Ammunition Guide: 4 Things That Actually Matter When Buying

Walk into any gun store and the ammo wall is a wall of nearly identical boxes. Same caliber, wildly different prices, cryptic abbreviations, and a counter guy who’s either genuinely helpful or moving slow inventory. This ammunition guide is the map — what all of it means, which distinctions actually matter, and how to stop guessing.

We’re not going to tell you what to buy here. That’s what our caliber-specific guides are for, and we’ll point you to them. What this page does is teach you to read the wall — so the next time you’re standing in front of it, you know exactly which two boxes you’re choosing between and why. Call it ammo explained without the tribalism.

How we researched this

This guide draws on manufacturer specifications, SAAMI standards, published ballistic testing, and the recurring questions we see shooters ask across forums and in our own inboxes. Where a topic deserves more depth than a hub can give it, we link to our dedicated guide rather than compressing it into a paragraph. We did not conduct our own testing.

On this page

Which Ammo Do You Need?
What a Cartridge Actually Is
Bullet Types (This Is the Big One)
Five Ammo Myths
The Caliber Landscape
How to Read an Ammo Box
+P, +P+, and Pressure
Why Barrel Length Changes Everything
How to Actually Buy Ammo
Storage and Shelf Life
Common Mistakes
Our Ammo Guides
FAQ

Which Ammo Do You Need?

Two questions get you 95% of the way there. Start at the top.

▸ What’s it for?

→ PRACTICE — buy FMJ. Cheapest brass-cased ball ammo you can find in stock. Buy it by the case. At seven yards there’s no meaningful difference between budget and premium ball, so don’t overthink it. Done — go shoot.

→ DEFENSE — you need a jacketed hollow point. Now answer one more question…

▸ How long is your barrel?

→ About 3 inches (P365, Hellcat, Glock 43X, Shield, Max-9) — you need a load built for short barrels, or one with velocity margin to spare. This is where the wrong choice quietly costs you expansion. See our 9mm self defense ammo guide.

→ 4 inches or longer (Glock 19, Glock 17, M&P, P320) — any quality premium JHP will do its job. You have velocity to spare. Buy a proven load and move on.

▸ Then, regardless of which branch you landed on: run 50 rounds of your actual carry load through your actual gun before you trust it. Not 50 rounds of FMJ — 50 rounds of the exact ammo you’ll carry. This step is not optional and it’s the one everyone skips.

What a Cartridge Actually Is

Four parts. That’s it. Understanding them makes everything else on this page click.

The case — the brass (or steel, or aluminum) housing that holds everything together. It expands under pressure to seal the chamber, then gets extracted.

The primer — a small impact-sensitive charge in the base. The firing pin hits it, it sparks, and that spark ignites the powder. This is the only part of a cartridge that’s genuinely dangerous to mishandle.

The powder — the propellant. It doesn’t explode; it burns rapidly, generating expanding gas that pushes the bullet down the barrel. How fast it burns, and how completely, is why barrel length matters so much later on this page.

The bullet — the projectile that actually leaves the gun. Everything else is a delivery system for this. When someone says “bullet” meaning the whole cartridge, they’re wrong, and now you know why.

That’s the whole machine. Our guide to the basic parts of ammunition goes deeper on each component if you want the full picture.

Bullet Types (This Is the Big One)

If you learn one thing from this page, learn this section. Bullet construction is the difference between ammo that does its job and ammo that doesn’t — and of all the types of ammunition on that wall, it’s the distinction people most often get wrong.

FMJ — Full Metal Jacket

A lead core wrapped in a copper jacket. It doesn’t expand. It punches a caliber-sized hole and keeps going.

What it’s for: practice. Nothing else. It’s cheap, it feeds reliably in almost anything, and it’s what you should be burning through by the hundreds at the range.

What it’s not for: defense. An FMJ that doesn’t stop in your target continues into whatever’s behind it — a wall, a neighbor, a bystander. That’s not a hypothetical concern; it’s the reason hollow points exist.

JHP — Jacketed Hollow Point

Same construction, but with a cavity in the nose. On impact, that cavity fills and forces the jacket to peel back — the bullet mushrooms to roughly double its diameter.

Why that matters, twice over: a wider bullet creates a wider wound channel, and an expanded bullet slows down dramatically, which means it’s far more likely to stop inside the target instead of exiting into the world behind it. Expansion is both an effectiveness feature and a safety feature.

