A box of ammunition is covered in numbers, abbreviations, and stamps that look like they require a glossary. They do — and most shooters never get one. You learn that 115gr 9mm runs your carry gun fine, you ignore the rest, and life moves on.
But those markings encode real, useful information. The difference between M193 and M855. Whether a load is rated for your gun. Whether a case is Boxer-primed and reloadable. What "+P" actually does to chamber pressure. Where the brass came from and what year it was made.
Here's a plain-English breakdown of everything on a typical box of ammo and the case head itself, what each marking means, and where it actually matters.
What's on the Front of the Box
The front of an ammo box is mostly marketing, but a handful of fields are doing real work.
Caliber Designation
The caliber line is the most important marking on the box. It looks simple — "9mm Luger," "5.56x45 NATO," ".308 Winchester" — but the suffix matters more than most shooters realize.
9mm Luger / 9mm Parabellum / 9x19mm are all the same cartridge. "9mm Makarov" (9x18) is a different round entirely and will not run in a 9mm Luger pistol.
.223 Remington and 5.56 NATO look interchangeable but aren't. 5.56 NATO is loaded to higher pressure (~62,000 psi vs ~55,000 psi for .223). A rifle marked ".223 Rem" only is not safe with 5.56 NATO ammo. A rifle marked "5.56 NATO" or ".223 Wylde" runs both. Check the barrel stamp before you mix them.
.308 Winchester and 7.62x51 NATO are similar but not identical. Most modern bolt rifles handle both, but older military surplus rifles chambered in 7.62 NATO have looser chambers that can cause case head separation with full-pressure commercial .308 loads. When in doubt, match the stamp.
.38 Special and .357 Magnum share case dimensions but not length. A .357 revolver shoots both; a .38 Special revolver does not safely shoot .357 Magnum.
If the cartridge name has a slash (.45 ACP/Auto, .38 Special/.38 SPL), those are just abbreviations of the same round.
Bullet Weight in Grains
Weight is listed in grains (gr), where 1 grain = 1/7000 of a pound. This is the weight of the bullet itself, not the whole cartridge.
Lighter bullets generally fly faster with less recoil. Heavier bullets carry more energy at distance and cycle differently in semi-autos.
Common standard weights:
- 9mm: 115gr (light, fast), 124gr (NATO standard), 147gr (heavy, often subsonic)
- .45 ACP: 185gr, 200gr, 230gr (standard ball)
- 5.56 / .223: 55gr (M193), 62gr (M855), 69gr and 77gr (match)
- .308: 147gr, 150gr, 168gr (match), 175gr (long-range)
Bullet weight matters for two practical reasons: rifle barrels prefer specific weights based on twist rate, and pistols can fail to cycle reliably with weights outside their design range. A 1:7 twist 5.56 barrel stabilizes 77gr match bullets well; a 1:9 twist barrel may not. A 9mm carry gun tuned for 115gr might short-cycle with 147gr subsonic.
Bullet Type Abbreviations
This is where most boxes get cryptic. The dominant abbreviations:
- FMJ — Full Metal Jacket. Lead core, copper jacket fully covering the bullet. Penetrates deeply, doesn't expand. Standard range and military round.
- TMJ — Total Metal Jacket. Like FMJ but the lead base is also covered. Reduces airborne lead at indoor ranges. Sometimes required by indoor range rules.
- JHP — Jacketed Hollow Point. Hollow cavity in the nose causes expansion on impact. Standard for self-defense.
- HP — Hollow Point (often unjacketed lead). Common in revolvers and .22 LR.
- SP / JSP — Soft Point / Jacketed Soft Point. Exposed lead tip for controlled expansion. Common in hunting loads.
- OTM / HPBT — Open Tip Match / Hollow Point Boat Tail. Match-grade rifle bullets. The opening is for manufacturing precision, not expansion. Don't assume an OTM is a defensive bullet.
- BTHP — Boat Tail Hollow Point. Similar to OTM, optimized for long-range accuracy.
- LRN / LFP — Lead Round Nose / Lead Flat Point. Unjacketed lead. Common in cowboy action and revolver target loads.
- FN / TC — Flat Nose / Truncated Cone. Bullet profile descriptors, often on revolver and reloading bullets.
- AP / API / APIT — Armor Piercing / Incendiary / Tracer. Mostly restricted to military or surplus, with legal restrictions on civilian sale.
- Frangible — Designed to disintegrate on hard targets. Used at steel-target ranges and for some defensive applications.
