A color-casehardened Anson & Deeley Boxlock
Boxlock - A type of action (receiver) for a break-open gun where the lockwork is contained within a box-shaped housing. A boxlock is superior to a sidelock because, although more metal needs to be removed from the action body, less wood needs to be removed from the head of the stock---and wood is generally more vulnerable than metal. The Anson & Deeley boxlock, patented in 1875, the simplest, most reliable and most successful action design, is identified by two pins spanning the width of the action, one at the bottom rear and one slightly forward and higher, upon which the sears and hammers, respectively, rotate.
Sidelock - A type of action on a break-open gun where the lockwork (hammer, sear, mainspring etc) is mounted to the back side (inside) of a plate (or pair of plates for a double gun). A sidelock is superior to a boxlock because: 1. Less steel needs be removed from the bar of the action; the action is therefore stronger. 2. The lock plates provide a larger canvas for the engraver's art. 3. Sidelocks have generally been considered a more aesthetically pleasing form. 4. They are often made with secondary, or intercepting, safety sears. 5. Trigger pulls theoretically may be adjusted more precisely. 6. Because of all the above, most makers building a range of guns have usually reserved the sidelock action for their better grades of guns; this last being the most relevant reason why sidelocks are generally considered superior to boxlocks. In this photo the left lockplate is in place; right lockplate is removed to the show internal mechanism.
A Hand-Detachable, Back-Action Sidelock with right-hand lockplate removed to show internal mechanism. This layout requires less steel to be removed from the bar of the action than does a Bar Action. It is consequently stronger suitable for use in double rifles.
A Bar-Action sidelock removed from gun. Note that the mainspring runs forward into the bar of the action. This layout allows a faster lock time than does a Back Action.
A Hand-Detachable Sidelock with concealed release. A fingernail into the faintly-visible crescent slot on the upper rear of the lockplate pulls open a hinged tab which allows the lockplate to be unscrewed without the use of tools.
Casehardening Colors - mottled blue/green/brown/grey colors on a shotgun or double rifle action, the by-product of a heat-treating process that incorporates carbon into the surface molecular structure of the steel, providing a hard-wearing surface without making the entire action brittle.