The catch: expansion isn’t guaranteed. It requires velocity — modern JHPs are generally engineered to open somewhere around 900–950 fps, though that’s a design target range that varies by bullet, not a universal cutoff. It also requires that the cavity not get plugged. Heavy clothing can pack the hollow point solid, and a plugged JHP behaves exactly like an FMJ: no expansion, deep penetration, all the downsides.

Premium defensive ammo exists to solve that specific problem. That’s what you’re paying for.

Soft Point

Exposed lead nose, jacketed body. Expands more slowly and less dramatically than a hollow point, penetrates deeper. Mostly a rifle-hunting proposition.

Frangible

Compressed metal powder instead of a solid core. Designed to disintegrate on hard surfaces rather than ricochet — used at steel-target ranges and in some training environments. Not a defensive round.

Polymer-Tipped

A hollow point with a plastic plug in the cavity. Clever solution: the polymer prevents clothing from plugging the cavity, then gets driven backward on impact to force expansion. Hornady’s FTX and FlexLock bullets are the best-known examples.

Lead Round Nose / Wadcutter

Unjacketed lead. Common in .22 LR and revolver ammunition. Cheap and accurate, but it fouls barrels faster than jacketed ammo — which matters a lot if you’re shooting suppressed. Our .22 suppressor guide covers why lead makes rimfire cans so filthy.

The rule that covers 95% of shooters

Practice with FMJ. Carry JHP. That’s it. FMJ is a fraction of the price and functionally identical for punching paper at seven yards — around $0.28/round versus $1.10+ for premium defensive loads. Buy practice ammo by the case and defensive ammo by the box, and never mix up which is which.

Five Ammo Myths

Myth: Bigger caliber always wins. Modern defensive ammunition has narrowed the gap between service calibers to the point where the differences are academic next to shot placement. A shooter who’s put a thousand rounds through a 9mm beats someone who bought a .45 and shot it twice. The caliber that wins is the one you’ll practice with.

Myth: FMJ is fine for defense. It isn’t, and this one’s a genuine safety issue rather than a preference. FMJ doesn’t stop in the target — it continues into whatever’s behind it. That’s the specific problem hollow points were invented to solve. In an emergency any ammo beats none, but if you’re choosing, choose JHP.

Myth: +P is always better. +P buys velocity at the cost of recoil, muzzle flash, and gun wear — and not every firearm is rated for it. From a short barrel where you need velocity margin, it’s often the right call. From a full-size gun where standard pressure already expands reliably, +P mostly buys deeper penetration you may not want.

Myth: Grain weight equals stopping power. Grain is just bullet mass. Heavier bullets go slower and penetrate deeper; lighter ones go faster and expand more readily. Neither is inherently superior — the right weight depends on your barrel length and what you’re trying to accomplish. A heavier bullet is not a more powerful bullet.

Myth: More expensive always means better. True for defensive ammo, false for practice ammo. Premium JHPs earn their price through engineering that defeats clothing plugging and reliable expansion. Practice FMJ is a commodity — at seven yards, budget ball shoots the same as premium ball. Spend the money where it does something.

The Caliber Landscape

Caliber debates are the oldest argument in shooting and mostly a waste of time. Here’s the honest landscape without the tribalism.

Rimfire

.22 LR — the most-produced cartridge in the world, and the one everyone should own something in. Almost no recoil, cheap enough to shoot all day, and genuinely the best training tool that exists. Not a serious defensive round, and it’s dirty — unjacketed lead over waxy powder fouls everything it touches. Our complete .22 LR guide covers the platform end to end, and our .22 LR ammo guide covers what to feed it.

Defensive Handgun

9mm Luger — the default answer for good reason. Modern defensive 9mm performs close enough to bigger calibers that the capacity and recoil advantages win. It’s the most common, most available, and cheapest centerfire defensive round. Start here unless you have a specific reason not to.

.380 ACP — 9mm’s smaller sibling. Exists because it lets guns be smaller. Modern .380 defensive loads are legitimate, but it’s less forgiving of poor ammo choice than 9mm.

.45 ACP — big, slow, heavy. Devoted following, and it works. Fewer rounds, more recoil, more expensive.

