Key Takeaway

Caliber is the size of the bullet. Grain is the weight of the bullet. FMJ is for the range, hollow point is for self-defense. Match the caliber stamped on your gun to the caliber on the ammo box. That is the most important rule.

1. What Is Caliber?

Caliber is the internal diameter of a gun's barrel, which also tells you the diameter of the bullet it fires. It is measured in either inches (American convention) or millimeters (metric convention).

When someone says "9mm," they mean the bullet is 9 millimeters in diameter. When someone says ".45 ACP," the bullet is 0.452 inches in diameter. Different naming conventions, same concept: it tells you how wide the bullet is.

The caliber stamped on your gun must match the caliber on the ammo box. A 9mm handgun shoots 9mm ammunition. A .45 ACP shoots .45 ACP. Using the wrong caliber can cause a catastrophic failure. This is non-negotiable.

Same Diameter, Different Cartridge

Some calibers share the same bullet diameter but are completely different cartridges. For example, .380 ACP and 9mm Luger both use a .355" bullet, but the .380 has a shorter case and lower pressure. They are NOT interchangeable. Always match the full cartridge name, not just the bullet size.

2. What Does "Grain" Mean?

Grain (abbreviated "gr" on ammo boxes) is a unit of weight. One pound equals exactly 7,000 grains. The grain weight listed on ammunition refers to the weight of the bullet (the projectile), not the entire cartridge.

So when you see "115 gr FMJ" on a box of 9mm, it means each bullet weighs 115 grains, which is about 7.5 grams or a quarter of an ounce. Light for something that can save your life.

Lighter vs Heavier Bullets

  • Lighter bullets (lower grain): Travel faster, produce slightly less recoil, can lose energy at distance. In 9mm, 115 gr is the lightest common weight. Good for target shooting and practice.
  • Heavier bullets (higher grain): Travel slower but carry more momentum, penetrate deeper, and can be more consistent through barriers. In 9mm, 147 gr is the heaviest common weight. Often preferred for self-defense.
  • Middle ground: In 9mm, 124 gr is the Goldilocks option. Balanced velocity, recoil, and penetration. Many law enforcement agencies use 124 gr +P hollow points.

Practical Advice

For your first few range trips, buy whatever 9mm FMJ is cheapest in 115 gr or 124 gr. The difference at beginner distances is negligible. Save the premium stuff for when you have chosen a defensive load.

3. Bullet Types: FMJ, Hollow Point, and Frangible

Full Metal Jacket (FMJ)

A lead core wrapped in a copper jacket. FMJ bullets maintain their shape on impact and penetrate deeply without expanding. This is what you buy for range practice. It is the cheapest option, feeds reliably in virtually every firearm, and is available everywhere. The downside for defense: FMJ can pass through a target (over-penetrate), which is a liability in a home with family members or neighbors nearby.

Hollow Point (JHP)

A bullet with a cavity in the tip that causes it to expand ("mushroom") on impact. This expansion creates a larger wound channel and transfers more energy to the target. Equally important, it reduces the risk of over-penetration. Hollow points are the standard for self-defense and law enforcement. They cost more than FMJ, but when you are loading a magazine for home defense or concealed carry, this is what goes in it.

Frangible

Made from compressed metal powder instead of solid lead and copper. Frangible bullets break apart into small fragments on impact with hard surfaces, virtually eliminating ricochet. They are used for close-quarters training, shoot houses, and steel target shooting at close range. They are also lead-free, which makes them popular at indoor ranges. You probably will not need frangible ammo as a new shooter, but it is good to know what it is if you see it on the shelf.

4. Common Calibers at a Glance

Here are the calibers you will see most often at the shop and the range, with approximate current retail prices for practice-grade FMJ ammunition.

Handgun Calibers

  • .22 LR: The smallest and cheapest. About $0.06 to $0.12 per round. Great for learning fundamentals, plinking, and introducing new shooters. Not recommended for self-defense. Note: .22 LR is rimfire, not centerfire like the other calibers listed here.
  • .380 ACP: A step up from .22 but still mild recoil. About $0.28 to $0.45 per round. Common in small pocket pistols. Same bullet diameter as 9mm (.355") but shorter case and lower pressure. Adequate for self-defense at close range with quality hollow points.
  • 9mm Luger: The most popular handgun caliber in the world. About $0.20 to $0.28 per round for practice ammo at retail. Best balance of recoil, capacity, cost, and terminal performance. This is what most first-time handgun owners should buy.
  • .40 S&W: A declining caliber. About $0.28 to $0.45 per round. Developed as a compromise between 9mm and .45 ACP. Snappier recoil than 9mm with less capacity. Many law enforcement agencies that adopted it in the 1990s have since switched back to 9mm.
  • .45 ACP: The classic big-bore round. About $0.30 to $0.50 per round. Heavier recoil, lower capacity, more expensive to shoot. The standard 230 gr load is naturally subsonic. Modern 9mm defensive ammo has largely closed the performance gap, but the .45 still has its dedicated following.

