A new hunter who buys a 12-gauge shotgun and a box of "12-gauge shells" at random is more likely to end up with a load mismatched to their hunt than not, because "12-gauge" describes the barrel and chamber the gun is built around — not what's inside the shell. The same gun can fire a load built to pattern tightly on a dove at thirty yards, a load built to throw a dense swarm of small pellets at a rooster pheasant flushing close, or a single solid slug built to punch through a deer's vital cavity. Gauge tells you what fits the gun. Shot size and shot type tell you what the gun actually does when you pull the trigger.
Gauge is an old measurement based on how many lead balls of that bore diameter it would take to add up to one pound — a 12-gauge bore is sized so that twelve such balls weigh a pound, a 20-gauge bore so that twenty do, which is why a smaller gauge number means a larger bore diameter. A 10-gauge has a wider bore than a 12, which has a wider bore than a 20. A wider bore can hold more shot and more powder per shell, generally giving larger gauges more pellets downrange and more energy per shot, at the cost of more recoil and a heavier gun to carry all day. Gauge sets the ceiling on what a shell can hold; it doesn't determine what's actually loaded inside any individual shell.
Shot size is where most new hunters get tripped up, because the numbering is inverted from what feels intuitive: a larger number means smaller individual pellets, and a smaller number means larger pellets. Size 8 or 9 shot is tiny — appropriate for dove or quail, where a dense pattern of small pellets is what actually breaks a small, fast bird, since any single pellet doesn't need to carry much energy. Size 4 or BB shot is much larger — fewer pellets per shell, but each one carries enough mass and energy to be effective on a bigger-bodied bird like a goose at longer range. The logic scales directly with the target: small, fragile, close-range game wants shot density; larger, tougher, farther-away game wards off a hit unless each individual pellet is carrying real energy.
A shotshell doesn't fire one projectile — it fires a swarm that spreads into a pattern as it travels, and that pattern has to be dense enough that at least one pellet reaches a vital area on a moving target the hunter often has only a second to track. More, smaller pellets fill that pattern more densely at a given range, which is why upland and waterfowl loads favor smaller shot. But smaller pellets also decelerate faster and carry less individual energy, so past a certain range they simply don't penetrate deep enough to be lethal even if they connect. That's the actual tradeoff behind every shot size choice: density of coverage against penetration per pellet, and the right balance depends entirely on how big, how tough, and how far away the target is.
For deer-sized game, a swarm of small pellets isn't an option at all — no individual pellet would carry enough energy to reach the vitals reliably. Shotgun deer hunting instead uses either buckshot, a much smaller number of considerably larger pellets designed to each act almost like a mini-projectile, or a slug, a single solid projectile that turns the shotgun into something that performs much more like a rifle for that one shot. States that restrict rifle use in certain zones — often for population-density and downrange safety reasons, since a rifle bullet travels far past a miss in a way a shotgun slug generally doesn't — lean on slug guns specifically because the shell can be built to deliver rifle-like energy without a rifle's much longer stray-round range.
For decades, lead was the standard shot material because it's dense, cheap, and easy to form. Waterfowl hunting changed that: ducks and geese feed by dabbling in sediment, ingesting spent shot along with grit, and lead shot ingested this way causes lead poisoning that was measurably affecting waterfowl populations. That's why non-toxic shot — steel, and later bismuth, tungsten, and other alloys — became a legal requirement for waterfowl hunting rather than a personal choice. Steel is significantly less dense than lead, so steel shot needs to be a size or two larger than the lead equivalent to carry comparable energy, which is why waterfowl shot size charts don't line up directly with the upland shot sizes hunters may already be used to.
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