.40 S&W — the compromise nobody wants anymore. Law enforcement has largely gone back to 9mm; .40 is on the way down.

.38 Special — the revolver standard. Simple, proven, and still the answer for anyone who wants a gun with no slide to rack.

Rifle

5.56 NATO / .223 Remington — the AR-15 cartridge. Note that these are not interchangeable in the direction most people assume: .223 is safe in a 5.56 chamber, but 5.56 in a .223-only chamber is a pressure problem. Check your barrel markings. Our .22 LR vs .223 comparison covers how these two very different rounds get confused.

.308 Winchester / 7.62×51 — the bigger, longer-range answer. Similar chambering caveat applies.

.300 Blackout — designed for short barrels and suppressors. Subsonic loads are the whole point.

The caliber debate, settled

Shot placement and training beat caliber, every time, and it isn’t close. A shooter who’s put a thousand rounds through a 9mm will outperform someone who bought a .45 and shot it twice. Modern defensive ammunition has narrowed the performance gap between service calibers to the point where the differences are academic compared to whether you can hit what you’re aiming at. Buy the caliber you’ll actually practice with. That’s the whole answer.

How to Read an Ammo Box

Every box tells you the same five things. Once you can read them, comparison shopping takes ten seconds.

Caliber — “9mm Luger,” “.223 Rem,” “.22 LR.” This must match your gun exactly. Not approximately. Exactly.

Grain weight — the bullet’s mass. 115gr, 124gr, 147gr in 9mm. A grain is 1/7000th of a pound, which is a useless fact except that it tells you these are small differences with real consequences. Heavier bullets go slower and penetrate deeper; lighter bullets go faster and expand more readily.

Bullet type — FMJ, JHP, SP, FTX, whatever. This is the section above. It’s the most important line on the box.

Velocity — usually stated in fps from a specific test barrel length. That last part matters enormously, and almost nobody reads it. A box claiming 1,150 fps from a 4-inch test barrel will do meaningfully less from your 3-inch carry gun.

The SKU — the boring alphanumeric string nobody looks at. It’s the single most useful thing on the box for shopping, because ammo listings are wildly inconsistent about naming. Search the SKU, not the brand, and you’ll know you’re comparing identical products.

+P, +P+, and Pressure

+P means the round is loaded to higher pressure than standard SAAMI spec for that caliber. More pressure means more velocity, which means more expansion and more penetration.

What it costs you: more recoil, more muzzle flash, and accelerated wear on your gun. It’s also not universally safe — some firearms, especially older ones and some lightweight aluminum-frame guns, are not rated for +P. Check your manual before you assume.

+P+ exceeds even +P and isn’t a SAAMI-standardized designation at all. Treat it with real caution.

When +P actually makes sense: short barrels. If you’re carrying a 3-inch micro-compact and running near the bottom of a bullet’s design velocity window, +P buys back the margin you lost to the barrel. From a full-size gun where standard pressure already expands reliably, +P mostly buys deeper penetration you may not want.

Why Barrel Length Changes Everything

This is the concept that separates people who understand ammo from people who just read the box.

Powder doesn’t burn instantly — it burns over a period, accelerating the bullet the whole way down the barrel. A longer barrel means more time for the powder to finish burning and more time to push. Cut the barrel and you lose velocity. You also get unburned powder exiting the muzzle, which is why short-barreled guns flash so much more.

Here’s why that’s not just trivia: hollow points need velocity to expand. A load that opens reliably from a 4-inch barrel can drop below the velocity it was designed around from a 3-inch barrel and simply not open — at which point you’ve paid premium prices for FMJ performance.

This is why “the best 9mm defensive load” is an incomplete question. The right question is “the best 9mm defensive load for my barrel,” and the answer genuinely changes. Our 9mm self defense ammo guide is built entirely around this problem, with the velocity numbers by barrel length.

The counterintuitive part

Heavy bullets lose a smaller percentage of their velocity in a short barrel than light ones — they started slower, and the powder has already done most of its work. Light, fast bullets are the ones designed around a longer burn. So heavy-for-caliber loads aren’t automatically wrong for short barrels; they’re just running closer to their own design floor. This is exactly why manufacturers make short-barrel-specific loads, and why you should believe them when they do — their engineers know their bullet’s threshold better than any rule of thumb does.

