Ammo Glossary: The Complete Guide to Every Ammunition Term

If you’ve ever stood in front of a wall of ammo boxes squinting at terms like “FMJ,” “+P,” or “grain weight” and felt completely lost, this ammo glossary is for you. Buying ammunition shouldn’t require a engineering degree, but the industry is packed with abbreviations, slang, and technical terms that nobody ever sits down and explains from scratch.

This ammo glossary does exactly that. It’s organized into six sections — caliber basics, bullet types, case and ignition types, pressure and power terms, shotgun-specific terms, and practical buying terms — so you can jump straight to what you need or read start to finish and walk away fluent. Bookmark this ammo glossary; you’ll be back.

Caliber & Cartridge Basics (Ammo Glossary, Part 1)

This part of the ammo glossary covers the foundational words you’ll see on every box of ammunition, no matter the caliber.

Caliber — The diameter of a bullet, usually measured in hundredths of an inch (like .308) or millimeters (like 9mm). Caliber alone doesn’t tell you everything about a round — cartridge length, pressure, and bullet weight all vary within the same caliber family.

Cartridge — The complete, ready-to-fire unit: case, primer, powder, and bullet assembled together. “Round” and “cartridge” are used interchangeably in everyday conversation.

Case (or “brass”) — The container that holds the powder and primer and seats the bullet. Cases are most commonly made of brass, though steel and aluminum cases exist too (more on that below).

Bullet — Just the projectile itself — the part that actually leaves the barrel and travels to the target. Calling the whole cartridge a “bullet” is a common but technically incorrect habit; the bullet is only one component.

Grain (gr) — The unit used to measure bullet weight. One grain equals 1/7,000th of a pound. A 115-grain 9mm bullet is lighter and generally faster than a 147-grain 9mm bullet of the same caliber. Our full breakdown of what grain weight changes in a round lives in our grain weight guide.

Gauge — The sizing system used for shotgun shells instead of caliber. Counterintuitively, a lower gauge number means a larger bore — a 12 gauge is bigger than a 20 gauge. Bore sizing is covered in detail in our shotgun gauges guide, and you can shop every gauge we carry in our shotgun ammo category.

Bore — The interior diameter of a barrel. “Bore” and “caliber” are closely related but not perfectly synonymous once you get into shotgun gauge math.

Bullet Types (Ammo Glossary, Part 2)

The bullet-type section of this ammo glossary is the one shoppers reach for most, since it decides how a round performs on impact.

FMJ (Full Metal Jacket) — A bullet with a lead core fully wrapped in a harder metal jacket (usually copper). FMJ bullets don’t expand on impact; they’re the standard choice for target practice and training because they’re reliable and inexpensive. Most of our bulk 9mm and 5.56 NATO range ammunition is FMJ. For the full breakdown of when FMJ makes sense versus other bullet types, see our hollow point vs FMJ guide.

JHP (Jacketed Hollow Point) — A bullet with a hollow cavity in the nose designed to expand (“mushroom”) on impact. Expansion increases the wound channel and helps transfer energy, which is why JHP ammo is the standard recommendation for self-defense. You’ll find JHP defensive loads throughout our handgun ammo range, especially in 9mm and .45 ACP.

Soft Point — A middle-ground bullet design with an exposed lead tip instead of a hollow cavity. Soft points expand more than FMJ but less dramatically than a true hollow point, and they’re popular in hunting rifle ammunition where controlled expansion matters.

Ball Ammo — Military slang for FMJ ammunition, dating back to early round musket balls. “Ball ammo” and “FMJ” are used interchangeably today, especially in rifle calibers like 5.56.

Boat Tail — A bullet design where the rear tapers inward instead of staying flat. The tapered base reduces aerodynamic drag, which is why boat-tail bullets are common in long-range and match-grade ammunition.

Spitzer — A pointed bullet nose shape designed to cut through air more efficiently than a round or flat nose. Most modern rifle bullets use some form of spitzer point.

Bonded Bullet — A hollow point or soft point where the lead core is chemically bonded to the jacket so the two don’t separate on impact. Bonded bullets retain more weight after hitting a barrier, which matters for hunting and some defensive applications.

Monolithic / Solid Copper Bullet — A bullet machined from a single piece of copper (or copper alloy) rather than a lead core with a jacket. These are lead-free, which matters in some hunting jurisdictions, and they tend to penetrate deeply while retaining nearly 100% of their weight.

Case & Ignition Types (Ammo Glossary, Part 3)

This section of the ammo glossary explains how a cartridge actually ignites, and why that detail affects whether brass can be reloaded.

Rimfire — An ignition system where the primer compound is loaded into the rim of the case itself, rather than a separate primer. The firing pin strikes the rim to set off the round. .22 LR is by far the most common rimfire cartridge — browse our full rimfire ammo range. Rimfire cases generally can’t be reloaded.

