Röchling shells were bunker-busting artillery shells, developed by German engineer August Cönders during World War II, based on the theory of increasing the sectional density to improve penetration.^[1]

The subcaliber shells made from chrome-vanadium steel were able to penetrate much more than 4 metres (13 ft) of reinforced concrete roof before burying the shell through the floor and into earth.^[2] They resembled fin-stabilized arrow shells, but had a discarding flange acting as a driving band instead of fins.^[3] Despite high penetration, these shells had a low muzzle velocity, and thus a high dispersion, 36 m (118 ft) on 1,000 m (3,300 ft) range. As a result, they saw very limited use during World War II; only about 200 shells were ever fired even though 6,000 such shells were made for the very large howitzers like the 21 cm mortar.^{[citation needed]}

Röchling shells were developed for the 21 cm Mörser 18, a captured French 34 cm railway gun 674(f), and the 35.5 cm Haubitze M1 only. In addition, many experimental HE shells were made for the 3.7 cm Pak 36 and 5 cm Pak 38 anti-tank guns.^[4]

Röchling shells were tested in 1942 and 1943 first against the Belgian Fort de Battice then against the Belgian Fort d'Aubin-Neufchâteau.^[5][6] They were also tested against the fortresses of Hůrka,^[7] Hanička and Dobrošov^[8][9] (today's Czech Republic),^[10] the Gössler wall, Toplitzsee (Austria) and at the Hillersleben test facility (Germany).

They were regarded as a German secret weapon, and there is speculation that their use was limited in order to reduce the chance of dud shells being recovered and exploited by the Allied forces.^[11] A more likely reason, however, is their poor accuracy.^[12]

Preserved small-size fin-stabilized prototype of Röchling shell (36 cm long) is since 2020 exhibited in the Museum of Czechoslovakian fortifications,^[13] inside the fort Hůrka [cs]. Remnants of full-sized test shells are still visible in the walls of the fort.^[14]

• Kinetic energy penetrator
• Impact depth