The 21 cm RK L/19 was the later name of a rifled breech loader gun of the Prussian Navy. This gun started with a massive gun barrel, cast from steel in one piece. In 1868 a built-up gun barrel version was tested in Prussia and found to be much more powerful. Many of the massive guns were then changed to built-up guns. These changed guns became known as kurze Ring Kanone, and later as 21 cm Ring Kanone L/19.

SMS Friedrich Carl  was an ironclad warship built for the Prussian Navy in the mid-1860s. The ship was constructed in the French Société Nouvelle des Forges et Chantiers de la Méditerranée shipyard in Toulon; her hull was laid in 1866 and launched in January 1867. The ship was commissioned into the Prussian Navy in October 1867. The ship was the third ironclad ordered by the Prussian Navy, after Arminius and Prinz Adalbert, though the fourth ship to be acquired, Kronprinz, was ordered after but commissioned before Friedrich Carl.