Semi-Automatic
Semi-automatic shotguns come in two flavors: gas-operated and inertia-driven. Your decision on which one to buy is largely affected by its use.
These types of shotguns are called semi-automatics because they automate several functions. With a break-open shotgun — such as an over-and-under or a side-by-side — you have to manually load the gun, generally after two shots. Semi-automatics got their name by mechanically ejecting a spent shell casing after each shot and self-loading a new shotgun shell.
When it comes to clays shooting, some people prefer the semi-automatic because they are less expensive than most break-open shotguns — lowering the financial barrier to entry. Also, certain types of semi-automatic shotguns deliver less felt recoil than a break-open.
Whether it's a gas-operated or inertia-driven semi-automatic shotgun, the shell itself is the driving factor in completing the self-loading shot cycle. When a shotgun shell is ignited, it produces two byproducts: expended gases from the gun powder and recoil (or kick) from the explosion. Some semi-automatic designers believe that controlling the gases is one way to cycle the next shot into the empty chamber, while others opt to harness the energy from the recoil (inertia-driven actions).
Naturally, both shotguns have different internal components and often provide distinctive shooting experiences. That means you should really know the primary use of the shotgun before going into your purchase.
With a gas-powered semi-automatic, the expanding gases from the shell move up the barrel into a gas port that regulates how much pressure is exerted against a piston under the forend. This piston drives back the bolt to cycle the shotgun for the next shot. Siphoning off the gasses to operate the gun provides a low-recoil and balanced shooting experience. In the clays sports, where a shooter can fire hundreds of shots per day, the reduced recoil contributes to more control and less fatigue than an inertia-driven shotgun or even a break-open shotgun.
Without gas ports and carbon-gunked pistons that could potentially clog, the inertia-driven semi-automatic tends to stay much cleaner and therefore more reliable. The chief downside, though, is that inertia-driven shotguns can kick hard. If you're wingshooting, the infrequent number of shots you'll take won't matter that much in terms of recoil fatigue.
Over/Under or Break-Action
An over-under shotgun is a double-barreled, break action shotgun. Where most double-barreled shotguns have the barrels placed side by side, this shotgun has one placed on top of the other.
The specific features will vary based on gauge. Common gauges are 20 gauge, 28 gauge, and 12 gauge. The one chosen should always be based on what works best for you, and suits your preferences. Some will have two triggers, while others will have a single trigger and a switch that allows you to select which barrel will be fired so you can alternate between double and single shots.
There are very few moving parts to these shotguns, which makes them reliable, particularly in areas with inclement weather. They aren’t limited in their ammunition as these shotguns can process any shell that fits, without jamming or short stroking. Each barrel can be loaded with a different shell, which gives the shooter additional choices and the ability to use strategy in competitions.
Having the barrels one on top of the other improves the balance and overall handling of the piece, which makes shooting accurately easier. In cases of wet weather or dust, the action is fully enclosed which leads to it being relatively weatherproof, as long as the firearm isn’t dragged through the mud.
With the lack of moving parts, this also leads to increased durability, as there’s very little that can go wrong with a break action shotgun. With proper care, break-action shotguns can last thousands of rounds. Even cleaning is simple, as it requires only opening the gun, then the barrels are readily available.
Even More Firearm Information
Here's a link to a document that goes into great detail about what to consider when purchasing a new firearm.
Firearms to Consider