A railgun uses electromagnetic force instead of chemical propellants to launch a projectile at extreme speeds. The basic concept is straightforward: two parallel conducting rails carry a massive pulse of electric current through a sliding projectile (called an armature), generating a magnetic field that accelerates the armature down the barrel. Military-grade systems have pushed projectiles past 3,000 m/s, roughly Mach 9. Building one that actually works, even at a small scale, requires solving several difficult engineering problems at once: storing enough energy, delivering it fast enough, and keeping the device from destroying itself in the process.
How the Physics Works
A railgun operates on the Lorentz force, the same principle that makes electric motors spin. When current flows down one rail, through the armature, and back along the opposite rail, it creates a magnetic field between the rails. That field pushes the current-carrying armature forward, accelerating it along the barrel’s length. The force depends on three things: how much current is flowing, how far apart the rails are, and the geometry of the barrel itself.
The key relationship is that force scales with the square of the current. Double the current and you quadruple the force on the projectile. This is why railguns are power-hungry devices. A modest increase in performance demands a disproportionate increase in electrical energy. The projectile’s acceleration also changes as it travels down the rails, because the geometry of the magnetic field shifts as the armature moves. MIT’s analytical model of railgun physics accounts for this by treating rail length, rail separation, and current as time-dependent variables in a single differential equation.
The Power Supply: Capacitor Banks
Railguns need enormous bursts of current delivered in milliseconds. Batteries can’t discharge fast enough, so the standard approach is a capacitor bank: a group of capacitors charged to high voltage and then dumped all at once through the rails. For a 2-meter research railgun, published models use a capacitor bank rated at 800 farads charged to 200 volts. That’s a staggering amount of stored energy.
Smaller hobbyist-scale builds use banks of electrolytic capacitors or pulse-rated film capacitors wired in parallel to increase total capacitance and in series to increase voltage. The tradeoff is always between how much energy you can store (capacitance times voltage squared, divided by two) and how quickly you can release it. Capacitors rated for pulse discharge have low internal resistance, which lets them deliver peak current faster. Standard capacitors designed for power filtering will overheat or fail catastrophically if used this way.
Switching: Releasing the Current
The switch between the capacitor bank and the rails has to handle tens of thousands of amps in a fraction of a millisecond. Mechanical switches arc and weld shut under these conditions, so the standard solution is a semiconductor switch, most commonly a thyristor (also called an SCR). Thyristors are triggered by a gate pulse and then stay conducting until the current through them drops to zero, making them well suited for single-discharge applications.
Industrial thyristors built for pulsed discharge can handle peak currents of 120,000 amps with current rise rates up to 1,500 amps per microsecond. More advanced GTO-type thyristors reach 150,000 amps and rise rates of 50,000 amps per microsecond. Triggering these devices requires a strong gate pulse, typically around 500 amps. For large systems like the 200-megajoule railgun designs studied by defense agencies, multiple thyristors are wired in series and parallel to handle 260,000 amps at 15,000 volts. At a hobbyist scale, smaller thyristors or even IGBTs (transistor-based switches) can work, but the component must be rated well above the expected peak current to survive.
Rail and Barrel Construction
The rails themselves are typically copper or a copper alloy, chosen for high electrical conductivity and reasonable mechanical strength. They need to be thick enough to carry enormous currents without melting and rigid enough to resist the magnetic forces trying to push them apart. In research launchers, rails are separated by epoxy-glass laminated insulators, a material that combines electrical insulation with high structural strength. The bore of one military test system uses a 40-mm-wide square cross-section with 3.5-meter-long rails held in place by these laminated insulators.
The armature, the piece that slides between the rails and carries current from one to the other, is usually aluminum alloy. Some designs add a polycarbonate bore rider in front of the armature to keep it aligned and reduce friction. The fit between armature and rails matters enormously. Too loose and the electrical contact is poor, leading to arcing. Too tight and friction eats up kinetic energy and damages the bore.
