100 yards can move a bullet a foot or more at 500. Higher BC and higher velocity both reduce drift.

Reading it

• Flags, grass, leaves, and dust show wind across the whole distance, not just at your position.
• Through a spotting scope or rifle scope, heat mirage leans and flows with the wind near the target.
• A handheld wind meter is accurate at your position, but conditions downrange can differ.

Air density changes how much drag a bullet sees. Thinner air means less drag and less drop. Three things affect it, and ballistic calculators often roll them into one number called density altitude.

Ammo temperature

Separate from the air, the ammunition's own temperature can change its velocity. Some powders burn faster when warm and slower when cold. Temperature-stable powders reduce this, and many match and hunting loads advertise it. If you zero in summer and hunt in winter, confirm your zero in the cold.

Uphill and downhill shots

Gravity only acts over the horizontal distance to the target, so steep shots in either direction hit high if you hold for the straight-line distance. The fix, often called the rifleman's rule, is to use the horizontal distance instead. Many rangefinders calculate this for you. A 500-yard shot at a 30-degree angle drops like a shot of about 433 yards.

Terminal ballistics is what the bullet does when it arrives. For targets, it barely matters. For hunting and defense, it's the whole point.

Expansion needs velocity

Hollow points, soft points, and polymer-tip bullets are designed to expand within a velocity window. Too slow, and they may punch through without opening. Many traditional hunting bullets are designed to expand reliably down to roughly 1,800 to 2,000 feet per second, and solid copper bullets often need more. Bullet makers publish these figures. Check them against your impact velocity at your longest planned shot.

Energy is a guideline, not a guarantee

Kinetic energy, in foot-pounds, comes from bullet weight and velocity squared. You can calculate it as grains times velocity squared, divided by 450,240. A 150-grain bullet at 2,800 fps carries about 2,610 ft-lb at the muzzle. Hunters often cite about 1,000 ft-lb at impact as a minimum for deer-sized game. That's a rough rule. Shot placement and bullet construction matter far more.

Published charts are a starting point. Your rifle, your ammunition lot, and your conditions produce your own numbers, and the shooters who hit consistently at distance are the ones who record them.

• Chronograph your load. Real velocity from your barrel is the most important input to any calculator.
• Use a ballistic calculator. Enter bullet weight, BC, velocity, sight height, zero distance, and current conditions to get your drop and wind corrections.
• True it at distance. Shoot at real ranges, compare impacts to predictions, and adjust the calculator's velocity until it matches.
• Keep a data card. Often called DOPE, a short chart of corrections by distance taped to your stock or scope cap is faster than a phone in the field.
• Stick with one load. Once you've built data, buy that load by the case, ideally from the same lot, so your numbers stay valid.