Ammunition naming conventions have evolved over more than a century, and the shorthand used at a gun-store counter — "ten-gauge," "ten-mil," "seventeen HMR" — compresses a substantial amount of technical meaning into a few syllables. This reference explains what each term actually specifies: bore, case dimension, projectile, and intended use.

Gauge Nomenclature and Shell Length

Gauge is not a diameter measurement; it is a historical weight ratio. "10 gauge" means ten pure-lead balls of barrel diameter sum to one pound, which works out to a 0.775-inch bore. The shorthand 10ga appears on box labels and catalog listings as the same specification written compactly.

For the most common shotshell family, the 12 gauge, shotgun shells are manufactured in 2¾-inch, 3-inch, and 3½-inch lengths. Chamber length must match or exceed shell length; running a 3-inch shell in a 2¾-inch chamber is a pressure event that damages guns.

Buckshot Sizing

Buckshot uses its own numeric system where "0" designations indicate larger pellets. A pellet of 12 ga buckshot in 00 size measures 0.33 inches; in 000 size, 0.36 inches; in #1 buck, 0.30 inches. Pellet count per shell varies accordingly — a standard 2¾-inch 00 buck shell carries 9 pellets; a 000 buck shell carries 8; a #1 buck shell carries 16.

Slug Terminology

When you see slugs for 12 gauge on a shelf, the specification tier is usually one of three: Foster rifled slugs (hollow-base lead for smoothbores), sabot slugs (sub-caliber projectile in a plastic sleeve for rifled barrels), or Brenneke-style slugs (solid lead attached to a wad column, compatible with both barrel types).

Non-Toxic Shot Terminology

Federal waterfowl regulations drove the 1991 transition to non-toxic shot. A waterfowl shotgun shell will be labeled "steel," "bismuth," "tungsten-matrix," or "HEVI-shot" — each a specific non-toxic alloy. Steel is the most common; tungsten and bismuth deliver lead-like density at 3 to 6 times the per-round cost.

Classic Gauge Nomenclature

The sixteen gauge shotgun shells classification refers to a 0.662-inch bore — between 12 (0.729 inches) and 20 (0.615 inches). The 16 gauge was the dominant American upland bore from 1900 through the 1960s and survives today in specialty platforms from Browning, CZ, Savage, and Ithaca.

Handgun Cartridge Naming

The "10mm" in bullet 10mm references the projectile diameter (10mm ≈ .40 inches). The cartridge is properly "10mm Auto," standardized by SAAMI in 1983 at 37,500 psi maximum operating pressure. Case length is 25.2mm; overall cartridge length is 32mm.

Grain Weight and Hardcast Designations

Projectile weight is measured in grains (7,000 grains per pound). A 220 grain 10mm projectile weighs 31.6 grams — the heavy end of practical 10mm loadings, usually a hard-cast flat-nose gas-checked design for hunting or bear-defense use.

Within the Underwood catalog, underwood 10mm ammo appears in weight designations from 115 grains (Xtreme Penetrator) through 230 grains (hardcast subsonic). Load selection is driven by intended barrel length and use case: lighter projectiles produce higher velocity; heavier projectiles penetrate deeper and retain more energy at distance.

Rimfire Pricing Shorthand

Listings marked 17 hmr ammo for sale typically specify projectile weight (17-grain standard, 20-grain heavy), construction (V-Max, XTP, FMJ), and case count. A 500-round brick of V-Max runs $100 to $135 depending on brand and lot; individual