AMMO FAQ: The Basics of Ammunition

Rifle, Handgun & Shotgun Ammunition

There are basically two types of ammunition, the cartridge and the shotshell. Rifles and handguns use a cartridge containing a projectile (bullet). Shotguns use a shotshell which contains either a number of small projectiles (known as shot) or a single projectile (known as a slug).

The components of both types of ammo are pretty similar; they both have the same basic components which include the case, the primer, gunpowder, and a projectile.

How does the ammo fire?

Ammunition is fired when a gun’s firing pin hits the ammo’s primer. This causes a spark from the primer to ignite the gunpowder. Gas is then converted from the burning powder which forces the projectile out of the case and down the barrel of the gun.

The whole sequence happens in roughly one to two milliseconds, start to finish — faster than you can consciously perceive. Modern smokeless powder doesn’t actually “explode” in the dramatic sense most people picture; it burns extremely fast in a controlled, progressive way, generating expanding gas that builds pressure behind the bullet. That pressure, not an explosion, is what pushes the bullet down the barrel.

Reading a Cartridge: What the Numbers Actually Mean

If you’ve ever looked at a box of ammo and wondered what “.223 Remington,” “9mm Luger,” or “.308 Winchester” actually tells you, here’s the breakdown most FAQs skip:

• The first number is typically the bullet diameter, in either inches (.223, .308, .45) or millimeters (9mm, 5.56mm). A “.308” bullet is roughly 0.308 inches across.
• The name after the number usually tells you who designed the cartridge or which company standardized it (Remington, Winchester, Luger), which matters because two cartridges with the same diameter aren’t always interchangeable — a .223 Remington and a 5.56 NATO are nearly the same bullet diameter but differ in chamber pressure and case dimensions, and shouldn’t be treated as identical despite looking nearly the same on a shelf.
• Grain weight (often printed as “115gr,” “147gr,” etc.) refers to the weight of the bullet itself, with one grain equal to 1/7,000th of a pound. Heavier grain bullets of the same caliber generally move slower but carry more momentum; lighter ones move faster but lose energy to drag sooner. Neither is universally “better” — it depends entirely on what you’re using it for.
• “+P” or “+P+” on a box means the ammunition is loaded to a higher pressure than standard for that caliber, producing more velocity and energy — but also more recoil and wear on your firearm. Always confirm your specific gun is rated for +P ammunition before using it; not all are.

What Is the Best Self-Defense Round?

This is one of the most argued-about questions in the entire firearms world, and honestly, much of the internet argument is built on incomplete or outdated information. Here’s where the actual data stands today.

Shot placement matters more than caliber. This part of the old conventional wisdom holds up: a center-mass hit from a smaller caliber will generally outperform a poorly placed hit from a larger one. No cartridge reliably guarantees an instant stop regardless of where it lands.

But “any caliber works the same” oversimplifies it. The idea that a .22LR performs identically to a 9mm or .45 ACP in a defensive situation doesn’t hold up against more rigorous, modern testing standards — which brings us to the actual industry benchmark worth knowing about.

The FBI Ballistic Protocol

In 1986, eight FBI agents engaged two heavily armed suspects in what’s now known as the Miami shootout. Standard-issue 9mm and .38 Special ammunition failed to stop the suspects effectively even with multiple hits on target, which exposed a serious gap in law enforcement ammunition performance. Two agents were killed and five others were wounded in the incident.

In response, the FBI spent the following years developing a standardized testing protocol, and by 1989 it had become what most shooters now consider the gold standard for evaluating handgun ammunition. Here’s how it actually works:

Bullets are fired into bare ballistic gelatin, then through a series of barrier tests — four layers of heavy winter clothing, half-inch wallboard, two pieces of 20-gauge sheet steel, three-quarter-inch plywood, and automobile windshield glass — into gelatin behind each barrier, fired from a range of ten feet. Thirty total shots are fired, five for each portion of the test, and the results feed into a scoring formula.

The penetration standard that came out of this testing is genuinely useful information for anyone serious about defensive ammunition:

The FBI requires a minimum penetration of 12 inches and a maximum of 18 inches in ballistic gelatin, a range intended to ensure the bullet reaches vital organs even after first passing through an extremity or an obstacle like clothing or glass. Penetration under 12 inches is heavily penalized as a real risk of failing to incapacitate a threat, while penetration beyond 18 inches is also penalized due to overpenetration risk; 14 to 16 inches is generally considered the ideal range.