Rifle Calibers

  • .223 Remington / 5.56 NATO: The AR-15 caliber. About $0.38 to $0.55 per round for brass-cased practice ammo. These two are closely related but not identical. You can safely fire .223 in a 5.56 NATO chamber. Firing 5.56 in a .223-only chamber can be dangerous because the shorter throat in .223 chambers can cause pressure spikes. Check your barrel markings.
  • .308 Winchester / 7.62 NATO: The full-power rifle round. About $0.55 to $0.80 per round. Used for hunting, precision shooting, and military applications. Effective out to 800+ yards. Significantly more recoil than .223/5.56.

5. Shotgun Gauge: Why the Numbers Are Backwards

Shotgun sizing uses the gauge system, which confuses everyone at first because a smaller number means a bigger bore. Here is why.

The gauge number tells you how many lead balls of the bore's diameter it takes to weigh one pound. A 12-gauge bore fits a lead ball that weighs 1/12 of a pound. A 20-gauge bore fits a lead ball that weighs 1/20 of a pound. Since the 12-gauge ball is heavier, it must be bigger. So 12 gauge is larger than 20 gauge.

  • 12 gauge (0.729" bore): The standard for home defense, hunting, and sport shooting. The vast majority of tactical and defensive shotguns are 12 gauge.
  • 20 gauge (0.615" bore): Lighter, less recoil. Good for smaller-framed shooters or anyone who finds 12 gauge uncomfortable. Effective for home defense at indoor distances.
  • .410 bore (0.410" bore): The exception. This one is measured in caliber (inches), not gauge. If it were expressed in gauge, it would be about 67 gauge. Minimal recoil but limited defensive effectiveness compared to 12 or 20 gauge.

6. What to Buy: Range vs Self-Defense

For the Range

Buy FMJ (full metal jacket) in bulk. For 9mm, 115 gr or 124 gr FMJ from brands like Federal American Eagle, Blazer Brass, Winchester White Box, or Fiocchi. The cheapest brass-cased FMJ you can find is usually fine. Buy in boxes of 50 or cases of 500 to 1,000 for the best per-round price.

For Self-Defense

Buy jacketed hollow points (JHP) from a premium manufacturer. The FBI standard for defensive ammunition is 12 to 18 inches of penetration in calibrated ballistic gelatin with reliable expansion. These loads meet that standard:

  • Federal HST: The most widely used law enforcement round. Consistent expansion and penetration.
  • Speer Gold Dot: Bonded core prevents jacket separation. Another law enforcement standard.
  • Hornady Critical Defense: Flex Tip insert prevents the hollow point from clogging with fabric. Designed for short-barreled concealed carry guns.

Match Your Practice to Your Carry

If you carry 147 gr hollow points for self-defense, practice with 147 gr FMJ at the range. Matching bullet weight keeps the recoil feel similar, so your training transfers directly to your carry setup.

7. Massachusetts: Ammo Purchase Rules

Massachusetts requires a firearms license to purchase ammunition. Here is the short version.

  • FID card holders: Can purchase ammunition for non-large-capacity, non-semi-automatic rifles and shotguns. Under Chapter 135 of the Acts of 2024, semi-automatic firearms now require an LTC, and so does their ammunition.
  • LTC holders: Can purchase all ammunition types, including handgun ammo.
  • Hollow points: Legal in Massachusetts. There are no state restrictions on hollow point, frangible, or other expanding ammunition. Buy what you need for self-defense without concern.
  • In-person purchase required: Massachusetts law (MGL c.140 s.131E) requires ammunition to be sold in a face-to-face transaction by a licensed dealer. As a result, most online retailers will not ship ammunition directly to Massachusetts residents. Buy your ammo at the shop.

Full Legal Details

For the complete rundown on Massachusetts firearms and ammunition law, visit massgunlaws.com.

8. Common Myths

A few things you will hear at the range or read online that are not true.

Shot placement matters more than bullet diameter. A well-placed 9mm is more effective than a poorly placed .45. The FBI switched back to 9mm in 2015 because modern hollow point ammunition has closed the terminal performance gap, and agents shoot 9mm more accurately due to lower recoil.
Modern centerfire ammunition stored in cool, dry conditions can remain reliable for decades. Avoid high humidity, extreme temperature swings, and direct sunlight. If your ammo has been sitting in a climate-controlled space, it is almost certainly fine.
This is potentially dangerous. A .223 Remington chamber has a shorter throat than a 5.56 NATO chamber, which can cause pressure spikes when firing 5.56 ammo. Only fire 5.56 in chambers rated for it. The reverse (.223 in a 5.56 chamber) is always safe. Check the markings on your barrel.
Steel-cased ammo is harder on extractors over time and cannot be reloaded, but it functions well in most modern firearms. If you are buying in bulk for range practice and want to save money, steel case is a reasonable option. Many ranges do not allow it, though, so check first.

Ready to Stock Up?

We carry a full selection of handgun, rifle, and shotgun ammunition in the shop. If you are not sure what to buy for your gun, bring it in (unloaded, in a case) and we will match you up with the right ammo for your purpose.

Need ammunition?

Stop by Fudd Free Firearms in Tewksbury. We stock practice ammo, premium defensive loads, and everything in between.

Visit the Shop

Buying your first gun? Start with our First Handgun Buying Guide or our Home Defense Shotgun Setup guide.

This guide is for informational purposes only. Prices are approximate and fluctuate. Massachusetts firearms laws change. Verify current legal requirements with your local police department, a licensed attorney, or massgunlaws.com.