The housing that holds everything together must be non-conductive and strong. Polycarbonate and G10 fiberglass are common choices because they resist both electrical breakdown and the mechanical stress of magnetic repulsion between the rails during firing.
Why Efficiency Is So Low
One of the most sobering realities of railgun design is how little of the stored electrical energy actually ends up as kinetic energy in the projectile. For short, low-velocity railguns, the maximum achievable efficiency is around 1.6%. That means roughly 98% of the energy initially stored in the capacitor bank is lost, mostly as heat in the plasma arcs that form at the armature-rail contact points and in resistive losses throughout the circuit.
This isn’t just a minor engineering problem to optimize away. It’s a fundamental characteristic of how short railguns work. The ratio of the rail inductance to the total circuit inductance sets an upper bound on efficiency, and for compact designs that ratio is inherently small. Longer rails, higher currents, and lower-resistance circuits all help, but even military programs with multimillion-dollar budgets struggle with thermal management. For a hobbyist build, this means most of your capacitor bank’s energy will become heat in the rails and armature rather than velocity in the projectile.
Rail Erosion and Longevity
Every time a railgun fires, the sliding electrical contact between the armature and rails damages both surfaces. When contact is lost even momentarily, an electric arc forms. That arc is hot enough to pit the rail surface and gouge the armature, a process called “transition.” Over repeated firings, this erosion degrades accuracy and eventually destroys the rails entirely. The U.S. Navy identified rail erosion as the primary obstacle preventing railguns from entering service.
Research at the Naval Postgraduate School found that coating grooved rails with silver paste significantly reduces this damage. The silver improves electrical contact between the armature and rail surfaces, preventing the arcing that causes erosion. In tests, shots fired with silver-coated rails showed no visible damage to either the rails or the projectile. Without the coating, both surfaces showed clear pitting and gouging after a single shot. For any build intended to fire more than a few times, maintaining good electrical contact at the rail-armature interface is the single most important factor in longevity.
Capacitor Safety
The capacitor bank is the most dangerous component by a wide margin. A charged capacitor bank stores enough energy to kill, and unlike a battery, it can release all of that energy in microseconds. Virginia Tech’s electrical safety guidelines outline several non-negotiable precautions for working with high-energy capacitors.
Every capacitor needs a permanently connected bleeder resistor that drains stored charge when the system is powered down. The resistor must bring the voltage below 50 volts within 5 minutes or less, ideally faster than the time it takes to physically access the terminals. For systems storing more than 5 joules (which any functional railgun capacitor bank will far exceed), an automatic mechanical discharge device should activate whenever the enclosure is opened. Manual grounding hooks should have crimped and soldered conductors with impedance below 0.1 ohms to ground, and the cable conductor should be visible through its insulation so you can verify it hasn’t been damaged. Capacitors wired in series each need their own bleeder resistor, because a break in a shared bleeder circuit can leave individual capacitors fully charged even when the bank appears safe.
Practical Scale and Expectations
If you’re approaching this as a learning project, be realistic about what a small-scale build can achieve. Research railguns reaching 2,100 to 3,000 m/s use capacitor banks storing tens of thousands to millions of joules, precision-machined bores, and sophisticated pulse-forming networks. A tabletop build with a few hundred joules of stored energy will launch a small projectile at modest speeds, enough to demonstrate the principle but nowhere near the dramatic performance of military systems.
The core components you need are a capacitor bank with pulse-rated capacitors, a thyristor or similar high-current switch, two parallel copper rails mounted in an insulating housing, and a conductive armature that fits snugly between them. The engineering challenge is getting all of these to work together: the switch must handle the peak current without failing, the rails must stay parallel under magnetic stress, the armature must maintain contact without jamming, and the entire system must be safely enclosed with proper grounding and discharge circuits. Each of these problems is solvable, but each one will teach you something about why railguns remain one of the hardest electromagnetic devices to build reliably.