Worth noting: this protocol was never designed to measure “stopping power” in the dramatic, one-shot-drop sense often discussed online — it measures whether a round will reliably reach vital organs under realistic, imperfect real-world conditions. That’s a more useful, more honest framing than the “magic bullet” debates that dominate online forums.

What this means practically: modern, quality hollow-point defensive ammunition in mainstream calibers (9mm, .40 S&W, .45 ACP, .380 ACP from a reputable manufacturer) that’s designed and tested to meet FBI protocol standards will perform far more predictably than older anecdote-driven comparisons suggest. The “9mm vs .45” debate that dominated forums for decades has cooled considerably in serious ballistics circles precisely because modern bullet construction closed most of the practical gap between calibers — assuming you’re using established, protocol-tested defensive ammunition rather than cheap practice-grade FMJ rounds, which don’t expand and behave very differently on impact.

A Note on Rimfire (.22LR) for Defense

The honest answer here: a .22LR is dramatically better than no firearm at all, and shot placement with any working firearm beats no firearm in a true worst-case scenario. But rimfire ammunition has real, well-documented limitations for defensive use — lower reliability of ignition (a dud rate that’s measurably higher than quality centerfire ammunition), less consistent penetration depth, and less reliable functioning in some semi-auto platforms under stress. If you’re building out a dedicated defensive battery and have the means to do so, a centerfire handgun in a mainstream defensive caliber is the better-supported choice based on current ballistic data — but a .22LR you’re actually trained and comfortable with will always beat an unfamiliar or unavailable larger caliber.

Is Old Ammunition Safe to Shoot?

It really depends on how the ammo was stored. Improperly stored ammo can be damaged by environmental conditions, especially humidity. Primers can lose their sensitivity, charges can deteriorate, and cartridges can become brittle and even rupture when fired. You also have to watch out for corrosion problems that can actually change the size or characteristics of the ammunition.

If you’re buying old or surplus ammunition, here’s a quick visual inspection checklist before you ever load it:

• Case corrosion or discoloration — green or white powdery residue on brass is a red flag, especially around the primer pocket
• Loose or “cocked” primers — a primer that looks tilted or sits proud of the case head can indicate prior firing pin contact or improper seating
• Case cracks or splits, especially near the case mouth — brittle, season-cracked brass
• Bulged or dented cases — can indicate the round was already chambered and extracted improperly, or stored under pressure or impact
• Discolored or crusty powder visible through the case mouth (on disassembled or damaged rounds) — a sign of significant propellant breakdown

When in doubt, don’t fire it — set it aside, or have it inspected by someone experienced in reloading or ammunition disposal.

How Should You Store Your Ammo?

With ammunition supply and pricing swinging wildly with the news cycle, more and more people are thinking seriously about long-term storage. The short version: humidity is the real enemy, rotation matters, and the cardboard boxes ammo ships in are fine for the short term but not for serious long-term storage.

We go into significantly more depth — including the actual chemistry of why heat and humidity degrade primers, powder, and brass, real target humidity and temperature numbers, and the truth about whether desiccant belongs inside a sealed ammo can — in our dedicated guide: How to Properly Store Ammo for Long-Term Survival.

One thing worth flagging here specifically: there’s a genuine, ongoing debate among experienced operators about whether desiccant packets belong inside a sealed ammo can at all. Some military veterans with decades of hands-on munitions storage experience argue that smokeless powder needs to retain a small amount of inherent moisture to burn at its intended, predictable rate — and that aggressive desiccant use inside a fully sealed can may pull that moisture out of the powder itself over long periods, not just out of the surrounding air. This isn’t settled with the kind of hard public data that would make it a slam-dunk either way, but it’s a real enough concern among people with serious storage experience that it’s worth knowing both sides exist rather than treating “more desiccant is always better” as gospel. The safer middle ground most serious long-term storage guides land on: use a reasonable amount of desiccant, recharge or replace it periodically rather than letting it sit indefinitely, and prioritize a stable, low-humidity room over trying to bone-dry the inside of every individual can.

The Lowdown on Firearm Ammunition

Ammunition isn’t complicated once you understand what each component is actually doing — primer ignites, powder burns and generates gas, gas pushes the bullet. Where people get into trouble is skipping the “why” behind the rules: why penetration depth matters more than muzzle energy bragging rights, why a cracked case is more dangerous than an old one, why humidity is the real enemy of a stockpile rather than time itself.

Building out your home defense plan? Check out our breakdown of why a shotgun belongs in your home defense arsenal, and make sure your stockpile is actually protected long-term with our full guide to ammo storage.