Centerfire — An ignition system where a separate, replaceable primer sits in the center of the case base. Nearly every handgun and rifle cartridge besides rimfire — 9mm, 5.56, .308, and so on — is centerfire. Centerfire cases can typically be reloaded. Our rimfire vs centerfire comparison covers this split in more depth.

Primer — The small, pressure-sensitive component that ignites when struck by the firing pin, which in turn ignites the gunpowder. Primers come in different sizes (small pistol, large pistol, small rifle, large rifle) matched to the cartridge.

Brass-Cased Ammo — Ammunition using a brass case — the traditional, most common material. Brass is reloadable, feeds smoothly, and handles heat well, though it costs more to manufacture than steel.

Steel-Cased Ammo — Ammunition using a steel case, usually coated in a lacquer or polymer finish to reduce corrosion and friction. Steel is cheaper than brass, which is why steel-cased ammo (common in 5.56 and 7.62×39) tends to be noticeably more affordable per round. See our full steel vs brass breakdown for the performance and reliability trade-offs.

Headspace — The precise distance a firearm’s chamber allows a cartridge to sit before the bolt or breech closes against it. Correct headspace is a safety-critical fit between chamber and cartridge; it’s not something you check yourself.

+P — A pressure designation meaning a cartridge is loaded to higher-than-standard pressure for that caliber, producing more velocity and generally more recoil. “+P+” goes a step further still. Not every firearm is rated for +P ammunition — check your owner’s manual first.

Pressure & Power Terms (Ammo Glossary, Part 4)

These ammo glossary entries describe how fast and how hard a round hits — the numbers you’ll see quoted on manufacturer spec sheets.

Muzzle Velocity — The speed of a bullet as it leaves the barrel, usually measured in feet per second (fps). Higher muzzle velocity generally means flatter trajectory and more energy delivered downrange.

Muzzle Energy — The kinetic energy a bullet carries as it leaves the barrel, usually measured in foot-pounds (ft-lbs). Muzzle energy is a product of both bullet weight and velocity, which is why two rounds with very different weights can still carry similar energy.

Subsonic — Ammunition loaded to travel slower than the speed of sound (about 1,125 fps at sea level), which eliminates the sharp “crack” a bullet makes breaking the sound barrier. Subsonic ammo is the standard pairing for suppressed shooting.

Supersonic — Ammunition that travels faster than the speed of sound. Most standard rifle and handgun ammunition is supersonic; a supersonic round will still produce a loud crack even through a suppressor.

Chamber Pressure — The pressure generated inside the cartridge case and chamber as powder burns, usually measured in psi. SAAMI (the industry body that standardizes U.S. ammunition) sets maximum safe chamber pressures for every cartridge.

Shotgun-Specific Terms (Ammo Glossary, Part 5)

Shotgun shoppers need their own corner of this ammo glossary, since shotshells use an entirely different vocabulary than rifle or handgun ammunition.

Shot — The small round pellets fired from a shotgun shell instead of a single bullet. Shot size is numbered — smaller numbers mean larger individual pellets.

Birdshot — Small-diameter shot (numbered sizes, like #7½ or #8) used for upland birds, waterfowl, and clay targets. Birdshot carries many small pellets but limited penetration, which is why it’s a poor home-defense choice at distance.

Buckshot — Larger-diameter shot (like 00 Buck) used for deer-sized game and home defense. Buckshot carries far more energy per pellet than birdshot, with fewer total pellets per shell. Browse buckshot, birdshot, and slugs in our shotgun ammo range. Our buckshot vs birdshot guide breaks down exactly when each makes sense.

Slug — A single solid projectile fired from a shotgun shell instead of multiple pellets. Slugs behave more like a rifle round and are the standard choice for shotgun deer hunting in states that restrict rifle use.

Choke — A constriction at the muzzle end of a shotgun barrel that controls how tightly the shot pattern spreads. Tighter chokes (like Full) produce denser patterns at range; more open chokes (like Cylinder or Improved Cylinder) spread wider and suit close-range work.

Wad — A plastic (or sometimes fiber) cup inside a shotgun shell that separates the powder from the shot and helps seal gas behind the payload as it travels down the barrel.

Hull — The outer casing of a shotgun shell, typically plastic with a brass base. “Hull” is the shotgun-world equivalent of “case” in rifle and handgun ammunition.

Practical & Buying Terms (Ammo Glossary, Part 6)

The final stretch of this ammo glossary covers the terms you’ll run into while actually shopping, storing, and troubleshooting ammunition.

Cost Per Round (CPR) — The total price of a box of ammunition divided by the number of rounds, which is the only honest way to compare ammo prices across different box sizes and calibers. We break this down caliber-by-caliber in our ammo price index.

Bulk Ammo — Ammunition sold in larger quantities (typically 250+ rounds) at a lower cost per round than small retail boxes. Bulk buying is the standard strategy for high-volume range shooters — 9mm and .22 LR are where bulk pricing stretches a training budget furthest.

Reloading — The process of reusing a fired brass case by replacing the primer, powder, and bullet to create a new, functional round. Reloading can meaningfully lower cost per round for high-volume shooters but requires equipment, time, and strict attention to safety.