If you see a combination — "55gr FMJBT," "147gr JHP +P" — read each abbreviation independently.
Pressure Ratings: +P, +P+, NATO
Pressure markings tell you whether the load runs hotter than standard.
Standard pressure is the SAAMI specification for that cartridge. No marking needed.
+P indicates the load is rated above standard pressure but within an industry-recognized higher spec. For 9mm, +P pushes from ~35,000 psi to ~38,500 psi. Most modern firearms handle +P fine; check your manual to confirm. Older or lightweight guns may be excluded.
+P+ has no SAAMI specification. It's a manufacturer-defined "above +P" load with no standardized ceiling. Some guns explicitly forbid it. Treat +P+ as ammunition for a specific application (some duty/LE loads), not as a general upgrade.
NATO (often shown as a cross-in-circle ⊕) means the cartridge meets NATO military specifications, which generally run hotter than SAAMI civilian specs. 9mm NATO is roughly equivalent to 9mm +P in pressure. 5.56 NATO is significantly hotter than .223 Remington. M80 7.62 NATO sits at the upper end of .308 pressure tolerance.
A rifle stamped for the NATO equivalent will run both. A rifle stamped only for the civilian designation may not be safe with the NATO load.
Velocity and Energy
Most boxes list muzzle velocity (fps) and muzzle energy (ft-lbs) — sometimes for several ranges out to 100 yards.
These are factory-test numbers from a specific barrel length, almost always longer than your barrel. A 9mm load advertised at 1,150 fps from a 4-inch test barrel will probably run 1,050–1,100 fps from a 3.1-inch carry gun. Use the published velocity as a relative comparison between loads, not an absolute prediction for your gun.
Lot Number
A small alphanumeric code, sometimes on the box flap or end label. The lot identifies which production run the ammo came from. If you find a load your rifle shoots well, write down the lot number — the next box of the same product line might shoot to a different point of impact. Match shooters buy in bulk by lot for this reason.
Reading the Headstamp
Flip a cartridge over and look at the case head. The markings stamped into the metal are the headstamp, and they tell you who made the case and (sometimes) when.
Civilian Headstamps
Commercial brass typically shows the manufacturer name and the cartridge designation:
- WIN 9MM LUGER — Winchester, 9mm Luger
- R-P .308 WIN — Remington-Peters, .308 Winchester
- FC 5.56 — Federal Cartridge, 5.56
- PMC ⋅ 223 REM — PMC, .223 Remington
- HORNADY 6.5 CREEDMOOR — self-explanatory
If the headstamp says "LUGER" or "PARA" instead of "9MM," you're still looking at 9mm Luger — older naming convention.
Military Headstamps
Military brass uses two-letter codes for the manufacturing plant and a two-digit year:
- LC 22 — Lake City Army Ammunition Plant, 2022
- WCC 81 — Winchester Western Cartridge Company, 1981
- FC 19 — Federal Cartridge contract, 2019
- WIN 23 — Winchester (commercial-format date stamp on military contract brass)
Lake City brass is desirable for reloaders because it's well-made and consistent. The crimped primer pocket needs to be reamed or swaged before it can be reprimed, but the cases are durable through many reloads.
A circle-with-cross symbol (⊕) on military brass means NATO-spec.
Foreign and Surplus Headstamps
Imported and surplus ammunition uses codes that often aren't intuitive:
- S&B — Sellier & Bellot (Czech Republic)
- PPU or PRVI — Prvi Partizan (Serbia)
- GGG — Giraitės Ginkluotės Gamykla (Lithuania)
- MFS — MFS 2000 (Hungary)
- IMI or TZZ — Israeli Military Industries
- HXP — Hellenic (Greek) military
- RG — Radway Green (UK military)
These are useful to know when buying surplus or import-brand ammunition, especially if you're sorting brass for reloading or want to track which production sources your gun shoots best.
Steel-Case Markings
Steel-cased ammunition (Wolf, Tula, Barnaul) typically has minimal headstamp markings — often just a year and a factory code. Steel cases are not reloadable in any practical sense, so the headstamp matters less for tracking.
Specific Markings Worth Knowing
A few markings appear on specific cartridges that warrant their own explanation.
5.56: M193 vs M855 vs Mk262
These are military load designations that have crossed over into civilian markets:
- M193 — 55gr FMJ, the original 5.56 round. Standard for general training and AR-15 use.
