Reddit can often point me to the most interesting of things. Latest is an old patent form Charles John Cooke, who formally patented (though arguably did not actually make the first one), a repeating firearm for trench warfare. Of principal interest to me is the extended magazine, which shows a single-stack capacity of almost 30 rounds.

The introduction to the patent is particularly profound, with its formal introduction. Below is the introduction but you can click here to be taken to Google’s archive of the patent. 

Be it known that 1, CHARLES JOHN Cooxn, a subject of the King of England, residing at Hongkong, China, have invented certain new and useful Improvements in Repeating Firearms for Trench Warfare; and I do hereby declare the following to be a full, clear, and exact description of the invention, such as will enable others skilled in the art to which it appertains to make and use the same.

My invention relates to firearms, but more particularly to machine guns, or small arms of the rapid-fire or magazine type, as also to novel and improved means for mounting and manipulating the same. It is designed, arranged and especially well adapted to meet defensive requirements of trench warfare, the arm proper being directly mounted upon an elongated magazine of large capacity, which in turn is adjustably mounted upon a supporting rod or pike,thus providing for trenches of varying depths,and further equipped with a suitable periscope by means of which latter the gun may be sighted without exposing an intrenched operator to fire of the enemy.

In modern warfare it is a well recognized fact that an efiort is usually made to concentrate the greatest number of siege guns pos sible upon a comparatively short front in order to prepare the way for invading forces, and for trench blasting operations. When the ai:illery fire ceases an effort is then usually made to capture he enemys works by charging infantry in close or mass formation, so that in the last analysis a military invasion becomes practically a hand to hand conflict, either with bayonets or close range small arms. Under these conditions it is apparent that intrenched defenders provided with an equipment such as contemplated by the present invention, must of necessity have an immense advantage over an advancing adversary in mass formation, in that a veritable stream of death-dealing missiles may thus be literally poured into the invading ranks, from numberless rapidfire machine arms, of the class indicated, without exposing their individual operators.

\Vith these and other objects and advantages in view the present invention will now be particularly described, and then set forth in the claims following.