Armor Penetration
Simplified kinetic energy estimate for hobby comparisons.
About the Armor Penetration
Estimate the kinetic energy a projectile carries, in foot-pounds, from its mass in grains and its velocity. Energy is the usual stand-in for penetrating power when comparing loads, which is why it is the number quoted on ammunition boxes.
How to use it
- Enter the projectile mass in grains.
- Enter the velocity in feet per second.
- Read the energy in foot-pounds.
The formula
This computes muzzle energy — the kinetic energy a projectile carries — which is the standard way hobbyists compare loads on paper.
Energy (ft-lb) = Mass (grains) × Velocity² (fps) ÷ 450,240
The constant converts grains and feet per second into foot-pounds; it bundles the 7,000 grains in a pound and the gravitational constant into one number. The physics underneath is the familiar ½mv².
Velocity is squared and mass is not, which is the single most important thing on this page. Doubling the weight doubles the energy; doubling the speed quadruples it. That is why a light, fast rifle bullet carries several times the energy of a heavy, slow pistol bullet.
Energy is not penetration. What a projectile actually does on impact depends on sectional density, construction, expansion behaviour and what it hits. This figure is a comparison number for hobby use, not a model of any material.
Worked examples
| Bullet | Weight | Velocity | Energy | Typical use |
|---|---|---|---|---|
| .45 ACP | 230 gr | 850 fps | 369 ft-lb | pistol target and defence load |
| .223 Remington | 55 gr | 3,240 fps | 1,282 ft-lb | varmint and target |
| .308 Winchester | 150 gr | 2,800 fps | 2,612 ft-lb | general-purpose hunting |
| .30-06 Springfield | 180 gr | 2,700 fps | 2,914 ft-lb | large game |
The first two rows make the point about velocity. The .223 bullet weighs less than a quarter of the .45 and carries more than three times the energy, purely because it is nearly four times faster and velocity is squared. It also shows the limit of energy as a measure: those two rounds behave completely differently on impact, and no single number captures that.
Common mistakes
- Reading energy as stopping power or penetration. It is one input among several. Bullet construction, sectional density, expansion and the medium struck all matter, and two loads with identical energy can perform nothing alike.
- Using muzzle velocity for distant targets. Velocity falls continuously downrange, and energy falls with its square. A load carrying 2,600 ft-lb at the muzzle may be under 1,500 at 300 yards.
- Mixing grams and grains. There are 15.43 grains to a gram, and 7,000 grains to a pound. Entering grams gives an answer roughly fifteen times too small.
- Comparing published figures from different barrel lengths. Velocity depends heavily on barrel length. A load chronographed in a 24-inch test barrel will be markedly slower from a 16-inch carbine, and the energy figure follows.
Terms explained
- Grain
- A unit of mass: 7,000 to the pound, about 0.0648 grams. The standard unit for projectile weight.
- Muzzle energy
- Kinetic energy as the projectile leaves the barrel. Its peak value, before drag begins reducing it.
- Foot-pound
- The energy needed to raise one pound by one foot. The customary unit here.
- Sectional density
- Mass divided by the square of diameter. A better predictor of penetration depth than energy is.
- Ballistic coefficient
- How well a projectile resists drag. Higher means it retains velocity, and therefore energy, further downrange.
- Terminal ballistics
- What actually happens on impact — expansion, fragmentation, penetration depth. The part this calculation does not address.
Common questions
- What does this actually calculate?
- Kinetic energy at the muzzle, in foot-pounds, from projectile weight and velocity. It is a standard comparison figure and not a model of what happens on impact.
- Why does velocity matter more than weight?
- Because it is squared in the formula. Doubling mass doubles energy; doubling velocity quadruples it, which is why light fast projectiles carry so much on paper.
- Does more energy mean more penetration?
- No. Penetration depends far more on sectional density and construction. A heavy, narrow, non-expanding projectile penetrates deeper than a lighter one of equal energy designed to expand.
- How much energy does hunting require?
- Guidance varies by species and jurisdiction, and many places set legal minimums. Check local regulations, and treat shot placement and appropriate bullet construction as more important than the energy figure.
- How do I convert grams to grains?
- Multiply grams by 15.43. A 10-gram projectile is about 154 grains.
- Why is my figure different from the box?
- Published figures come from specific test barrels, usually longer than a typical sporting rifle. Shorter barrels give lower velocity and therefore noticeably lower energy.
- Does energy drop off quickly?
- Yes. Drag reduces velocity continuously and energy falls with its square, so a load can lose 40% or more of its muzzle energy by 300 yards depending on its ballistic coefficient.
- What is the difference between energy and momentum?
- Energy is ½mv² and momentum is mv. Momentum weights mass more heavily, which is why the two measures rank light-fast and heavy-slow projectiles differently.