Ammunition Explained: What the Numbers on the Box Mean

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September 24, 2026 · Dan Ribacoff, NRA-Certified Instructor

The anatomy of a cartridge

A modern cartridge has four components:

• Case — usually brass, occasionally steel or aluminium. Holds everything together and seals the chamber during firing.
• Primer — a small impact-sensitive charge in the case head that ignites when struck by the firing pin.
• Propellant — smokeless powder, which burns rapidly to generate expanding gas.
• Bullet — the projectile that leaves the barrel. Note that "bullet" refers to this component only; the whole assembly is a cartridge or round.

Understanding these makes the terminology on a box comprehensible rather than mysterious.

Calibre

Calibre nominally describes bullet diameter, but the naming conventions are inconsistent and occasionally misleading. Some are metric (9mm), some are decimal inches (.45), and several different cartridges share similar names while being entirely incompatible.

Some combinations are permitted in one direction only: a .357 Magnum revolver can fire .38 Special, but a .38 Special revolver must not fire .357 Magnum. Likewise a 5.56 NATO rifle can generally fire .223 Remington, but not the reverse. These asymmetries are about pressure.

Grain weight

Grain is a unit of mass. There are 7,000 grains in a pound. On an ammunition box, the grain figure refers to the weight of the bullet.

It is not a measure of power, and heavier is not automatically better. In general terms, within a given calibre:

	Lighter bullet	Heavier bullet
Velocity	Higher	Lower
Recoil impulse	Sharper, less push	More push
Penetration	Generally less	Generally more
Point of impact	Often lower	Often higher
Typical 9mm weights	115 gr	124 gr, 147 gr

The practical implication: different grain weights shoot to different points of impact from the same gun. If you practise with 115 grain and carry 147 grain, confirm where your defensive load actually lands.

Bullet construction

• Full metal jacket (FMJ) — lead core in a copper jacket. Inexpensive, reliable feeding, minimal expansion. The standard practice round. Over-penetrates in defensive use.
• Jacketed hollow point (JHP) — designed to expand on impact, increasing effectiveness and reducing over-penetration. The standard defensive choice.
• Total metal jacket / plated — fully enclosed lead, reducing airborne lead exposure. Preferred at some indoor facilities.
• Frangible — designed to break up on hard surfaces. Used for steel target training and certain specialised applications.
• Wadcutter and semi-wadcutter — flat-nosed target designs that cut clean holes in paper.

Practice vs. defensive ammunition

These are different products designed for different jobs and both are necessary.

Practice ammunition is inexpensive FMJ intended to be fired in volume. Defensive ammunition is engineered hollow point costing several times as much per round, designed for terminal performance and reliability.

• Buy defensive ammunition from a reputable manufacturer with a documented track record.
• Test it in your firearm. Fire at least one or two boxes to confirm reliable feeding and extraction. Some pistols are picky about hollow point profiles, and finding out at the wrong moment is unacceptable.
• Confirm point of impact, which will differ from your practice load.
• Replace carried ammunition periodically. Repeatedly chambering the same round can push the bullet back into the case, which raises pressure dangerously.
• Do not use unknown or unverified reloaded ammunition in a defensive firearm.

Pressure ratings and +P

"+P" indicates ammunition loaded to a higher pressure than standard for that cartridge, producing higher velocity, more recoil and more wear on the firearm.

Not all firearms are rated for +P, and some are rated for occasional rather than continuous use. Check your manual. Some older firearms and some lightweight revolvers should never be fed +P.

"+P+" exceeds even the +P specification and is not standardised. Approach it with caution and only in firearms explicitly rated for it.

Storage in a humid climate

Modern ammunition is fairly robust but not indifferent to conditions, and South Florida is the harder end of the spectrum.

• Store in a cool, dry place. Not an attic, not a garage, not a car.
• Sealed containers with desiccant are worth the small effort.
• Avoid direct sunlight and large temperature swings.
• Inspect for corrosion on cases and primers periodically.
• Rotate stock — use the oldest first.
• Ammunition carried daily in a pocket or a sweaty holster environment should be replaced more frequently than boxed stock, and inspected regularly.

Ammunition selection and testing come up in essentially every lesson, and it is a subject where good information saves real money — both by avoiding the wrong purchases and by understanding what genuinely matters.

Buying decisions worth slowing down

Firearms and accessories are a market with enormous marketing spend and very little independent guidance, which makes it easy to accumulate equipment that does nothing for your shooting. A few principles that hold up over time.

Spend on fit and function before you spend on appearance. A gun that fits your hand, a belt stiff enough to carry it, a holster that covers the trigger guard completely, and quality hearing protection will do more for your shooting than any optic, trigger job or muzzle device.

Test everything personally. Recoil spring weights, trigger reach, grip texture and sight visibility vary enormously between models and cannot be evaluated from a specification sheet or a video review. An afternoon at a rental counter is the cheapest research available.

Be sceptical of anything that promises to substitute for training. Optics, lasers, compensators and premium ammunition all have legitimate uses, and none of them fix a grip, a flinch or a trigger press. The order of operations is fundamentals first, equipment second, and reversing it is how people end up with expensive guns they cannot shoot.

Train with a certified instructor in South Florida

Reading about shooting is useful. Being watched while you shoot is transformative — because the faults that limit you are, almost by definition, the ones you cannot see yourself. Delray Firearms Training provides private, NRA-certified instruction with Dan Ribacoff throughout Palm Beach County and Broward County, including Delray Beach, Boca Raton, Boynton Beach, West Palm Beach, Fort Lauderdale, Coral Springs and Weston.

Lessons are one-on-one by default, arranged by appointment seven days a week, and shaped around your experience level rather than a fixed script. Whether you have never fired a shot or you have carried for twenty years and want a specific problem solved, the starting point is a short conversation.

Call 516-315-2809 Request a Time

About the instructor

Dan Ribacoff is an NRA-certified firearms instructor, licensed private investigator and career security professional who has spent decades in investigations, executive protection and private security. He teaches handgun, rifle, carbine, shotgun, concealed carry, home defense and maintenance courses across South Florida. Read more about Dan →

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