FFL (Federal Firearms License) — The federal license required to deal in firearms. Ammunition purchases generally don’t require an FFL, but some state and local laws add their own requirements — our guide on buying ammo online legally covers the details.

NFA (National Firearms Act) — The federal law governing regulated items like suppressors, short-barreled rifles, and machine guns. If you’re exploring suppressed shooting, our suppressor tax stamp guide explains the current rules.

Squib Load — A dangerously underpowered round that doesn’t generate enough pressure to fully push the bullet out of the barrel, potentially leaving it lodged inside. Firing another round behind a squib-obstructed barrel can destroy the firearm and injure the shooter — if a shot sounds or feels abnormally weak, stop and inspect the barrel immediately.

Hangfire — A noticeable, abnormal delay between the trigger pull and the round firing. If a hangfire occurs, keep the firearm pointed in a safe direction for a full 30 seconds before investigating, since the round can still fire late.

Misfire — A complete failure of the round to fire after the primer is struck. Treat every misfire as a possible hangfire first — don’t immediately open the action.

💬 Before You Go

Did this ammo glossary clear up a term that had been confusing you? We’d love to know. If you came here looking for something specific and didn’t find it, tell us — we’re always expanding this page based on real reader questions.

Leaving without finding what you needed? Drop a comment below or reach out through our contact page and let us know what term or topic we missed. This ammo glossary gets better every time a reader points out a gap.

Shop With Confidence Now That You Know the Ammo Glossary

Now that this ammo glossary has cleared up the jargon, put it to use. Browse 9mm ammo, 5.56 NATO ammo, .223 Remington ammo, .45 ACP ammo, .22 LR ammo, and shotgun ammo — or explore our full rifle, handgun, and rimfire catalogues for every caliber this ammo glossary covers.

Please buy and use ammunition responsibly and in accordance with all federal, state, and local laws. You must meet all legal requirements to purchase, and ammunition cannot be shipped to every jurisdiction — check your local regulations before ordering.

Frequently Asked Questions

Where can I find a complete ammo glossary online? Right here — this page covers every common caliber, bullet, case, pressure, shotgun, and buying term in plain English, organized into six sections you can jump between.

What does FMJ mean on ammo boxes? FMJ stands for Full Metal Jacket — a bullet with a lead core fully wrapped in a harder metal jacket, typically copper. FMJ bullets don’t expand on impact and are the standard choice for target practice.

What’s the difference between caliber and gauge? This is the single most common ammo glossary question. Caliber (used for rifles and handguns) measures bullet diameter directly, usually in inches or millimeters. Gauge (used for shotguns) is based on an older bore-sizing system where a lower number actually means a larger bore.

What does grain mean in ammo? Grain is one of the most-looked-up entries in any ammo glossary. It’s the unit used to measure bullet weight — 1/7,000th of a pound. A higher grain number means a heavier bullet, which generally trades some velocity for more weight and energy retention.

What is the difference between rimfire and centerfire? Rimfire ammunition (like .22 LR) has its primer compound built into the case rim and generally can’t be reloaded. Centerfire ammunition uses a separate, replaceable primer in the center of the case base and can typically be reloaded.

What does +P mean on a box of ammo? +P means the cartridge is loaded to higher-than-standard pressure for that caliber, producing more velocity and recoil. Always confirm your firearm is rated for +P before using it.

What is the difference between a bullet and a cartridge? The bullet is only the projectile that leaves the barrel. The cartridge is the complete assembled round — case, primer, powder, and bullet together. This ammo glossary’s caliber section covers both terms in more detail. People often say “bullet” when they mean the whole cartridge.

What does CPR mean in ammo shopping? CPR stands for Cost Per Round — the total price of a box divided by the number of rounds inside. It’s the only fair way to compare prices across different box sizes and calibers.

What is a squib load and why is it dangerous? A squib load is a round that fires with too little pressure to push the bullet all the way out of the barrel. If you fire another round behind an obstructed barrel, it can destroy the firearm and seriously injure the shooter. Any shot that sounds or feels abnormally weak should be treated as a possible squib.

What’s the difference between a hangfire and a misfire? A hangfire is a delayed firing after the trigger is pulled — the round still goes off, just late. A misfire is a complete failure to fire. Both require keeping the firearm pointed in a safe direction for at least 30 seconds before investigating.

What does ball ammo mean? Ball ammo is older military terminology for FMJ ammunition, dating back to round musket balls — one of the few ammo glossary terms that survives purely out of habit. The terms are used interchangeably today, especially for rifle calibers like 5.56.

Why is steel-cased ammo cheaper than brass-cased ammo? Steel costs less to manufacture than brass and can’t be reloaded as reliably, which is reflected in the lower retail price. Steel-cased ammo is a popular budget option for high-volume range shooting in calibers like 5.56 and 7.62×39. (See the case and ignition section of this ammo glossary for the full breakdown.)