How to Actually Buy Ammo

Price varies more than you’d believe

When we price-checked a single SKU of Federal HST 147gr across four major retailers, the spread ran from $21.99 to $36.99 — 41% on the identical box. Same manufacturer, same product, same twenty rounds.

Two takeaways. First, search the SKU rather than the brand, since listings are inconsistent. Second, a cheap listing isn’t a cheap price if it’s out of stock — we hit that twice on the same check.

Bulk versus boxes

Practice ammo is where bulk buying pays. Case pricing on FMJ is meaningfully cheaper per round, and FMJ doesn’t go bad. Defensive ammo is different — you need far less of it, and you want it fresh enough that you’re not carrying rounds you bought a decade ago.

If you’d rather automate the accumulation, subscription services exist. Our Ammo Squared review covers how that model works, and whether it’s actually worth it gets into the math.

The rule that matters more than price

Whatever defensive ammo you choose, run at least 50 rounds of it through your actual gun before you trust it. Not 50 rounds of FMJ — 50 rounds of the specific load you’ll carry. Hollow points have different profiles, pressures, and feed geometry than ball ammo, and guns that run flawlessly on FMJ do choke on JHP. Fifty rounds costs about $55. It’s the cheapest insurance in shooting.

Storage and Shelf Life

Modern ammunition is remarkably durable. Kept cool, dry, and stable, it lasts decades — people shoot 1960s surplus routinely.

The three enemies: heat, humidity, and temperature cycling. A hot car trunk is the worst place you can keep ammo, because it’s all three at once.

The one storage rule people skip: rotate your chambered round. Every time you unload and re-chamber the same cartridge, the bullet can get pushed slightly deeper into the case — that’s called setback, and it raises chamber pressure. Mark your chambered round, and retire it from the carry rotation after a handful of cycles rather than re-chambering it for years.

Our ammo storage guide covers humidity control, containers, and organizing a real stockpile.

Common Ammo Mistakes

Carrying defensive ammo you’ve never fired. The most common and the most consequential. Your gun doesn’t care what the internet says about a load — it cares whether it feeds it.

Assuming every hollow point expands. Below the velocity it was designed around, or with the cavity plugged by heavy clothing, a JHP behaves like an over-penetrating FMJ. That’s the failure premium ammo is engineered to prevent.

Ignoring barrel length. The best load for a full-size gun can be the wrong load for a micro-compact. Match the ammo to the barrel.

Using FMJ for home defense. It won’t stop in your target. Whatever is behind that target — a wall, a room, a family member — is downrange. This is a genuine safety issue, not a performance preference.

Buying based on water-jug videos. Water jugs, phone books, and ballistic soap are entertainment, not calibrated gel. Use published testing with a stated methodology, and even then treat it as comparison, not prophecy.

Mixing up 5.56 and .223. .223 is safe in a 5.56 chamber. 5.56 in a .223-only chamber is a pressure problem. Read your barrel markings.

Storing it in a car trunk. Heat, humidity, and temperature cycling — the three things that actually degrade ammunition, all at once.

Buying whatever the counter guy suggests. Sometimes he’s right. Sometimes he’s moving inventory that isn’t selling. Now you can evaluate the answer yourself.

Our Ammo Guides

This page is the map. These are the destinations.

Defensive ammo: our best 9mm self defense ammo guide covers the loads worth carrying, the FBI penetration standard, and the barrel-length problem in depth — including which loads actually work from a 3-inch micro-compact.

Rimfire: our best .22 LR ammo guide covers what to feed a rimfire and why bulk isn’t always the answer. The complete .22 LR guide covers the whole platform.

Fundamentals: the basic parts of ammunition goes deeper on case, primer, powder, and projectile.

Caliber comparison: .22 LR vs .223 sorts out two rounds that sound similar and aren’t.

Storage and stockpiling: ammo storage covers doing it right. If you’re considering a subscription service, our Ammo Squared review, value analysis, and shipping breakdown cover that model.

Less-lethal: Byrna ammo covers kinetic and chemical rounds for launchers, and non-lethal shotgun shells covers that category.

Suppressed shooting: ammo choice changes when you add a can. Our 9mm suppressors guide and .22 suppressors guide cover subsonic requirements and why rimfire fouls so badly.

Questions People Actually Ask

What’s the difference between FMJ and hollow point ammo?