- M855 — 62gr FMJ with a steel penetrator core (the "green tip"). Slightly better at defeating barriers, slightly less accurate than M193. Some indoor ranges ban it because the steel core damages backstops.
- M855A1 — Updated 62gr load with a copper-alloy core. Restricted to military distribution; civilian-market M855 is the older lead-core version.
- Mk262 Mod 1 — 77gr OTM match round developed for SOCOM. A premium match load when you can find it commercially labeled.
If you're shooting a 1:7 or 1:8 twist barrel, all three stabilize fine. A 1:9 twist barrel handles M193 and M855 but may struggle with 77gr. For a deeper breakdown of which loads actually pair well with which AR-15 setups, see our guide to the best ammo for AR-15s.
.300 Blackout: Subsonic vs Supersonic
.300 Blackout is unique because it's commonly loaded both subsonic (around 1,000 fps with 200–220gr bullets) and supersonic (around 2,200 fps with 110–125gr bullets). The box will say which.
This matters because subsonic .300 BLK is often suppressor-optimized and may not cycle reliably without a suppressor or adjusted gas system. Supersonic .300 BLK runs fine in any standard setup. Don't assume one will substitute for the other in your specific rifle. If you're specifically shopping the subsonic side of the market, our best .300 BLK subsonic loads breakdown covers what to look for.
Shotshell Markings
Shotgun ammunition uses its own conventions:
- Gauge or bore — 12, 20, 28, .410. Larger numbers mean smaller bores (except .410, which is a true caliber measurement).
- Shell length — 2¾", 3", 3½". Your shotgun's chamber length determines what you can shoot. A 3" chamber accepts 2¾" and 3" shells; a 2¾" chamber does not safely accept 3" shells.
- Dram equivalent — Old-school gunpowder measurement. "3 dram eq." means the load delivers ballistics roughly equivalent to 3 drams of black powder. Mostly historical, but still printed on hunting and clay loads.
- Shot size — #9, #8, #7½ for clay targets and small birds; #4, #2, BB for waterfowl; 00 buck for defense and deer. Smaller numbers mean larger pellets.
- Slug — Single-projectile shotgun load. Rifled slugs spin from a smooth bore; sabot slugs require a rifled barrel.
What the Markings Don't Tell You
A few things you might assume are on the box that often aren't:
Whether the primer is Boxer or Berdan. Most American-made commercial ammo is Boxer-primed and reloadable. Most European and military surplus is Berdan-primed and not practically reloadable. The headstamp doesn't say. The way to check is to look down the inside of a fired case — Boxer primers have a single central flash hole; Berdan primers have two or three small holes around a central anvil.
Bullet construction details for defensive loads. "147gr JHP" tells you almost nothing about how that bullet performs in ballistic gel. For carry ammo, look up the specific load by name (e.g., Federal HST 124gr +P, Speer Gold Dot 147gr) and check independent test results.
Whether the load is suppressor-safe. Subsonic isn't enough — bimetal jackets, frangible compositions, and steel cores all cause problems with suppressors. Match the load to your can's manufacturer recommendations.
Lot-to-lot variation. Two boxes of the same product line, same year, can group differently in the same rifle. The headstamp won't warn you. Match shooters buy in matched lots; everyone else just notes when a particular lot shoots well.
Why Any of This Matters for What You Buy
Most shooters don't need to memorize any of this. But knowing how to read a box pays off in a few specific situations:
Buying online without seeing the box first. Product listings vary in quality. If a listing says "5.56 62gr FMJ," knowing that's almost certainly M855 (and what M855 implies for indoor ranges and barrier performance) helps you make the right call.
Mixing ammo across guns. Knowing which guns can handle +P, NATO, or 5.56 vs .223 keeps you out of trouble.
Buying surplus or import. PPU, S&B, GGG, and IMI all make solid ammunition, but knowing what those headstamps mean lets you compare across sources rather than treating all "imports" as equivalent.
Reloading or casing collection. Brass with crimped military primers, Berdan-primed cases, and steel cases all have different reuse implications.
Building a stash. If you're keeping ammo for the long term, knowing the year on military headstamps and the lot codes on commercial boxes gives you a way to rotate stock and track what's been sitting longest.
The markings are dense, but they exist for a reason. Once you know the vocabulary, the box tells you what's inside before you open it — and that's the difference between buying ammo and buying the right ammo.
IronScout tracks ammunition prices across major online retailers for 14 calibers. To see current per-round pricing for any of the loads mentioned in this guide, visit our caliber pages or the ammo price index.