To go with the illustration from the Manual of British Rural Sports (2nd ed., 1856) above, here is Walsh's description of the Lang gun in its entirety:

The Breech-Loading Gun.– A gun of this description was introduced some years ago by a French maker, but which was little known in this country until recently taken up by an English gunmaker, Mr. Lang, of Cockspur-street, London, who has succeeding in improving upon the principle to such an extent as to produce a really useful gun, which can be discharged four times while a common fowling-piece can be loaded and discharged once. The principle is exceedingly simple and beautiful, but it requires good workmanship to carry it out; and certainly in that respect Mr. Lang has done full justice to the original idea. The following is a description of this most ingenious invention, which will be better understood by a reference to the engraving: – The barrels are united to the stock by a strong hinge, which is set at liberty by moving the lever (a, fig. 1), a quarter turn to the right, as shown in fig. 2. This turn releases a bolt which connects the barrels firmly to the stock when ready for firing, as shown in fig. 1. After turning the lever, the barrels may be raised from the stock, as shown in fig. 2, and are then ready for loading, for which purpose a cartridge is used, containing within itself all the requisites for the gun’s discharge. These cartridges, which will be presently described, are carried loosely in the pocket of the shooting-coat, or in a properly-made belt, and supersede the necessity for powder-flask, shot-pouch, cap-holder, wadding, &c., &c. In loading, the shooter turns the lever, the barrels then raise themselves by their own weight, and he places into each the cartridge, turns back the lever, and the gun is ready for use, the whole operation being only an affair of a few seconds. On reloading, it is necessary to draw out the discharged cartridge before inserting a fresh one; but the time required for this is scarcely more than that consumed in removing the exploded cap before putting on the fresh one in an ordinary gun. In Mr. Lang’s gun, the old breech-chamber is entirely done away with, and the barrels merely drop down against the flat surface of the false-breech, just as in the revolving pistol. They are bored larger at the breech-end to admit the cartridge, the calibre of which should exactly correspond with that of the rest of the barrel, and thus form with it a continuous tube; the barrel, therefore, has a shoulder at the part where the cartridge ends. The cartridge itself is composed of a thick brown paper-cylinder two inches long, and varying in bore according to that of the gun; one end is open to receive the charge, which is the same as in the ordinary gun, the other is closed by a brass capsule, which overlaps the paper nearly a quarter of an inch, forming a strong joint. In the middle of the inner surface of this capsule, is a small brass chamber, firmly supported in its place, and containing the small cap which fires the charge. A brass pin eight-tenths of an inch in length, and about a line in diameter, passes through the capsule and one side of the small chamber, and receives the cap on its point in the same way as the the ordinary nipple. The other end of the pin projects beyond the cartridge about one-fourth of an inch, and lies in a notch between the barrel and the false-breech, beyond which it stands up to receive the blow of the striker. All this will be more readily understood by a reference to the engraving (figs. 3 and 4). The cartridge-case is sold with the cap, all ready for receiving the powder and shot; and this is the best plan of using it, because then dry powder may always be obtained, and they are as easy to charge as an ordinary gun. Their price is a penny a-piece, containing a cap; but, as the cases may be recapped, and used many times, the additional price is very trifling indeed. The advantages of this gun are manifest, and to all those who value rapid and safe loading as highly as it deserves, I should strongly recommend a trial of it, since it appears to me to supply a great desideratum, and if as good in practice as it appears to me to be theoretically perfect, its invention will be almost as great an era in gun-making as that of the detonator itself. Time, however, and time alone, must decide its merits.

This is the earliest published description of the pin-fire game gun and pin-fire system in Britain that I have, written in 1855, a year after Lang's gun appeared on the market.