The most common types of locks used on drop-barrel guns are detachable box-locks, box-locks, droplocks and side-locks.
The lock is the mechanism that allows the transmission of the trigger movement, via the striker, to the primer of the cartridge. The actual trigger movement will release the hammers so that they drive the striker through the breach face into the primer.
All parts of the lock mechanism are mounted on a horizontally lock plate and the lock can thus be easily removed from the weapon. Detachable box-locks used to have the reputation of being cheap and, above all, poor lock mechanisms. However, nowadays there are very high quality detachable box-locks on the market as they have some advantages.
Since you can take very easily the locks out of the system and clean them, they are popular with safari rifles e.g. Westley Richards. Sporting shotguns, such as those made by Antonio Zoli or Perazzi, use detachable box-locks in order to be able to use different hammer weights and spring types. In this way, the locks are always optimally matched to the ammunition used.
Source: Drop-Lock Antonio Zoli Z-Gun
The box-lock has a very compact design in which the most important work is done "literally in the box". It sits mostly inside the action (box) and is based on the design of Anson & Deeley (approx. 1875). This design has become the standard action of almost all box-locks because of its simplicity and the positioning of the swivel mechanism. It has been much copied since its introduction in the 1870s because it was the first truly successful hammerless design. Westley Richards and Greener have also developed their own solutions based on Anson & Deeley.
Source: Beretta Shotgun; Norbert Klups
In 1897, Westley Richards introduced the manually removable drop lock. In this design, the lower part of the action can be removed to expose the locks, which can then be removed by hand. The design makes it possible to easily clean the locks and, in the case of major defect, to replace the locks completely.
The advantage of the box lock design, with its few components and simple construction, remains.
A side-lock, as the name implies, sits on the side of the action. With an average of over 50 parts, a side-lock is a much more complex lock than a box-lock.
It is difficult to compare the two designs in terms of their merits, as both have advantages and disadvantages. The box-lock is simpler in design and has fewer components that can malfunction. The side-lock adds more mass to the action, and therefore to the swing center. The side-locks are further back, which allows more material staying at the action and therefore, adds stiffness and weight to the system.
The geometry of a side-lock generally makes for a crisper trigger characteristic and the mechanism can usually be easily removed as on an detachable box-lock. Hence makes it easier to clean and repair.
In an over-and-under design, the firing pin holes within the breach face are centered in a middle vertical line, which makes a side-lock mechanism less efficient. This is because the hammers are positioned on the side plates at a slight angle or with a massive hammer head (pic. Below) to provide enough force to detonate the primers.
Fredrick Beasley, perfected the hammerless side-lock. His patents were later purchased by Purdey and then further developed by Holland and Holland.
Demi-bloc and mono-bloc are naming conventions for barrel constructions on shotguns and rifles. The names are helpful because "Demi", coming from the French, means half and Mono stands for one. In that means, a demi-bloc is a breach-bloc of two halves/parts and mono-bloc is one.
The bloc or breach-bloc is in both cases the first part of the barrels that fits into the action and is usually locked by the barrel lumps. This part of the barrel normally also holds the cartridge chamber.
A demi-bloc is often referred to as a "chopper lump barrel." Each barrel is formed from a single piece of steel. The homogeneous steel structure along the entire length (barrel + chamber) makes the piece very stable and material defects are unlikely.
The picture below shows the two assembled demi-blocs. The line in the center is the assembled solder joint, which must fit together exactly to ensure that the barrels are at the correct angle together and stably connected. It is through this angle and the Muzzle wedge that the Barrels are regulated.
The main advantage of a demi-bloc barrel is its strength. When a cartridge is fired, the gun is held closed by the body bolt or lumps. On a demi-bloc barrel the body bolt applies pressure equally to both halves of the barrel, providing additional strength.
However, this design strength only comes into play with side-by-side designs, as the body bolt only engages on the lower barrel with an over-under-design.
A mono-bloc barrel construction consists of three main parts. A receiving block, which is located in the action, and the two barrels, which are inserted into the mono-bloc.
The advantages of a mono-bloc derive from the accuracy of its manufacture, as it is perfectly machinable. Tolerances are often in the range of thousandths of a millimeter, and the position of the barrels relative to each other is much easier to adjust. Because mono-bloc barrels are easier to machine, they are also inherently less expensive to manufacture.
With today's manufacturing capabilities in CNC machining, most guns are made in a mono-bloc design. The quality is on par with demi-block guns, and a mono-bloc can also have advantages depending on the type of gun.
Nonetheless, a gun with handmade chopper lump barrels is a mark of quality and shows the skill of the gunmaker. Therefore, demi-bloc barrels or chopper lump barrels are also significantly more expensive than machine-made mono-bloc barrels.
Source: Boxall & Edmiston (UK)