FMJ has a solid copper jacket over a lead core and doesn’t expand — it punches a caliber-sized hole and keeps going. A jacketed hollow point has a cavity in the nose that fills on impact and forces the bullet to mushroom to roughly double its diameter. Expansion creates a wider wound channel and, critically, slows the bullet so it’s more likely to stop in the target rather than exiting. Use FMJ for practice and JHP for defense.

Can I use FMJ for self defense?

You shouldn’t. FMJ over-penetrates — it doesn’t stop in the target, which means whatever’s behind that target is in danger. That’s the specific problem hollow points were invented to solve. In a genuine emergency any ammo beats none, but if you’re choosing what to load your carry gun with, load hollow points.

What does grain mean in ammo?

It’s the bullet’s weight. One grain is 1/7000th of a pound, so 9mm bullets run roughly 115, 124, or 147 grains. Heavier bullets travel slower and generally penetrate deeper; lighter bullets travel faster and expand more readily. Neither is universally better — it depends on your barrel length and what you’re trying to accomplish. Grain weight is mass, not power.

What does +P mean?

The round is loaded to higher pressure than standard SAAMI spec, which buys velocity. That means more expansion and penetration, at the cost of more recoil, more muzzle flash, and faster wear on your gun. Not every firearm is rated for +P — check your manual. It makes the most sense in short-barreled guns where you need velocity margin.

Does ammunition expire?

Not really, if you store it properly. Cool, dry, and stable conditions will keep modern ammunition viable for decades — people shoot surplus from the 1960s routinely. Heat, humidity, and temperature cycling are what degrade it, which is why a car trunk is the worst place you can keep it.

Is expensive ammo worth it?

For defense, yes. For practice, no. Premium defensive ammo is engineered to expand reliably through heavy clothing at real-world velocities, and it has decades of testing and law enforcement data behind it. Practice FMJ is a commodity — at seven yards there’s no meaningful accuracy difference between budget and premium ball ammo. Save money on the range ammo, not the carry ammo.

Can I shoot 5.56 in a .223 rifle?

Not safely, and this trips people up because the reverse is fine. .223 Remington is safe in a 5.56 NATO chamber. 5.56 is loaded to higher pressure and shouldn’t be fired in a .223-only chamber. Check your barrel markings — many modern rifles are marked 5.56 or “.223 Wylde,” which handle both.

Why does barrel length affect which ammo I should use?

Powder burns over time, accelerating the bullet the whole way down the barrel. Shorter barrel means less time, which means less velocity. Since hollow points need velocity to expand — and each bullet design has its own threshold — a load that opens reliably from a 4-inch barrel might not open from a 3-inch one. If you carry a micro-compact, use a load built for short barrels.

How much ammo should I have?

Depends entirely on what you’re doing. For defense, a few boxes of your carry load plus what’s in your magazines is plenty — you should be shooting it periodically to verify function, not stockpiling it. For practice, buy as much as you’ll actually shoot; bulk pricing rewards volume and FMJ doesn’t go bad. The honest answer is that most people would be better served buying less ammo and shooting more of it.

What’s the best caliber for self defense?

9mm, for most people, most of the time — and the reason isn’t ballistics. It’s that 9mm gives you more rounds, less recoil, cheaper practice, and universal availability, and modern defensive 9mm performs close enough to bigger calibers that those advantages win. But the real answer is that shot placement and training beat caliber every time. Buy the caliber you’ll practice with.

The Bottom Line

Ammunition is simpler than the wall of boxes suggests. Four components. A handful of bullet types where really only two matter for most people. A caliber landscape where the debate is mostly noise.

If you take three things from this page:

Practice with FMJ, carry hollow points. They cost about a quarter of what defensive ammo costs and they’re functionally identical for punching paper. That’s how you afford to shoot enough to get good.

Match the load to your barrel. The best defensive round for a full-size pistol may not expand from a 3-inch carry gun. This is the single most overlooked thing in ammo selection.

Shoot your carry ammo before you carry it. Fifty rounds. Every time. No exceptions.

Everything else is detail — and when you want the detail, the guides above have it.

Authority reference: ammunition standards are set by SAAMI, the Sporting Arms and Ammunition Manufacturers’ Institute, which publishes the pressure and dimensional specifications manufacturers build to. Ammunition laws vary by state and locality — always verify current law in your jurisdiction.