Boxlocks and Sidelocks
When considering double shotguns, you will often hear the terms “boxlock” and “sidelock.” These are the two broad categories double guns fall into and describes where the action lockwork is located on the gun. It has nothing to do with how the action locks up for firing.
Sidelocks have removable plates on both rear sides of the action. Mounted on those plates are parts such as hammer, mainspring and sear. The hammers may be visible on the outside, like on a muzzleloader, or concealed inside like on a modern shotgun. Boxlock guns have their action parts contained within the hollowed-out body of the action.
The argument of which is better has gone on since Anson & Deeley patented the first successful boxlock in 1875. Sidelock fans point out that that it’s easier to remove sidelocks for cleaning or repair, and that may have been a good argument when guns were less reliable and more prone to breakage. More metal remains in the action of a sidelock, so the argument also exists that they’re “stronger.”
Boxlock fans will quickly point out that metal is stronger than wood, and since sidelocks are inletted into the stock and boxlocks aren’t, that boxlocks are stronger. Boxlocks are also less expensive to make, so you can often get a very good boxlock for less money than a not as good sidelock.
With modern manufacturing and materials, the only real practical difference is that sidelocks have more metal surface for engraving, so you’ll often see that more elaborately engraved (and expensive) shotguns are sidelocks. There are also shotguns with “false sideplates,” which are nothing more than boxlocks with fake sidelocks for the purpose of more space for engraving.